LLVM IR

Debugging now works in my toy MLIR compiler!

May 25, 2025 C/C++ development and debugging. No comments , , , , , , , , , ,

Screenshot

Screenshot

I’ve now got both line debugging (break, next, continue) working, and variable display (and modification) debugging now working for my toy language and compiler.

Here’s an example program:

BOOL i1;
i1 = TRUE;
PRINT i1;

INT8 i8;
i8 = 10;
PRINT i8;

INT16 i16;
i16 = 1000;
PRINT i16;

INT32 i32;
i32 = 100000;
PRINT i32;

INT64 i64;
i64 = 100000000000;
PRINT i64;

FLOAT32 f32;
f32 = 1.1;
PRINT f32;

FLOAT64 f64;
f64 = 2.2E-1;
PRINT f64;

It doesn’t do anything interesting, other than demonstrate that I got the DILocalVariableAttr declarations right for each supported type. Here’s the MLIR for this program:

"builtin.module"() ({
  "toy.program"() ({
    "toy.declare"() <{name = "i1", type = i1}> : () -> () loc(#loc)
    %0 = "arith.constant"() <{value = true}> : () -> i1 loc(#loc1)
    "toy.assign"(%0) <{name = "i1"}> : (i1) -> () loc(#loc1)
    %1 = "toy.load"() <{name = "i1"}> : () -> i1 loc(#loc2)
    "toy.print"(%1) : (i1) -> () loc(#loc2)
    "toy.declare"() <{name = "i8", type = i8}> : () -> () loc(#loc3)
    %2 = "arith.constant"() <{value = 10 : i64}> : () -> i64 loc(#loc4)
    "toy.assign"(%2) <{name = "i8"}> : (i64) -> () loc(#loc4)
    %3 = "toy.load"() <{name = "i8"}> : () -> i8 loc(#loc5)
    "toy.print"(%3) : (i8) -> () loc(#loc5)
    "toy.declare"() <{name = "i16", type = i16}> : () -> () loc(#loc6)
    %4 = "arith.constant"() <{value = 1000 : i64}> : () -> i64 loc(#loc7)
    "toy.assign"(%4) <{name = "i16"}> : (i64) -> () loc(#loc7)
    %5 = "toy.load"() <{name = "i16"}> : () -> i16 loc(#loc8)
    "toy.print"(%5) : (i16) -> () loc(#loc8)
    "toy.declare"() <{name = "i32", type = i32}> : () -> () loc(#loc9)
    %6 = "arith.constant"() <{value = 100000 : i64}> : () -> i64 loc(#loc10)
    "toy.assign"(%6) <{name = "i32"}> : (i64) -> () loc(#loc10)
    %7 = "toy.load"() <{name = "i32"}> : () -> i32 loc(#loc11)
    "toy.print"(%7) : (i32) -> () loc(#loc11)
    "toy.declare"() <{name = "i64", type = i64}> : () -> () loc(#loc12)
    %8 = "arith.constant"() <{value = 100000000000 : i64}> : () -> i64 loc(#loc13)
    "toy.assign"(%8) <{name = "i64"}> : (i64) -> () loc(#loc13)
    %9 = "toy.load"() <{name = "i64"}> : () -> i64 loc(#loc14)
    "toy.print"(%9) : (i64) -> () loc(#loc14)
    "toy.declare"() <{name = "f32", type = f32}> : () -> () loc(#loc15)
    %10 = "arith.constant"() <{value = 1.100000e+00 : f64}> : () -> f64 loc(#loc16)
    "toy.assign"(%10) <{name = "f32"}> : (f64) -> () loc(#loc16)
    %11 = "toy.load"() <{name = "f32"}> : () -> f32 loc(#loc17)
    "toy.print"(%11) : (f32) -> () loc(#loc17)
    "toy.declare"() <{name = "f64", type = f64}> : () -> () loc(#loc18)
    %12 = "arith.constant"() <{value = 2.200000e-01 : f64}> : () -> f64 loc(#loc19)
    "toy.assign"(%12) <{name = "f64"}> : (f64) -> () loc(#loc19)
    %13 = "toy.load"() <{name = "f64"}> : () -> f64 loc(#loc20)
    "toy.print"(%13) : (f64) -> () loc(#loc20)
    "toy.exit"() : () -> () loc(#loc)
  }) : () -> () loc(#loc)
}) : () -> () loc(#loc)
#loc = loc("types.toy":1:1)
#loc1 = loc("types.toy":2:6)
#loc2 = loc("types.toy":3:1)
#loc3 = loc("types.toy":5:1)
#loc4 = loc("types.toy":6:6)
#loc5 = loc("types.toy":7:1)
#loc6 = loc("types.toy":9:1)
#loc7 = loc("types.toy":10:7)
#loc8 = loc("types.toy":11:1)
#loc9 = loc("types.toy":13:1)
#loc10 = loc("types.toy":14:7)
#loc11 = loc("types.toy":15:1)
#loc12 = loc("types.toy":17:1)
#loc13 = loc("types.toy":18:7)
#loc14 = loc("types.toy":19:1)
#loc15 = loc("types.toy":21:1)
#loc16 = loc("types.toy":22:7)
#loc17 = loc("types.toy":23:1)
#loc18 = loc("types.toy":25:1)
#loc19 = loc("types.toy":26:7)
#loc20 = loc("types.toy":27:1)

and the generated LLVM-IR

; ModuleID = 'types.toy'
source_filename = "types.toy"
target datalayout = "e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-i128:128-f80:128-n8:16:32:64-S128"
target triple = "x86_64-unknown-linux-gnu"

declare void @__toy_print_f64(double)

declare void @__toy_print_i64(i64)

define i32 @main() !dbg !4 {
  %1 = alloca i1, i64 1, align 1, !dbg !8
    #dbg_declare(ptr %1, !9, !DIExpression(), !8)
  store i1 true, ptr %1, align 1, !dbg !11
  %2 = load i1, ptr %1, align 1, !dbg !12
  %3 = zext i1 %2 to i64, !dbg !12
  call void @__toy_print_i64(i64 %3), !dbg !12
  %4 = alloca i8, i64 1, align 1, !dbg !13
    #dbg_declare(ptr %4, !14, !DIExpression(), !13)
  store i8 10, ptr %4, align 1, !dbg !16
  %5 = load i8, ptr %4, align 1, !dbg !17
  %6 = sext i8 %5 to i64, !dbg !17
  call void @__toy_print_i64(i64 %6), !dbg !17
  %7 = alloca i16, i64 1, align 2, !dbg !18
    #dbg_declare(ptr %7, !19, !DIExpression(), !18)
  store i16 1000, ptr %7, align 2, !dbg !21
  %8 = load i16, ptr %7, align 2, !dbg !22
  %9 = sext i16 %8 to i64, !dbg !22
  call void @__toy_print_i64(i64 %9), !dbg !22
  %10 = alloca i32, i64 1, align 4, !dbg !23
    #dbg_declare(ptr %10, !24, !DIExpression(), !23)
  store i32 100000, ptr %10, align 4, !dbg !26
  %11 = load i32, ptr %10, align 4, !dbg !27
  %12 = sext i32 %11 to i64, !dbg !27
  call void @__toy_print_i64(i64 %12), !dbg !27
  %13 = alloca i64, i64 1, align 8, !dbg !28
    #dbg_declare(ptr %13, !29, !DIExpression(), !28)
  store i64 100000000000, ptr %13, align 8, !dbg !31
  %14 = load i64, ptr %13, align 8, !dbg !32
  call void @__toy_print_i64(i64 %14), !dbg !32
  %15 = alloca float, i64 1, align 4, !dbg !33
    #dbg_declare(ptr %15, !34, !DIExpression(), !33)
  store float 0x3FF19999A0000000, ptr %15, align 4, !dbg !36
  %16 = load float, ptr %15, align 4, !dbg !37
  %17 = fpext float %16 to double, !dbg !37
  call void @__toy_print_f64(double %17), !dbg !37
  %18 = alloca double, i64 1, align 8, !dbg !38
    #dbg_declare(ptr %18, !39, !DIExpression(), !38)
  store double 2.200000e-01, ptr %18, align 8, !dbg !41
  %19 = load double, ptr %18, align 8, !dbg !42
  call void @__toy_print_f64(double %19), !dbg !42
  ret i32 0, !dbg !8
}

; Function Attrs: nocallback nofree nosync nounwind speculatable willreturn memory(none)
declare void @llvm.dbg.declare(metadata, metadata, metadata) #0

attributes #0 = { nocallback nofree nosync nounwind speculatable willreturn memory(none) }

