flang

Tagged V3 of my toy compiler (playing with the MLIR -> LLVM-IR toolchain)

June 3, 2025 clang/llvm No comments , , , , , , , , , , , , , ,

Screenshot

 

I’ve added a number of elements to the language and compiler:

  • comparison operators (<, <=, EQ, NE) yielding BOOL values.  These work for any combinations of floating and integer types (including BOOL.)

  • integer bitwise operators (OR, AND, XOR).  These only for for integer types (including BOOL.)

  • a NOT operator, yielding BOOL.

  • Array + string declaration and lowering support, including debug instrumentation, and print support for string variables.

This version also fixes a few specific issues:

  • Fixed -g/-OX propagation to lowering.  If -g not specified, now don’t generate the DI.
  • Show the optimized .ll with –emit-llvm instead of the just-lowered .ll (unless not invoking the assembly printer, where the ll optimization passes are registered.)
  • Reorganize the grammar so that all the simple lexer tokens are last.  Rename a bunch of the tokens, introducing some consistency.
  • calculator.td: introduce IntOrFloat constraint type, replacing AnyType usage; array decl support, and string support.
  • driver: writeLL helper function, pass -g to lowering if set.
  • parser: handle large integer constants properly, array decl support, and string support.
  • simplest.cpp: This MWE is updated to include a global variable and global variable access.
  • parser: implicit exit: use the last saved location, instead of the module start.  This means the line numbers don’t jump around at the very end of the program anymore (i.e.: implicit return/exit)

I started with the comparison operators, thinking that I’d add if statement support, and loops, but got sidetracked.  In particular, I generated a number of really large test programs, and without some way to print a string message, it was hard to figure out where an error was occuring.  This led to implementing PRINT string-variable support as an interesting feature first.

As a side effect of adding STRING support, I’ve also got declaration support for arbitrary fixed size arrays for any type.  I haven’t implemented array access yet, but that probably won’t be too hard.

Here’s an example of a program that uses STRING variables:

STRING t[2];
STRING u[3];
STRING s[2];
INT8 s2;
s = "hi";
PRINT s;
t = "hi";
PRINT t;
u = "hi";
PRINT u;
u = "bye";
PRINT u;

This is the MLIR for the program:

module {
  toy.program {
    "toy.declare"() <{name = "t", size = 2 : i64, type = i8}> : () -> ()
    "toy.declare"() <{name = "u", size = 3 : i64, type = i8}> : () -> ()
    "toy.declare"() <{name = "s", size = 2 : i64, type = i8}> : () -> ()
    "toy.declare"() <{name = "s2", type = i8}> : () -> ()
    toy.string_assign "s" = "hi"
    %0 = toy.load "s" : !llvm.ptr
    toy.print %0 : !llvm.ptr
    toy.string_assign "t" = "hi"
    %1 = toy.load "t" : !llvm.ptr
    toy.print %1 : !llvm.ptr
    toy.string_assign "u" = "hi"
    %2 = toy.load "u" : !llvm.ptr
    toy.print %2 : !llvm.ptr
    toy.string_assign "u" = "bye"
    %3 = toy.load "u" : !llvm.ptr
    toy.print %3 : !llvm.ptr
    toy.exit
  }
}

It’s a bit clunky, because I cheated and didn’t try to implement PRINT support of string literals directly. I thought that since I had variable support already (which emits llvm.alloca), I could change that alloca trivially to an array from a scalar value.

I think that this did turn out to be a relatively easy way to do it, but this little item did take much more effort than I expected.

The DeclareOp builder is fairly straightforward:

builder.create<toy::DeclareOp>( loc, builder.getStringAttr( varName ), mlir::TypeAttr::get( ty ), nullptr ); // for scalars
...
auto sizeAttr = builder.getI64IntegerAttr( arraySize );
builder.create<toy::DeclareOp>( loc, builder.getStringAttr( varName ), mlir::TypeAttr::get( ty ), sizeAttr ); // for arrays

This matches the Optional size now added to DeclareOp for arrays:

def Toy_DeclareOp : Op<Toy_Dialect, "declare"> {
  let summary = "Declare a variable or array, specifying its name, type (integer or float), and optional size.";
  let arguments = (ins StrAttr:$name, TypeAttr:$type, OptionalAttr:$size);
  let results = (outs);