!llvm.module.flags = !{!0}
!llvm.dbg.cu = !{!1}
!llvm.ident = !{!3}

!0 = !{i32 2, !"Debug Info Version", i32 3}
!1 = distinct !DICompileUnit(language: DW_LANG_C, file: !2, producer: "toycalculator", isOptimized: false, runtimeVersion: 0, emissionKind: FullDebug)
!2 = !DIFile(filename: "types.toy", directory: ".")
!3 = !{!"toycalculator V2"}
!4 = distinct !DISubprogram(name: "main", linkageName: "main", scope: !2, file: !2, line: 1, type: !5, scopeLine: 1, spFlags: DISPFlagDefinition, unit: !1)
!5 = !DISubroutineType(types: !6)
!6 = !{!7}
!7 = !DIBasicType(name: "int", size: 32, encoding: DW_ATE_signed)
!8 = !DILocation(line: 1, column: 1, scope: !4)
!9 = !DILocalVariable(name: "i1", scope: !4, file: !2, line: 1, type: !10, align: 8)
!10 = !DIBasicType(name: "bool", size: 8, encoding: DW_ATE_boolean)
!11 = !DILocation(line: 2, column: 6, scope: !4)
!12 = !DILocation(line: 3, column: 1, scope: !4)
!13 = !DILocation(line: 5, column: 1, scope: !4)
!14 = !DILocalVariable(name: "i8", scope: !4, file: !2, line: 5, type: !15, align: 8)
!15 = !DIBasicType(name: "int8_t", size: 8, encoding: DW_ATE_signed)
!16 = !DILocation(line: 6, column: 6, scope: !4)
!17 = !DILocation(line: 7, column: 1, scope: !4)
!18 = !DILocation(line: 9, column: 1, scope: !4)
!19 = !DILocalVariable(name: "i16", scope: !4, file: !2, line: 9, type: !20, align: 16)
!20 = !DIBasicType(name: "int16_t", size: 16, encoding: DW_ATE_signed)
!21 = !DILocation(line: 10, column: 7, scope: !4)
!22 = !DILocation(line: 11, column: 1, scope: !4)
!23 = !DILocation(line: 13, column: 1, scope: !4)
!24 = !DILocalVariable(name: "i32", scope: !4, file: !2, line: 13, type: !25, align: 32)
!25 = !DIBasicType(name: "int32_t", size: 32, encoding: DW_ATE_signed)
!26 = !DILocation(line: 14, column: 7, scope: !4)
!27 = !DILocation(line: 15, column: 1, scope: !4)
!28 = !DILocation(line: 17, column: 1, scope: !4)
!29 = !DILocalVariable(name: "i64", scope: !4, file: !2, line: 17, type: !30, align: 64)
!30 = !DIBasicType(name: "int64_t", size: 64, encoding: DW_ATE_signed)
!31 = !DILocation(line: 18, column: 7, scope: !4)
!32 = !DILocation(line: 19, column: 1, scope: !4)
!33 = !DILocation(line: 21, column: 1, scope: !4)
!34 = !DILocalVariable(name: "f32", scope: !4, file: !2, line: 21, type: !35, align: 32)
!35 = !DIBasicType(name: "float", size: 32, encoding: DW_ATE_float)
!36 = !DILocation(line: 22, column: 7, scope: !4)
!37 = !DILocation(line: 23, column: 1, scope: !4)
!38 = !DILocation(line: 25, column: 1, scope: !4)
!39 = !DILocalVariable(name: "f64", scope: !4, file: !2, line: 25, type: !40, align: 64)
!40 = !DIBasicType(name: "double", size: 64, encoding: DW_ATE_float)
!41 = !DILocation(line: 26, column: 7, scope: !4)
!42 = !DILocation(line: 27, column: 1, scope: !4)

Interesting bits include:

source_filename = "types.toy"
target datalayout = "e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-i128:128-f80:128-n8:16:32:64-S128"
target triple = "x86_64-unknown-linux-gnu"

!llvm.module.flags = !{!0}
!llvm.dbg.cu = !{!1}
!llvm.ident = !{!3}

!0 = !{i32 2, !"Debug Info Version", i32 3}
!1 = distinct !DICompileUnit(language: DW_LANG_C, file: !2, producer: "toycalculator", isOptimized: false, runtimeVersion: 0, emissionKind: FullDebug)
!2 = !DIFile(filename: "types.toy", directory: ".")
!3 = !{!"toycalculator V2"}
!4 = distinct !DISubprogram(name: "main", linkageName: "main", scope: !2, file: !2, line: 1, type: !5, scopeLine: 1, spFlags: DISPFlagDefinition, unit: !1)
!5 = !DISubroutineType(types: !6)
!6 = !{!7}

Unlike flang’s AddDebugInfoPass DI instrumentation pass, I didn’t try to do anything fancy, instead just implemented a couple of helper functions.

One for the target triple:

void setModuleAttrs()
{
    std::string targetTriple = llvm::sys::getDefaultTargetTriple();
    llvm::Triple triple( targetTriple );
    assert( triple.isArch64Bit() &amp;amp;amp;amp;&amp;amp;amp;amp; triple.isOSLinux() );

    std::string error;
    const llvm::Target* target = llvm::TargetRegistry::lookupTarget( targetTriple, error );
    assert( target );
    llvm::TargetOptions options;
    auto targetMachine = std::unique_ptr&amp;amp;amp;lt;llvm::TargetMachine&amp;amp;amp;gt;( target-&amp;amp;amp;gt;createTargetMachine(
        targetTriple, "generic", "", options, std::optional&amp;amp;amp;lt;llvm::Reloc::Model&amp;amp;amp;gt;( llvm::Reloc::PIC_ ) ) );
    assert( targetMachine );
    std::string dataLayoutStr = targetMachine-&amp;amp;amp;gt;createDataLayout().getStringRepresentation();

    module-&amp;amp;amp;gt;setAttr( "llvm.ident", builder.getStringAttr( COMPILER_NAME COMPILER_VERSION ) );
    module-&amp;amp;amp;gt;setAttr( "llvm.data_layout", builder.getStringAttr( dataLayoutStr ) );
    module-&amp;amp;amp;gt;setAttr( "llvm.target_triple", builder.getStringAttr( targetTriple ) );
}

one for the DICompileUnitAttr, and DISubprogramAttr:

void createMain()
{
    auto ctx = builder.getContext();
    auto mainFuncType = LLVM::LLVMFunctionType::get( builder.getI32Type(), {}, false );
    mainFunc =
        builder.create&amp;amp;amp;lt;LLVM::LLVMFuncOp&amp;amp;amp;gt;( module.getLoc(), ENTRY_SYMBOL_NAME, mainFuncType, LLVM::Linkage::External );

    // Construct module level DI state:
    fileAttr = mlir::LLVM::DIFileAttr::get( ctx, driverState.filename, "." );
    auto distinctAttr = mlir::DistinctAttr::create( builder.getUnitAttr() );
    auto compileUnitAttr = mlir::LLVM::DICompileUnitAttr::get(
        ctx, distinctAttr, llvm::dwarf::DW_LANG_C, fileAttr, builder.getStringAttr( COMPILER_NAME ), false,
        mlir::LLVM::DIEmissionKind::Full, mlir::LLVM::DINameTableKind::Default );
    auto ta =
        mlir::LLVM::DIBasicTypeAttr::get( ctx, (unsigned)llvm::dwarf::DW_TAG_base_type, builder.getStringAttr( "int" ),
                                          32, (unsigned)llvm::dwarf::DW_ATE_signed );
    llvm::SmallVector&amp;amp;amp;lt;mlir::LLVM::DITypeAttr, 1&amp;amp;amp;gt; typeArray;
    typeArray.push_back( ta );
    auto subprogramType = mlir::LLVM::DISubroutineTypeAttr::get( ctx, 0, typeArray );
    subprogramAttr = mlir::LLVM::DISubprogramAttr::get(
        ctx, mlir::DistinctAttr::create( builder.getUnitAttr() ), compileUnitAttr, fileAttr,
        builder.getStringAttr( ENTRY_SYMBOL_NAME ), builder.getStringAttr( ENTRY_SYMBOL_NAME ), fileAttr, 1, 1,
        mlir::LLVM::DISubprogramFlags::Definition, subprogramType, llvm::ArrayRef&amp;amp;amp;lt;mlir::LLVM::DINodeAttr&amp;amp;amp;gt;{},
        llvm::ArrayRef&amp;amp;amp;lt;mlir::LLVM::DINodeAttr&amp;amp;amp;gt;{} );
    mainFunc-&amp;amp;amp;gt;setAttr( "llvm.debug.subprogram", subprogramAttr );

    // This is the key to ensure that translateModuleToLLVMIR does not strip the location info (instead converts
    // loc's into !dbg's)
    mainFunc-&amp;amp;amp;gt;setLoc( builder.getFusedLoc( { module.getLoc() }, subprogramAttr ) );
}

The ‘setLoc’ call above, right near the end is critical.  Without that, the call to mlir::translateModuleToLLVMIR strips out all the loc() references, instead of replacing them with !DILocation.

Finally, one for the variable DI creation:

void constructVariableDI( llvm::StringRef varName, mlir::Type&amp;amp;amp;amp; elemType, mlir::FileLineColLoc loc,
                          unsigned elemSizeInBits, mlir::LLVM::AllocaOp&amp;amp;amp;amp; allocaOp )
{
    auto ctx = builder.getContext();
    allocaOp-&amp;amp;amp;gt;setAttr( "bindc_name", builder.getStringAttr( varName ) );

    mlir::LLVM::DILocalVariableAttr diVar;

    if ( elemType.isa&amp;amp;amp;lt;mlir::IntegerType&amp;amp;amp;gt;() )
    {
        const char* typeName{};
        unsigned dwType = llvm::dwarf::DW_ATE_signed;
        unsigned sz = elemSizeInBits;

        switch ( elemSizeInBits )
        {
            case 1:
            {
                typeName = "bool";
                dwType = llvm::dwarf::DW_ATE_boolean;
                sz = 8;
                break;
            }
            case 8:
            {
                typeName = "int8_t";
                break;
            }
            case 16:
            {
                typeName = "int16_t";
                break;
            }
            case 32:
            {
                typeName = "int32_t";
                break;
            }
            case 64:
            {
                typeName = "int64_t";
                break;
            }
            default:
            {
                llvm_unreachable( "Unsupported float type size" );
            }
        }

        auto diType = mlir::LLVM::DIBasicTypeAttr::get( ctx, llvm::dwarf::DW_TAG_base_type,
                                                        builder.getStringAttr( typeName ), sz, dwType );

        diVar = mlir::LLVM::DILocalVariableAttr::get( ctx, subprogramAttr, builder.getStringAttr( varName ), fileAttr,
                                                      loc.getLine(), 0, sz, diType, mlir::LLVM::DIFlags::Zero );
    }
    else
    {
        const char* typeName{};

        switch ( elemSizeInBits )
        {
            case 32:
            {
                typeName = "float";
                break;
            }
            case 64:
            {
                typeName = "double";
                break;
            }
            default:
            {
                llvm_unreachable( "Unsupported float type size" );
            }
        }

        auto diType =
            mlir::LLVM::DIBasicTypeAttr::get( ctx, llvm::dwarf::DW_TAG_base_type, builder.getStringAttr( typeName ),
                                              elemSizeInBits, llvm::dwarf::DW_ATE_float );

        diVar =
            mlir::LLVM::DILocalVariableAttr::get( ctx, subprogramAttr, builder.getStringAttr( varName ), fileAttr,
                                                  loc.getLine(), 0, elemSizeInBits, diType, mlir::LLVM::DIFlags::Zero );
    }
            
    builder.setInsertionPointAfter( allocaOp );
    builder.create&amp;amp;amp;lt;mlir::LLVM::DbgDeclareOp&amp;amp;amp;gt;( loc, allocaOp, diVar );
        
    symbolToAlloca[varName] = allocaOp;
}

In this code, the call to builder.setInsertionPointAfter is critical.  When the lowering eraseOp takes out the DeclareOp, we need the replacement instructions to all end up in the same place.  Without that, the subsequent AssignOp lowering results in an error like this:

//===-------------------------------------------===//
Legalizing operation : 'toy.assign'(0x2745ab50) {
  "toy.assign"(%3) <{name = "x"}> : (i64) -> ()Fold {
  } -> FAILURE : unable to fold
Pattern : 'toy.assign -> ()' {
Trying to match "toy::AssignOpLowering"
Lowering AssignOp: toy.assign "x", %c5_i64 : i64
name: x
value: ImplicitTypeIDRegistry::lookupOrInsert(mlir::PromotableOpInterface::Trait<mlir::TypeID::get()::Empty>)
...
operand #0 does not dominate this use
mlir-asm-printer: 'builtin.module' failed to verify and will be printed in generic form
%3 = "arith.constant"() <{value = 5 : i64}> : () -> i64
valType: i64
elemType: f64
** Insert  : 'llvm.sitofp'(0x274a6ed0)
ImplicitTypeIDRegistry::lookupOrInsert(mlir::LLVM::detail::StoreOpGenericAdaptorBase::Properties)
** Insert  : 'llvm.store'(0x27437f30)
** Erase   : 'toy.assign'(0x2745ab50)
"toy::AssignOpLowering" result 1

My DI insertion isn’t fancy like flang’s, but I have only simple types to deal with, and don’t even support functions yet, so my simple way seemed like a reasonable choice. Regardless, getting working debugger support is nice milestone.

The quest for DWARF instrumentation in a toy MLIR compiler

May 25, 2025 C/C++ development and debugging. No comments , , , , , , ,

I’ve gotten my toy compiler project to the stage where I can generate a program, compile and run it.  Here’s a little example:

Screenshot

This is the MLIR that is generated for this little program:

"builtin.module"() ({
  "toy.program"() ({
    "toy.declare"() <{name = "x", type = f64}> : () -> () loc(#loc)
    %0 = "arith.constant"() <{value = 5 : i64}> : () -> i64 loc(#loc1)
    %1 = "arith.constant"() <{value = 3.140000e+00 : f64}> : () -> f64 loc(#loc1)
    %2 = "toy.add"(%0, %1) : (i64, f64) -> f64 loc(#loc1)
    "toy.assign"(%2) <{name = "x"}> : (f64) -> () loc(#loc1)
    %3 = "toy.load"() <{name = "x"}> : () -> f64 loc(#loc2)
    "toy.print"(%3) : (f64) -> () loc(#loc2)
    "toy.declare"() <{name = "y", type = f64}> : () -> () loc(#loc3)
    %4 = "toy.load"() <{name = "x"}> : () -> f64 loc(#loc4)
    %5 = "arith.constant"() <{value = 2 : i64}> : () -> i64 loc(#loc4)
    %6 = "toy.mul"(%4, %5) : (f64, i64) -> f64 loc(#loc4)
    "toy.assign"(%6) <{name = "y"}> : (f64) -> () loc(#loc4)
    %7 = "toy.load"() <{name = "y"}> : () -> f64 loc(#loc5)
    "toy.print"(%7) : (f64) -> () loc(#loc5)
    "toy.exit"() : () -> () loc(#loc)
  }) : () -> () loc(#loc)
}) : () -> () loc(#loc)
#loc = loc("test.toy":1:1)
#loc1 = loc("test.toy":2:5)
#loc2 = loc("test.toy":3:1)
#loc3 = loc("test.toy":4:1)
#loc4 = loc("test.toy":5:5)
#loc5 = loc("test.toy":6:1)

Notice how the location information is baked into the cake. After lowering of all the MLIR operations to LLVM-IR (except for the top most MLIR module), we have:

"builtin.module"() ({
  "llvm.func"() <{CConv = #llvm.cconv, function_type = !llvm.func, linkage = #llvm.linkage, sym_name = "__toy_print_f64", visibility_ = 0 : i64}> ({
  }) : () -> () loc(#loc)
  "llvm.func"() <{CConv = #llvm.cconv, function_type = !llvm.func, linkage = #llvm.linkage, sym_name = "__toy_print_i64", visibility_ = 0 : i64}> ({
  }) : () -> () loc(#loc)
  "llvm.func"() <{CConv = #llvm.cconv, function_type = !llvm.func, linkage = #llvm.linkage, sym_name = "main", visibility_ = 0 : i64}> ({
    %0 = "llvm.mlir.constant"() <{value = 1 : i64}> : () -> i64 loc(#loc)
    %1 = "llvm.alloca"(%0) <{alignment = 8 : i64, elem_type = f64}> : (i64) -> !llvm.ptr loc(#loc)
    %2 = "llvm.mlir.constant"() <{value = 5 : i64}> : () -> i64 loc(#loc1)
    %3 = "llvm.mlir.constant"() <{value = 3.140000e+00 : f64}> : () -> f64 loc(#loc1)
    %4 = "llvm.sitofp"(%2) : (i64) -> f64 loc(#loc1)
    %5 = "llvm.fadd"(%4, %3) <{fastmathFlags = #llvm.fastmath}> : (f64, f64) -> f64 loc(#loc1)
    "llvm.store"(%5, %1) <{ordering = 0 : i64}> : (f64, !llvm.ptr) -> () loc(#loc1)
    %6 = "llvm.load"(%1) <{ordering = 0 : i64}> : (!llvm.ptr) -> f64 loc(#loc2)
    "llvm.call"(%6) <{CConv = #llvm.cconv, TailCallKind = #llvm.tailcallkind, callee = @__toy_print_f64, fastmathFlags = #llvm.fastmath, op_bundle_sizes = array, operandSegmentSizes = array}> : (f64) -> () loc(#loc2)
    %7 = "llvm.alloca"(%0) <{alignment = 8 : i64, elem_type = f64}> : (i64) -> !llvm.ptr loc(#loc3)
    %8 = "llvm.load"(%1) <{ordering = 0 : i64}> : (!llvm.ptr) -> f64 loc(#loc4)
    %9 = "llvm.mlir.constant"() <{value = 2 : i64}> : () -> i64 loc(#loc4)
    %10 = "llvm.sitofp"(%9) : (i64) -> f64 loc(#loc4)
    %11 = "llvm.fmul"(%8, %10) <{fastmathFlags = #llvm.fastmath}> : (f64, f64) -> f64 loc(#loc4)
    "llvm.store"(%11, %7) <{ordering = 0 : i64}> : (f64, !llvm.ptr) -> () loc(#loc4)
    %12 = "llvm.load"(%7) <{ordering = 0 : i64}> : (!llvm.ptr) -> f64 loc(#loc5)
    "llvm.call"(%12) <{CConv = #llvm.cconv, TailCallKind = #llvm.tailcallkind, callee = @__toy_print_f64, fastmathFlags = #llvm.fastmath, op_bundle_sizes = array, operandSegmentSizes = array}> : (f64) -> () loc(#loc5)
    %13 = "llvm.mlir.constant"() <{value = 0 : i32}> : () -> i32 loc(#loc)
    "llvm.return"(%13) : (i32) -> () loc(#loc)
  }) : () -> () loc(#loc)
}) : () -> () loc(#loc)
#loc = loc("../samples/test.toy":1:1)
#loc1 = loc("../samples/test.toy":2:5)
#loc2 = loc("../samples/test.toy":3:1)
#loc3 = loc("../samples/test.toy":4:1)
#loc4 = loc("../samples/test.toy":5:5)
#loc5 = loc("../samples/test.toy":6:1)

All is still good.  See how all the location information has been retained through the lowering to LLVM-IR transformations. However, as soon as we run mlir::translateModuleToLLVMIR, our final LLVM-IR ends up stripped of all the location information:

; ModuleID = '../samples/test.toy'
source_filename = "../samples/test.toy"

declare void @__toy_print_f64(double)

declare void @__toy_print_i64(i64)

define i32 @main() {
  %1 = alloca double, i64 1, align 8
  store double 8.140000e+00, ptr %1, align 8
  %2 = load double, ptr %1, align 8
  call void @__toy_print_f64(double %2)
  %3 = alloca double, i64 1, align 8
  %4 = load double, ptr %1, align 8
  %5 = fmul double %4, 2.000000e+00
  store double %5, ptr %3, align 8
  %6 = load double, ptr %3, align 8
  call void @__toy_print_f64(double %6)
  ret i32 0
}

!llvm.module.flags = !{!0}

!0 = !{i32 2, !"Debug Info Version", i32 3}

If we build a simple standalone C program with clang, it’s a much different story:

int main() {
    int x = 42;
    return 0;
}


fedora:/home/pjoot/toycalculator/prototypes> clang -g -S -emit-llvm test.c
fedora:/home/pjoot/toycalculator/prototypes> cat test.s
; ModuleID = 'test.c'
source_filename = "test.c"
target datalayout = "e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-i128:128-f80:128-n8:16:32:64-S128"
target triple = "x86_64-redhat-linux-gnu"

; Function Attrs: noinline nounwind optnone uwtable
define dso_local i32 @main() #0 !dbg !8 {
  %1 = alloca i32, align 4
  %2 = alloca i32, align 4
  store i32 0, ptr %1, align 4
    #dbg_declare(ptr %2, !13, !DIExpression(), !14)
  store i32 42, ptr %2, align 4, !dbg !14
  ret i32 0, !dbg !15
}

attributes #0 = { noinline nounwind optnone uwtable "frame-pointer"="all" "min-legal-vector-width"="0" "no-trapping-math"="true" "stack-protector-buffer-size"="8" "target-cpu"="x86-64" "target-features"="+cmov,+cx8,+fxsr,+mmx,+sse,+sse2,+x87" "tune-cpu"="generic" }

!llvm.dbg.cu = !{!0}
!llvm.module.flags = !{!2, !3, !4, !5, !6}
!llvm.ident = !{!7}

!0 = distinct !DICompileUnit(language: DW_LANG_C11, file: !1, producer: "clang version 20.1.3 (Fedora 20.1.3-1.fc42)", isOptimized: false, runtimeVersion: 0, emissionKind: FullDebug, splitDebugInlining: false, nameTableKind: None)
!1 = !DIFile(filename: "test.c", directory: "/home/pjoot/toycalculator/prototypes", checksumkind: CSK_MD5, checksum: "8d269ab8efc2854a823fb05a50ccf4b0")
!2 = !{i32 7, !"Dwarf Version", i32 5}
!3 = !{i32 2, !"Debug Info Version", i32 3}
!4 = !{i32 1, !"wchar_size", i32 4}
!5 = !{i32 7, !"uwtable", i32 2}
!6 = !{i32 7, !"frame-pointer", i32 2}
!7 = !{!"clang version 20.1.3 (Fedora 20.1.3-1.fc42)"}
!8 = distinct !DISubprogram(name: "main", scope: !1, file: !1, line: 1, type: !9, scopeLine: 1, spFlags: DISPFlagDefinition, unit: !0, retainedNodes: !12)
!9 = !DISubroutineType(types: !10)
!10 = !{!11}
!11 = !DIBasicType(name: "int", size: 32, encoding: DW_ATE_signed)
!12 = !{}
!13 = !DILocalVariable(name: "x", scope: !8, file: !1, line: 2, type: !11)
!14 = !DILocation(line: 2, column: 9, scope: !8)
!15 = !DILocation(line: 3, column: 5, scope: !8)

This has a number of features that our automatically-lowered LLVM-IR does not have including:

  • !DICompileUnit
  • !DIFile
  • Dwarf Version
  • !DISubprogram
  • !DILocation
  • !llvm.dbg.cu
  • source_filename
  • target datalayout
  • target triple

It doesn’t surprise me that we don’t have some of these (like target triple, …) without doing extra work, but I’d have expected the location info to be converted into !DILocation.

It’s possible to emit DWARF info from an LLVM builder, which is how a non-MLIR project would do it.  I have brutal hack of an example of that in my project — that program is a little standalone beastie that constructs equivalent LLVM-IR for a simulated C program.  After MLIR lowering to LLVM-IR, I’m able to inject all the DI instrumentation (relying on the fact that I know the operations in this program, and the line numbers for those operations).  It would be very difficult to do this in my toy-language compiler driver, since that has to work for all programs.  It is totally and completely obvious that this is the WRONG way to do it.  However, the little program is at least debuggable, and perhaps worth something for that reason.

We see the same sort of DI artifacts if we build a Fortran program with flang (not classic-flang, but MLIR flang):

program main
  implicit none
  integer :: x
  x = 42
  print *, x
end program main

The flang LL has all the expected DI elements:

; ModuleID = 'FIRModule'
source_filename = "FIRModule"
target datalayout = "e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-i128:128-f80:128-n8:16:32:64-S128"
target triple = "x86_64-unknown-linux-gnu"

$_QQclXc451cd8b1f5eae110b31be7df6c0fcf9 = comdat any

@_QQclXc451cd8b1f5eae110b31be7df6c0fcf9 = linkonce constant [43 x i8] c"/home/pjoot/toycalculator/fortran/test.f90\00", comdat

define void @_QQmain() #0 !dbg !6 {
  %1 = alloca i32, i64 1, align 4, !dbg !9
    #dbg_declare(ptr %1, !10, !DIExpression(), !9)
  store i32 42, ptr %1, align 4, !dbg !12
  %2 = call ptr @_FortranAioBeginExternalListOutput(i32 6, ptr @_QQclXc451cd8b1f5eae110b31be7df6c0fcf9, i32 5), !dbg !13
  %3 = load i32, ptr %1, align 4, !dbg !14
  %4 = call i1 @_FortranAioOutputInteger32(ptr %2, i32 %3), !dbg !14
  %5 = call i32 @_FortranAioEndIoStatement(ptr %2), !dbg !13
  ret void, !dbg !15
}

declare !dbg !16 ptr @_FortranAioBeginExternalListOutput(i32, ptr, i32)

declare !dbg !21 zeroext i1 @_FortranAioOutputInteger32(ptr, i32)

declare !dbg !25 i32 @_FortranAioEndIoStatement(ptr)

declare !dbg !28 void @_FortranAProgramStart(i32, ptr, ptr, ptr)

declare !dbg !31 void @_FortranAProgramEndStatement()

define i32 @main(i32 %0, ptr %1, ptr %2) #0 !dbg !33 {
  call void @_FortranAProgramStart(i32 %0, ptr %1, ptr %2, ptr null), !dbg !36
  call void @_QQmain(), !dbg !36
  call void @_FortranAProgramEndStatement(), !dbg !36
  ret i32 0, !dbg !36
}

attributes #0 = { "frame-pointer"="all" "target-cpu"="x86-64" }

!llvm.module.flags = !{!0, !1, !2}
!llvm.dbg.cu = !{!3}
!llvm.ident = !{!5}

!0 = !{i32 2, !"Debug Info Version", i32 3}
!1 = !{i32 8, !"PIC Level", i32 2}
!2 = !{i32 7, !"PIE Level", i32 2}
!3 = distinct !DICompileUnit(language: DW_LANG_Fortran95, file: !4, producer: "flang version 20.1.5 (https://github.com/llvm/llvm-project.git 7b09d7b446383b71b63d429b21ee45ba389c5134)", isOptimized: false, runtimeVersion: 0, emissionKind: FullDebug)
!4 = !DIFile(filename: "test.f90", directory: "/home/pjoot/toycalculator/fortran")
!5 = !{!"flang version 20.1.5 (https://github.com/llvm/llvm-project.git 7b09d7b446383b71b63d429b21ee45ba389c5134)"}
!6 = distinct !DISubprogram(name: "main", linkageName: "_QQmain", scope: !4, file: !4, line: 1, type: !7, scopeLine: 1, spFlags: DISPFlagDefinition | DISPFlagMainSubprogram, unit: !3)
!7 = !DISubroutineType(cc: DW_CC_program, types: !8)
!8 = !{null}
!9 = !DILocation(line: 3, column: 14, scope: !6)
!10 = !DILocalVariable(name: "x", scope: !6, file: !4, line: 3, type: !11)
!11 = !DIBasicType(name: "integer", size: 32, encoding: DW_ATE_signed)
!12 = !DILocation(line: 4, column: 3, scope: !6)
!13 = !DILocation(line: 5, column: 3, scope: !6)
!14 = !DILocation(line: 5, column: 12, scope: !6)
!15 = !DILocation(line: 6, column: 1, scope: !6)
!16 = !DISubprogram(name: "_FortranAioBeginExternalListOutput", linkageName: "_FortranAioBeginExternalListOutput", scope: !4, file: !4, line: 5, type: !17, scopeLine: 5, spFlags: 0)
!17 = !DISubroutineType(cc: DW_CC_normal, types: !18)
!18 = !{!19, !11, !20, !11}
!19 = !DIDerivedType(tag: DW_TAG_pointer_type, baseType: !20, size: 64)
!20 = !DIBasicType(name: "integer", size: 8, encoding: DW_ATE_signed)
!21 = !DISubprogram(name: "_FortranAioOutputInteger32", linkageName: "_FortranAioOutputInteger32", scope: !4, file: !4, line: 5, type: !22, scopeLine: 5, spFlags: 0)
!22 = !DISubroutineType(cc: DW_CC_normal, types: !23)
!23 = !{!24, !20, !11}
!24 = !DIBasicType(name: "integer", size: 1, encoding: DW_ATE_signed)
!25 = !DISubprogram(name: "_FortranAioEndIoStatement", linkageName: "_FortranAioEndIoStatement", scope: !4, file: !4, line: 5, type: !26, scopeLine: 5, spFlags: 0)
!26 = !DISubroutineType(cc: DW_CC_normal, types: !27)
!27 = !{!11, !20}
!28 = !DISubprogram(name: "_FortranAProgramStart", linkageName: "_FortranAProgramStart", scope: !4, file: !4, line: 6, type: !29, scopeLine: 6, spFlags: 0)
!29 = !DISubroutineType(cc: DW_CC_normal, types: !30)
!30 = !{null, !11, !11, !11, !11}
!31 = !DISubprogram(name: "_FortranAProgramEndStatement", linkageName: "_FortranAProgramEndStatement", scope: !4, file: !4, line: 6, type: !32, scopeLine: 6, spFlags: 0)
!32 = !DISubroutineType(cc: DW_CC_normal, types: !8)
!33 = distinct !DISubprogram(name: "main", linkageName: "main", scope: !4, file: !4, line: 6, type: !34, scopeLine: 6, spFlags: DISPFlagDefinition, unit: !3)
!34 = !DISubroutineType(cc: DW_CC_normal, types: !35)
!35 = !{!11, !11, !11, !11}
!36 = !DILocation(line: 6, column: 1, scope: !33)

Ignoring all the language bootstrap gorp, this has all the expected DI elements, so we know that there’s a mechanism for flang to emit the DWARF instrumentation.

Looking through the flang source, the only place I found any sort of DI instrumentation was flang/lib/Optimizer/Transforms/AddDebugInfo.cpp, which, in about 600 lines, appears to implement all the DI instrumentation needed for variable and function declarations for the Fortran language (as a MLIR pass.)  There’s nothing in there for location translation that I can see, except for a couple of little sneaky bits

bool debugInfoIsAlreadySet(mlir::Location loc) {
  if (mlir::isa<mlir::FusedLoc>(loc)) {
    if (loc->findInstanceOf<mlir::FusedLocWith<fir::LocationKindAttr>>())
      return false;
    return true;
  }
  return false;
}

...
declOp->setLoc(builder.getFusedLoc({declOp->getLoc()}, localVarAttr));
...
globalOp->setLoc(builder.getFusedLoc({globalOp.getLoc()}, arrayAttr));
...

void AddDebugInfoPass::handleFuncOp(mlir::func::FuncOp funcOp,
                                    mlir::LLVM::DIFileAttr fileAttr,
                                    mlir::LLVM::DICompileUnitAttr cuAttr,
                                    fir::DebugTypeGenerator &typeGen,
                                    mlir::SymbolTable *symbolTable) {
  mlir::Location l = funcOp->getLoc();
  // If fused location has already been created then nothing to do
  // Otherwise, create a fused location.
  if (debugInfoIsAlreadySet(l))
    return;
...
// We have the imported entities now. Generate the final DISubprogramAttr.
  spAttr = mlir::LLVM::DISubprogramAttr::get(
      context, recId, /*isRecSelf=*/false, id2, compilationUnit, Scope,
      funcName, fullName, funcFileAttr, line, line, subprogramFlags,
      subTypeAttr, entities, /*annotations=*/{});
  funcOp->setLoc(builder.getFusedLoc({l}, spAttr));
...

It looks as if a fused location is used to replace any MLIR supplied location for each function, but all the rest of the location info is left as is.  My presumption is that translateModuleToLLVMIR() handles the plain old loc() references to !dbg.  To get an idea what happens in AddDebugInfoPass, let’s look at the MLIR dump at the beginning, and compare to afterwards.

To see this, I used the following gdb script:

set follow-fork-mode child
set detach-on-fork off
set breakpoint pending on
break AddDebugInfoPass::runOnOperation
run

Here’s the debug session to see the module dumps before and after AddDebugInfoPass

fedora:/home/pjoot/toycalculator/fortran> gdb -q -x b --args /usr/local/llvm-20.1.5/bin/flang-new -g -S -emit-llvm test.f90 -o test.ll
...
[Switching to Thread 0x7fffda437640 (LWP 119612)]

Thread 2.1 "flang" hit Breakpoint 1.2, (anonymous namespace)::AddDebugInfoPass::runOnOperation (this=0x6368e0)
    at /home/pjoot/llvm-project/flang/lib/Optimizer/Transforms/AddDebugInfo.cpp:520
520	  mlir::ModuleOp module = getOperation();
(gdb) n
521	  mlir::MLIRContext *context = &getContext();
(gdb) p module->dump()
module attributes {dlti.dl_spec = #dlti.dl_spec : vector<2xi64>, !llvm.ptr<270> = dense<32> : vector<4xi64>, f16 = dense<16> : vector<2xi64>, f64 = dense<64> : vector<2xi64>, i32 = dense<32> : vector<2xi64>, i16 = dense<16> : vector<2xi64>, i8 = dense<8> : vector<2xi64>, i1 = dense<8> : vector<2xi64>, !llvm.ptr = dense<64> : vector<4xi64>, f80 = dense<128> : vector<2xi64>, i128 = dense<128> : vector<2xi64>, i64 = dense<64> : vector<2xi64>, !llvm.ptr<271> = dense<32> : vector<4xi64>, !llvm.ptr<272> = dense<64> : vector<4xi64>, "dlti.endianness" = "little", "dlti.stack_alignment" = 128 : i64>, fir.defaultkind = "a1c4d8i4l4r4", fir.kindmap = "", fir.target_cpu = "x86-64", llvm.data_layout = "e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-i128:128-f80:128-n8:16:32:64-S128", llvm.ident = "flang version 20.1.5 (https://github.com/llvm/llvm-project.git 7b09d7b446383b71b63d429b21ee45ba389c5134)", llvm.target_triple = "x86_64-unknown-linux-gnu"} {
  func.func @_QQmain() attributes {fir.bindc_name = "main"} {
    %c5_i32 = arith.constant 5 : i32
    %c6_i32 = arith.constant 6 : i32
    %c42_i32 = arith.constant 42 : i32
    %0 = fir.alloca i32 {bindc_name = "x", uniq_name = "_QFEx"}
    %1 = fircg.ext_declare %0 {uniq_name = "_QFEx"} : (!fir.ref) -> !fir.ref
    fir.store %c42_i32 to %1 : !fir.ref
    %2 = fir.address_of(@_QQclXc451cd8b1f5eae110b31be7df6c0fcf9) : !fir.ref<!fir.char<1,43>>
    %3 = fir.convert %2 : (!fir.ref<!fir.char<1,43>>) -> !fir.ref
    %4 = fir.call @_FortranAioBeginExternalListOutput(%c6_i32, %3, %c5_i32) fastmath : (i32, !fir.ref, i32) -> !fir.ref
    %5 = fir.load %1 : !fir.ref
    %6 = fir.call @_FortranAioOutputInteger32(%4, %5) fastmath : (!fir.ref, i32) -> i1
    %7 = fir.call @_FortranAioEndIoStatement(%4) fastmath : (!fir.ref) -> i32
    return
  }
  func.func private @_FortranAioBeginExternalListOutput(i32, !fir.ref, i32) -> !fir.ref attributes {fir.io, fir.runtime}
  fir.global linkonce @_QQclXc451cd8b1f5eae110b31be7df6c0fcf9 constant : !fir.char<1,43> {
    %0 = fir.string_lit "/home/pjoot/toycalculator/fortran/test.f90\00"(43) : !fir.char<1,43>
    fir.has_value %0 : !fir.char<1,43>
  }
  func.func private @_FortranAioOutputInteger32(!fir.ref, i32) -> i1 attributes {fir.io, fir.runtime}
  func.func private @_FortranAioEndIoStatement(!fir.ref) -> i32 attributes {fir.io, fir.runtime}
  func.func private @_FortranAProgramStart(i32, !llvm.ptr, !llvm.ptr, !llvm.ptr)
  func.func private @_FortranAProgramEndStatement()
  func.func @main(%arg0: i32, %arg1: !llvm.ptr, %arg2: !llvm.ptr) -> i32 {
    %c0_i32 = arith.constant 0 : i32
    %0 = fir.zero_bits !fir.ref<tuple<i32, !fir.ref<!fir.array<0xtuple<!fir.ref, !fir.ref>>>>>
    fir.call @_FortranAProgramStart(%arg0, %arg1, %arg2, %0) fastmath : (i32, !llvm.ptr, !llvm.ptr, !fir.ref<tuple<i32, !fir.ref<!fir.array<0xtuple<!fir.ref, !fir.ref>>>>>) -> ()
    fir.call @_QQmain() fastmath : () -> ()
    fir.call @_FortranAProgramEndStatement() fastmath : () -> ()
    return %c0_i32 : i32
  }
}
(gdb) c
Continuing.

Thread 2.1 "flang" hit Breakpoint 3.2, Fortran::frontend::CodeGenAction::generateLLVMIR (this=0x485530) at /home/pjoot/llvm-project/flang/lib/Frontend/FrontendActions.cpp:889
889	  timingScopeMLIRPasses.stop();

(gdb) mlirModule.op.dump()
module attributes {dlti.dl_spec = #dlti.dl_spec : vector<2xi64>, !llvm.ptr<270> = dense<32> : vector<4xi64>, f16 = dense<16> : vector<2xi64>, f64 = dense<64> : vector<2xi64>, i32 = dense<32> : vector<2xi64>, i16 = dense<16> : vector<2xi64>, i8 = dense<8> : vector<2xi64>, i1 = dense<8> : vector<2xi64>, !llvm.ptr = dense<64> : vector<4xi64>, f80 = dense<128> : vector<2xi64>, i128 = dense<128> : vector<2xi64>, i64 = dense<64> : vector<2xi64>, !llvm.ptr<271> = dense<32> : vector<4xi64>, !llvm.ptr<272> = dense<64> : vector<4xi64>, "dlti.endianness" = "little", "dlti.stack_alignment" = 128 : i64>, fir.defaultkind = "a1c4d8i4l4r4", fir.kindmap = "", fir.target_cpu = "x86-64", llvm.data_layout = "e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-i128:128-f80:128-n8:16:32:64-S128", llvm.ident = "flang version 20.1.5 (https://github.com/llvm/llvm-project.git 7b09d7b446383b71b63d429b21ee45ba389c5134)", llvm.target_triple = "x86_64-unknown-linux-gnu"} {
  llvm.func @_QQmain() attributes {fir.bindc_name = "main", frame_pointer = #llvm.framePointerKind, target_cpu = "x86-64"} {
    %0 = llvm.mlir.constant(1 : i64) : i64
    %1 = llvm.alloca %0 x i32 {bindc_name = "x"} : (i64) -> !llvm.ptr
    %2 = llvm.mlir.constant(5 : i32) : i32
    %3 = llvm.mlir.constant(6 : i32) : i32
    %4 = llvm.mlir.constant(42 : i32) : i32
    %5 = llvm.mlir.constant(1 : i64) : i64
    llvm.intr.dbg.declare #llvm.di_local_variable, id = distinct[1]<>, compileUnit = , sourceLanguage = DW_LANG_Fortran95, file = <"test.f90" in "/home/pjoot/toycalculator/fortran">, producer = "flang version 20.1.5 (https://github.com/llvm/llvm-project.git 7b09d7b446383b71b63d429b21ee45ba389c5134)", isOptimized = false, emissionKind = Full>, scope = #llvm.di_file<"test.f90" in "/home/pjoot/toycalculator/fortran">, name = "main", linkageName = "_QQmain", file = <"test.f90" in "/home/pjoot/toycalculator/fortran">, line = 1, scopeLine = 1, subprogramFlags = "Definition|MainSubprogram", type = >, name = "x", file = <"test.f90" in "/home/pjoot/toycalculator/fortran">, line = 3, type = #llvm.di_basic_type> = %1 : !llvm.ptr
    llvm.store %4, %1 : i32, !llvm.ptr
    %6 = llvm.mlir.addressof @_QQclXc451cd8b1f5eae110b31be7df6c0fcf9 : !llvm.ptr
    %7 = llvm.call @_FortranAioBeginExternalListOutput(%3, %6, %2) {fastmathFlags = #llvm.fastmath} : (i32, !llvm.ptr, i32) -> !llvm.ptr
    %8 = llvm.load %1 : !llvm.ptr -> i32
    %9 = llvm.call @_FortranAioOutputInteger32(%7, %8) {fastmathFlags = #llvm.fastmath} : (!llvm.ptr, i32) -> i1
    %10 = llvm.call @_FortranAioEndIoStatement(%7) {fastmathFlags = #llvm.fastmath} : (!llvm.ptr) -> i32
    llvm.return
  }
  llvm.func @_FortranAioBeginExternalListOutput(i32, !llvm.ptr, i32) -> !llvm.ptr attributes {fir.io, fir.runtime, frame_pointer = #llvm.framePointerKind, sym_visibility = "private", target_cpu = "x86-64"}
  llvm.mlir.global linkonce constant @_QQclXc451cd8b1f5eae110b31be7df6c0fcf9() comdat(@__llvm_comdat::@_QQclXc451cd8b1f5eae110b31be7df6c0fcf9) {addr_space = 0 : i32} : !llvm.array<43 x i8> {
    %0 = llvm.mlir.constant("/home/pjoot/toycalculator/fortran/test.f90\00") : !llvm.array<43 x i8>
    llvm.return %0 : !llvm.array<43 x i8>
  }
  llvm.comdat @__llvm_comdat {
    llvm.comdat_selector @_QQclXc451cd8b1f5eae110b31be7df6c0fcf9 any
  }
  llvm.func @_FortranAioOutputInteger32(!llvm.ptr, i32) -> (i1 {llvm.zeroext}) attributes {fir.io, fir.runtime, frame_pointer = #llvm.framePointerKind, sym_visibility = "private", target_cpu = "x86-64"}
  llvm.func @_FortranAioEndIoStatement(!llvm.ptr) -> i32 attributes {fir.io, fir.runtime, frame_pointer = #llvm.framePointerKind, sym_visibility = "private", target_cpu = "x86-64"}
  llvm.func @_FortranAProgramStart(i32, !llvm.ptr, !llvm.ptr, !llvm.ptr) attributes {frame_pointer = #llvm.framePointerKind, sym_visibility = "private", target_cpu = "x86-64"}
  llvm.func @_FortranAProgramEndStatement() attributes {frame_pointer = #llvm.framePointerKind, sym_visibility = "private", target_cpu = "x86-64"}
  llvm.func @main(%arg0: i32, %arg1: !llvm.ptr, %arg2: !llvm.ptr) -> i32 attributes {frame_pointer = #llvm.framePointerKind, target_cpu = "x86-64"} {
    %0 = llvm.mlir.constant(0 : i32) : i32
    %1 = llvm.mlir.zero : !llvm.ptr
    llvm.call @_FortranAProgramStart(%arg0, %arg1, %arg2, %1) {fastmathFlags = #llvm.fastmath} : (i32, !llvm.ptr, !llvm.ptr, !llvm.ptr) -> ()
    llvm.call @_QQmain() {fastmathFlags = #llvm.fastmath} : () -> ()
    llvm.call @_FortranAProgramEndStatement() {fastmathFlags = #llvm.fastmath} : () -> ()
    llvm.return %0 : i32
  }
}

Even before the AddDebugInfoPass we have target_cpu, data_layout, ident, and target_triple set, so AddDebugInfoPass doesn’t add those. We do have all of these, either added by AddDebugInfoPass, or something after it:

 llvm.intr.dbg.declare #llvm.di_local_variable, id = distinct[1]<>, compileUnit = , sourceLanguage = DW_LANG_Fortran95, file = <"test.f90" in "/home/pjoot/toycalculator/fortran">, producer = "flang version 20.1.5 (https://github.com/llvm/llvm-project.git 7b09d7b446383b71b63d429b21ee45ba389c5134)", isOptimized = false, emissionKind = Full>, scope = #llvm.di_file<"test.f90" in "/home/pjoot/toycalculator/fortran">, name = "main", linkageName = "_QQmain", file = <"test.f90" in "/home/pjoot/toycalculator/fortran">, line = 1, scopeLine = 1, subprogramFlags = "Definition|MainSubprogram", type = >, name = "x", file = <"test.f90" in "/home/pjoot/toycalculator/fortran">, line = 3, type = #llvm.di_basic_type> = %1 : !llvm.ptr

So, it looks like we need to add a compileUnit, subprogram, and if we want debug variables, instrumentation for those too.

In some standalone code (prototypes/simplest.cpp) adding those wasn’t enough to ensure that translateModuleToLLVMIR() didn’t strip out the location info.

What did make the difference was adding this bit:

func->setLoc( builder.getFusedLoc( { loc }, subprogramAttr ) );

with that, the generated LLVM-IR, not only doesn’t have the loc() stripped, but everything is properly converted to !dbg:

; ModuleID = 'test'
source_filename = "test"
target datalayout = "e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-i128:128-f80:128-n8:16:32:64-S128"
target triple = "x86_64-unknown-linux-gnu"

define i32 @main() !dbg !4 {
  %1 = alloca i64, i64 1, align 4, !dbg !8
  store i32 42, ptr %1, align 4, !dbg !8
    #dbg_declare(ptr %1, !9, !DIExpression(), !8)
  ret i32 0, !dbg !10
}

; Function Attrs: nocallback nofree nosync nounwind speculatable willreturn memory(none)
declare void @llvm.dbg.declare(metadata, metadata, metadata) #0

attributes #0 = { nocallback nofree nosync nounwind speculatable willreturn memory(none) }

!llvm.module.flags = !{!0}
!llvm.dbg.cu = !{!1}
!llvm.ident = !{!3}

!0 = !{i32 2, !"Debug Info Version", i32 3}
!1 = distinct !DICompileUnit(language: DW_LANG_C, file: !2, producer: "testcompiler", isOptimized: false, runtimeVersion: 0, emissionKind: FullDebug)
!2 = !DIFile(filename: "test.c", directory: ".")
!3 = !{!"toycompiler 0.0"}
!4 = distinct !DISubprogram(name: "main", linkageName: "main", scope: !2, file: !2, line: 1, type: !5, scopeLine: 1, spFlags: DISPFlagDefinition, unit: !1)
!5 = !DISubroutineType(types: !6)
!6 = !{!7}
!7 = !DIBasicType(name: "int", size: 32, encoding: DW_ATE_signed)
!8 = !DILocation(line: 2, column: 3, scope: !4)
!9 = !DILocalVariable(name: "x", scope: !4, file: !2, line: 2, type: !7, align: 32)
!10 = !DILocation(line: 3, column: 3, scope: !4)

I’m also able to debug a faked source file corresponding to the LLVM-IR generated by this MWE program:

fedora:/home/pjoot/toycalculator/prototypes> ../build/simplest > output.ll
fedora:/home/pjoot/toycalculator/prototypes> clang -g -o output output.ll -Wno-override-module
fedora:/home/pjoot/toycalculator/prototypes> gdb -q ./output
Reading symbols from ./output...
(gdb) b main
Breakpoint 1 at 0x400450: file test.c, line 2.
(gdb) run
Starting program: /home/pjoot/toycalculator/prototypes/output 
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib64/libthread_db.so.1".

Breakpoint 1, main () at test.c:2
2           int x = 42;
(gdb) n
3           return 0;
(gdb) p x
$1 = 42
(gdb) rerun
Undefined command: "rerun".  Try "help".
(gdb) run
The program being debugged has been started already.
Start it from the beginning? (y or n) y
Starting program: /home/pjoot/toycalculator/prototypes/output 
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib64/libthread_db.so.1".

Breakpoint 1, main () at test.c:2
2           int x = 42;
(gdb) b 3
Breakpoint 2 at 0x400458: file test.c, line 3.
(gdb) c
Continuing.

Breakpoint 2, main () at test.c:3
3           return 0;
(gdb) q
A debugging session is active.

        Inferior 1 [process 119835] will be killed.

Quit anyway? (y or n) y

Now that I have this working in a standalone test case, it’s time to do it in the toy compiler itself, but that’s a task for another day.

Adding fixed size types to my toy MLIR compiler

May 17, 2025 C/C++ development and debugging. No comments , ,

In toycalculator, an MLIR/LLVM compiler experiment, I described a rudimentary MLIR based compiler.

I’ve now added fixed size integer types, a boolean type and boolean constants (but not boolean operators), and two floating point types. For the time being, the untyped ‘DCL’ declaration type (FLOAT64) is still in the grammar and the parser/MLIR-builder.

There’s lots still in the TODO list, including control flow, functions and dwarf support. I was quite surprised that given the location requirements for all MLIR statements, we don’t get DWARF instrumentation for free when doing LLVM lowering, and I wonder if I am missing some sort of setup for that.

New Language/Compiler enhancements

Here are the new elements added to the language:

  • Declare variables with BOOL, INT8, INT16, INT32, INT64, FLOAT32, FLOAT64 types: 

    BOOL b;
INT16 i;
FLOAT32 f;
  • TRUE, FALSE, and floating point constants: 

    b = TRUE;
f = 5 + 3.14E0;
  • An EXIT builtin to return a Unix command line value (must be the last statement in the program): 

    EXIT 1;
EXIT x;
  • Expression type conversions: 

    INT16 x;
FLOAT32 y;
y = 3.14E0;
x = 1 + y;

    The type conversion rules in the language are not like C.
    Instead, all expression elements are converted to the type of the destination before the operation, and integers are truncated.
    Example:

    INT32 x;
x = 1.78 + 3.86E0;
FLOAT64 f;
f = x;
PRINT f;
f = 1.78 + 3.86E0;
PRINT f;

    The expected output for this program is:

    4.000000
5.640000

MLIR

Example 1

The MLIR for the language now matches the statements of the language much more closely. Consider test.toy for example:

DCL x;
x = 5 + 3.14E0;
PRINT x;
DCL y;
y = x * 2;
PRINT y;

for which the MLIR is now free of memref dialect:

"builtin.module"() ({
  "toy.program"() ({
    "toy.declare"() <{name = "x", type = f64}> : () -> () loc(#loc)
    %0 = "arith.constant"() <{value = 5 : i64}> : () -> i64 loc(#loc1)
    %1 = "arith.constant"() <{value = 3.140000e+00 : f64}> : () -> f64 loc(#loc1)
    %2 = "toy.add"(%0, %1) : (i64, f64) -> f64 loc(#loc1)
    "toy.assign"(%2) <{name = "x"}> : (f64) -> () loc(#loc1)
    %3 = "toy.load"() <{name = "x"}> : () -> f64 loc(#loc2)
    "toy.print"(%3) : (f64) -> () loc(#loc2)
    "toy.declare"() <{name = "y", type = f64}> : () -> () loc(#loc3)
    %4 = "toy.load"() <{name = "x"}> : () -> f64 loc(#loc4)
    %5 = "arith.constant"() <{value = 2 : i64}> : () -> i64 loc(#loc4)
    %6 = "toy.mul"(%4, %5) : (f64, i64) -> f64 loc(#loc4)
    "toy.assign"(%6) <{name = "y"}> : (f64) -> () loc(#loc4)
    %7 = "toy.load"() <{name = "y"}> : () -> f64 loc(#loc5)
    "toy.print"(%7) : (f64) -> () loc(#loc5)
    "toy.exit"() : () -> () loc(#loc)
  }) : () -> () loc(#loc)
}) : () -> () loc(#loc)
#loc = loc("test.toy":1:1)
#loc1 = loc("test.toy":2:5)
#loc2 = loc("test.toy":3:1)
#loc3 = loc("test.toy":4:1)
#loc4 = loc("test.toy":5:5)
#loc5 = loc("test.toy":6:1)

Variable references still use llvm.alloca, but are symbol based in the builder, and llvm.alloca now doesn’t show up until lowering to LLVM-IR:

; ModuleID = 'test.toy'
source_filename = "test.toy"

declare void @__toy_print(double)

define i32 @main() {
  %1 = alloca double, i64 1, align 8
  store double 8.140000e+00, ptr %1, align 8
  %2 = load double, ptr %1, align 8
  call void @__toy_print(double %2)
  %3 = alloca double, i64 1, align 8
  %4 = load double, ptr %1, align 8
  %5 = fmul double %4, 2.000000e+00
  store double %5, ptr %3, align 8
  %6 = load double, ptr %3, align 8
  call void @__toy_print(double %6)
  ret i32 0
}

!llvm.module.flags = !{!0}

!0 = !{i32 2, !"Debug Info Version", i32 3}

Here is an example generated assembly, for the program above:

0000000000000000 <main>:
   0:   push   %rax
   1:   movsd  0x0(%rip),%xmm0        
   9:   call   e <main+0xe>
   e:   movsd  0x0(%rip),%xmm0       
  16:   call   1b <main+0x1b>
  1b:   xor    %eax,%eax
  1d:   pop    %rcx
  1e:   ret

Example 2

Here’s an example that highlights the new type support a bit better:

DESKTOP-16N83AG:/home/pjoot/toycalculator/samples> cat addi.toy 
INT32 x;
x = 5 + 3.14E0;
FLOAT64 f;
f = x;
PRINT f;

The MLIR for this is:

"builtin.module"() ({
  "toy.program"() ({
    "toy.declare"() <{name = "x", type = i32}> : () -> () loc(#loc)
    %0 = "arith.constant"() <{value = 5 : i64}> : () -> i64 loc(#loc1)
    %1 = "arith.constant"() <{value = 3.140000e+00 : f64}> : () -> f64 loc(#loc1)
    %2 = "toy.add"(%0, %1) : (i64, f64) -> i32 loc(#loc1)
    "toy.assign"(%2) <{name = "x"}> : (i32) -> () loc(#loc1)
    "toy.declare"() <{name = "f", type = f64}> : () -> () loc(#loc2)
    %3 = "toy.load"() <{name = "x"}> : () -> i32 loc(#loc3)
    "toy.assign"(%3) <{name = "f"}> : (i32) -> () loc(#loc3)
    %4 = "toy.load"() <{name = "f"}> : () -> f64 loc(#loc4)
    "toy.print"(%4) : (f64) -> () loc(#loc4)
    "toy.exit"() : () -> () loc(#loc)
  }) : () -> () loc(#loc)
}) : () -> () loc(#loc)
#loc = loc("addi.toy":1:1)
#loc1 = loc("addi.toy":2:5)
#loc2 = loc("addi.toy":3:1)
#loc3 = loc("addi.toy":4:5)
#loc4 = loc("addi.toy":5:1)

for which the LLVM IR lowering result is:

; ModuleID = 'addi.toy'
source_filename = "addi.toy"

declare void @__toy_print(double)

define i32 @main() {
  %1 = alloca i32, i64 1, align 4
  store i32 8, ptr %1, align 4
  %2 = alloca double, i64 1, align 8
  %3 = load i32, ptr %1, align 4
  %4 = sitofp i32 %3 to double
  store double %4, ptr %2, align 8
  %5 = load double, ptr %2, align 8
  call void @__toy_print(double %5)
  ret i32 0
}

!llvm.module.flags = !{!0}

!0 = !{i32 2, !"Debug Info Version", i32 3}

Because of lack of side effects, most of that code is obliterated in the assembly printing stage, leaving just:

0000000000000000 <main>:
   0:   push   %rax
   1:   movsd  0x0(%rip),%xmm0
   9:   call   e <main+0xe>
   e:   xor    %eax,%eax
  10:   pop    %rcx
  11:   ret

toycalculator, an MLIR/LLVM compiler experiment.

April 27, 2025 C/C++ development and debugging. , , , , ,

Over the last 8 years, I’ve been intimately involved in building a pair of LLVM based compilers for the COBOL and PL/I languages.  However, a lot of my work was on the runtime side of the story. This was non-trivial work, with lots of complex interactions to figure out, but it also meant that I didn’t get to play with the fun (codegen) part of the compiler.

Over the last month or so, I’ve been incrementally building an MLIR based compiler for a toy language.  I thought this would be a great way to get my hands dirty, and learn as I went.  These were the elements required:

As it turns out, MLIR tablegen is pretty finicky when you have no clue what you are doing.  Once you get the basic infrastructure in place it makes a lot more sense, and you can look at the generated C++ classes associated with your tablegen, to get a good idea what is happening under the covers.

Here’s a couple examples that illustrate the toy language:

// empty.toy
// This should be allowed by the grammar.



// dcl.toy
DCL x; // the next simplest non-empty program (i.e.: also has a comment)



// foo.toy
DCL x;
x = 3;
// This indenting is to test location generation, and to verify that the resulting columnar position is right.
     PRINT x;



// unary.toy
DCL x;
x = 3;
x = +x;
x = -x;
PRINT x;



// test.toy
DCL x;
DCL y;
x = 5 + 3;
y = x * 2;
PRINT x;

There is also a RETURN statement, not in any of those examples explicitly. I added that language element to simplify the LLVM lowering process. Here’s a motivating example:

> ../build/toycalculator empty.toy  --location
"builtin.module"() ({
  "toy.program"() ({
  ^bb0:
  }) : () -> () loc(#loc1)
}) : () -> () loc(#loc)
#loc = loc(unknown) 
#loc1 = loc("empty.toy":2:1)

Notice the wierd looking ‘^bb0:’ in the MLIR dump. This is a representation of an empty basic block, and was a bit of a pain to figure out how to lower properly. What I ended up doing is inserting a RETURN operation into the program if there was no other statements. I wanted to support such a dumb trivial program with no actual statements as a first test for the lowering, and see that things worked end to end before getting some of the trickier lowering working. With a return statement, empty.toy’s MLIR now looks like:

"builtin.module"() ({ 
  "toy.program"() ({
    "toy.return"() : () -> () loc(#loc1)
  }) : () -> () loc(#loc1)
}) : () -> () loc(#loc)
#loc = loc(unknown)
#loc1 = loc("../samples/empty.toy":2:1)

The idea behind the lowering operations is that each MLIR operation can be matched with a rewriter operation, and the program can be iterated over, gradually replacing all the MLIR operators with LLVM operations.

For example, an element like:

"toy.return"() : () -> ()

Can be replaced by

  %0 = "llvm.mlir.constant"() <{value = 0 : i32}> : () -> i32
  "llvm.return"(%0) : (i32) -> ()

Once that replacement is made, we can delete the toy.return element:

    // Lower toy.return to nothing (erase).
    class ReturnOpLowering : public ConversionPattern
    {
       public:
        ReturnOpLowering( MLIRContext* context )
            : ConversionPattern( toy::ReturnOp::getOperationName(), 1, context )
        {
        }           

        LogicalResult matchAndRewrite(
            Operation* op, ArrayRef operands,
            ConversionPatternRewriter& rewriter ) const override
        {   
            LLVM_DEBUG( llvm::dbgs()
                        << "Lowering toy.return: " << *op << '\n' );
            // ...
            rewriter.eraseOp( op );
            return success();
        }
    };

In the sample above the toy.program elememt also needs to be deleted. It gets replaced by a llvm basic block, moving all the MLIR BB elements into it. The last step is the removal of the outer most MLIR module, but there’s an existing Dialect for that. When all is said and done, we are left with the following LLVM IR:

define i32 @main() {
  ret i32 0
}

Here’s the MLIR for the foo.toy, which is slightly more interesting

"builtin.module"() ({
  "toy.program"() ({
    %0 = "memref.alloca"() <{operandSegmentSizes = array}> : () -> memref
    "toy.declare"() <{name = "x"}> : () -> ()
    %1 = "arith.constant"() <{value = 3 : i64}> : () -> i64
    %2 = "toy.unary"(%1) <{op = "+"}> : (i64) -> f64
    "memref.store"(%2, %0) : (f64, memref) -> ()
    "toy.assign"(%2) <{name = "x"}> : (f64) -> ()
    "toy.print"(%0) : (memref) -> ()
    "toy.return"() : () -> ()
  }) : () -> ()
}) : () -> ()

As we go through the lowering replacements, more an more of the MLIR operators get replaced with LLVM equivalents. Here’s an example part way through:

"llvm.func"() <{CConv = #llvm.cconv, function_type = !llvm.func, linkage = #llvm.linkage, sym_name = "main", visibility_ = 0 : i64}> ({
  %0 = "llvm.mlir.constant"() <{value = 1 : i64}> : () -> i64
  %1 = "llvm.alloca"(%0) <{alignment = 8 : i64, elem_type = f64}> : (i64) -> !llvm.ptr
  %2 = "memref.alloca"() <{operandSegmentSizes = array}> : () -> memref
  "toy.declare"() <{name = "x"}> : () -> ()
  %3 = "llvm.mlir.constant"() <{value = 3 : i64}> : () -> i64
  %4 = "arith.constant"() <{value = 3 : i64}> : () -> i64
  %5 = "toy.unary"(%4) <{op = "+"}> : (i64) -> f64
  "memref.store"(%5, %2) : (f64, memref) -> ()
  "toy.assign"(%5) <{name = "x"}> : (f64) -> ()
  "toy.print"(%2) : (memref) -> ()
  "toy.return"() : () -> ()
}) : () -> ()

and after a few more:

"llvm.func"() <{CConv = #llvm.cconv, function_type = !llvm.func, linkage = #llvm.linkage, sym_na
me = "main", visibility_ = 0 : i64}> ({
  %0 = "llvm.mlir.constant"() <{value = 1 : i64}> : () -> i64
  %1 = "llvm.alloca"(%0) <{alignment = 8 : i64, elem_type = f64}> : (i64) -> !llvm.ptr
  %2 = "memref.alloca"() <{operandSegmentSizes = array}> : () -> memref
  "toy.declare"() <{name = "x"}> : () -> ()
  %3 = "llvm.mlir.constant"() <{value = 3 : i64}> : () -> i64
  %4 = "arith.constant"() <{value = 3 : i64}> : () -> i64
  %5 = "llvm.sitofp"(%3) : (i64) -> f64
  %6 = "toy.unary"(%4) <{op = "+"}> : (i64) -> f64
  "llvm.store"(%5, %1) <{ordering = 0 : i64}> : (f64, !llvm.ptr) -> ()
  "memref.store"(%6, %2) : (f64, memref) -> ()
  "toy.assign"(%6) <{name = "x"}> : (f64) -> ()
  %7 = "llvm.load"(%1) <{ordering = 0 : i64}> : (!llvm.ptr) -> f64
  "llvm.call"(%7) <{CConv = #llvm.cconv, TailCallKind = #llvm.tailcallkind, callee = @__toy_print, fastmathFlags = #llvm.fastmath, op_bundle_sizes = array, operandSegmentSizes = array}> : (f64) -> ()
  "toy.print"(%2) : (memref) -> ()
  %8 = "llvm.mlir.constant"() <{value = 0 : i32}> : () -> i32
  "llvm.return"(%8) : (i32) -> ()
  "toy.return"() : () -> ()
}) : () -> ()

Eventually, after various LLVM IR blocks get merged (almost magically by one of the passes), we end up with:

declare void @__toy_print(double)

define i32 @main() {
  %1 = alloca double, i64 1, align 8
  store double 3.000000e+00, ptr %1, align 8
  %2 = load double, ptr %1, align 8
  call void @__toy_print(double %2)
  ret i32 0
}

Enabling an assembly printer pass, we get an object file

fedora:/home/pjoot/toycalculator/samples> objdump -dr foo.o

foo.o:     file format elf64-x86-64


Disassembly of section .text:

0000000000000000

: 0: 50 push %rax 1: 48 b8 00 00 00 00 00 movabs $0x4008000000000000,%rax 8: 00 08 40 b: 48 89 04 24 mov %rax,(%rsp) f: f2 0f 10 05 00 00 00 movsd 0x0(%rip),%xmm0 # 17 <main+0x17> 16: 00 13: R_X86_64_PC32 .LCPI0_0-0x4 17: e8 00 00 00 00 call 1c <main+0x1c> 18: R_X86_64_PLT32 __toy_print-0x4 1c: 31 c0 xor %eax,%eax 1e: 59 pop %rcx 1f: c3 ret
Here’s an end to end example of a full compile, link and build of this little module:

fedora:/home/pjoot/toycalculator/samples> ../build/toycalculator foo.toy 
Generated object file: foo.o
fedora:/home/pjoot/toycalculator/samples> clang -o foo foo.o -L ../build -l toy_runtime -Wl,-rpath,`pwd`/../build
fedora:/home/pjoot/toycalculator/samples> ./foo
3.000000

A month of work and 1800 lines of code, and now I can print a single constant number!

LLVM IR Null pointer constants and function pointers. A wild goose chase after a bad assumption.

March 30, 2017 clang/llvm , , , , , ,

With ELLCC, you can easily check out the LLVM IR for code like:

typedef void ( *f )( void );
void foo( void );

f bar() {
    return (f)foo;
}

That code is:

define nonnull void ()* @bar() local_unnamed_addr {
  ret void ()* @foo
}

declare void @foo()

I was trying to use @foo in a “struct” object, and was getting an error attempting this:

llvm/lib/IR/Constants.cpp:879:llvm::ConstantAggregate::ConstantAggregate(
 llvm::CompositeType*, llvm::Value::ValueTy, llvm::ArrayRef<llvm::Constant*>):
 Assertion `V[I]->getType() == T->getTypeAtIndex(I) &&
 "Initializer for composite element doesn't match!"' failed.

After adding:

fooFunc->dump();

where it shows the whole function body of foo(), I thought that’s where the error was coming from, and that I needed some other method to obtain just “@foo”, a global variable reference to the function, and not the function body itself.

The actual story is much simpler. Here the LLVM code to generate the IR for a foo() with this interface:

//------------
// void foo(){ }
//
auto vt = m_builder.getVoidTy();
auto voidFuncVoidType = FunctionType::get( vt, false /* varargs */ );

Function *fooFunc = Function::Create(
    voidFuncVoidType, Function::InternalLinkage, "foo",
    m_module );
BasicBlock *fooBB =
    BasicBlock::Create( m_context, "", fooFunc );
m_builder.SetInsertPoint( fooBB );
m_builder.CreateRetVoid();

My clue that the error is something else is that I am able to build a function that returns a foo function pointer:

//------------
// void(*)() bar() { return foo ; }
//
auto fpRetFuncType = FunctionType::get( voidFuncVoidType->getPointerTo(), false /* varargs */ );

Function *barFunc = Function::Create(
    fpRetFuncType, Function::ExternalLinkage, "bar",
    m_module );
BasicBlock *barBB =
    BasicBlock::Create( m_context, "", barFunc );
m_builder.SetInsertPoint( barBB );
m_builder.CreateRet( fooFunc );

The module at this point looks like:

define internal void @foo() {
   ret void
}

define void ()* @bar() {
   ret void ()* @foo
}

So why can I used fooFunc in a return statement, but don’t appear to be able to use it in a structure object? Here’s the code that created that structure type

//------------
//
// struct { int, void (*)(), char * }
auto i8t = m_builder.getInt8Ty();
auto i32t = m_builder.getInt32Ty();
std::vector<Type *> consStructMembers{
    i32t, voidFuncVoidType->getPointerTo(), i8t->getPointerTo()};
auto consStructType =
    StructType::create( m_context, consStructMembers, "" );

and my attempt to populate an object of this type:

//
// %struct { int, void (*)(), char * } = { 65535, foo, null };
//
auto consPriority = ConstantInt::get( i32t, 65535 );
auto consDataZero = ConstantInt::get( i8t->getPointerTo(), 0 );

std::vector<Constant *> v{consPriority, fooFunc, consDataZero};
Constant *g = ConstantStruct::get( consStructType, v );

The actual error was in the third struct member initialization, and had nothing to do with the function pointer value. In retrospect, this makes sense since llvm::Function is derived from llvm::Constant, so there shouldn’t logically be a mismatch there.

What actually fixed the error was simply:

auto consDataZero = ConstantPointerNull::get( i8t->getPointerTo() );

It appears that the numeric zero value isn’t the same thing as an LLVM ‘null’. With that corrected, my variable declaration is:

%"type 0x10ea0c0" { i32 65535, void ()* @foo, i8* null }

… so I should now be able to proceed with the actual task at hand.