There’s a new AssignStringOp complementing AssignOp:

def Toy_AssignOp : Op<Toy_Dialect, "assign"> {
  let summary = "Assign a (non-string) value to a variable associated with a declaration";
  let arguments = (ins StrAttr:$name, AnyType:$value);
  let results = (outs);

  // toy.assign "x", %0 : i32
  let assemblyFormat = "$name `,` $value `:` type($value) attr-dict";
}

def Toy_AssignStringOp : Op<Toy_Dialect, "string_assign"> {
  let summary = "Assign a string literal to an i8 array variable";
  let arguments = (ins StrAttr:$name, Builtin_StringAttr:$value);
  let results = (outs);
  let assemblyFormat = "$name `=` $value attr-dict";
}

I also feel this is a cludge. I probably really want a string literal type like flang’s. Here’s a fortran hello world:

program hello
  print *, "Hello, world!"
end program hello

and selected parts of the flang fir dialect MLIR for it:

    %4 = fir.declare %3 typeparams %c13 {fortran_attrs = #fir.var_attrs, uniq_name = "_QQclX48656C6C6F2C20776F726C6421"} : (!fir.ref<!fir.char<1,13>>, index) -> !fir.ref<!fir.char<1,13>>
    %5 = fir.convert %4 : (!fir.ref<!fir.char<1,13>>) -> !fir.ref
    %6 = fir.convert %c13 : (index) -> i64
    %7 = fir.call @_FortranAioOutputAscii(%2, %5, %6) fastmath : (!fir.ref, !fir.ref, i64) -> i1
  ...

  fir.global linkonce @_QQclX48656C6C6F2C20776F726C6421 constant : !fir.char<1,13> {
    %0 = fir.string_lit "Hello, world!"(13) : !fir.char<1,13>
    fir.has_value %0 : !fir.char<1,13>
  }

I had to specialize the LoadOp builder too so that it didn’t create a scalar load. That code looks like:

mlir::Type varType;
mlir::Type elemType = declareOp.getTypeAttr().getValue();
        
if ( declareOp.getSizeAttr() )    // Check if size attribute exists
{       
    // Array: load a generic pointer 
    varType = mlir::LLVM::LLVMPointerType::get( builder.getContext(), /*addressSpace=*/0 );
}       
else    
{       
    // Scalar: load the value
    varType = elemType;
}       
        
auto value = builder.create<toy::LoadOp>( loc, varType, builder.getStringAttr( varName ) );

Lowering didn’t require too much. I needed a print function object:

auto ptrType = LLVM::LLVMPointerType::get( ctx );
auto printFuncStringType = LLVM::LLVMFunctionType::get( LLVM::LLVMVoidType::get( ctx ),
                                                        { pr_builder.getI64Type(), ptrType }, false );
pr_printFuncString = pr_builder.create<LLVM::LLVMFuncOp>( pr_module.getLoc(), "__toy_print_string",
                                                          printFuncStringType, LLVM::Linkage::External );

With LoadOp now possibly having pointer valued return

if ( loadOp.getResult().getType().isa<mlir::LLVM::LLVMPointerType>() )
{           
    // Return the allocated pointer
    LLVM_DEBUG( llvm::dbgs() << "Loading array address: " << allocaOp.getResult() << '\n' );
    rewriter.replaceOp( op, allocaOp.getResult() );
}           
else        
{           
    // Scalar load
    auto load = rewriter.create<LLVM::LoadOp>( loc, elemType, allocaOp );
    LLVM_DEBUG( llvm::dbgs() << "new load op: " << load << '\n' );
    rewriter.replaceOp( op, load.getResult() );
}

assign-string lowering basically just generates a memcpy from a global:

Type elemType = allocaOp.getElemType();
int64_t numElems = 0;
if ( auto constOp = allocaOp.getArraySize().getDefiningOp<LLVM::ConstantOp>() )
{           
    auto intAttr = mlir::dyn_cast<IntegerAttr>( constOp.getValue() );
    numElems = intAttr.getInt();
}           
LLVM_DEBUG( llvm::dbgs() << "numElems: " << numElems << '\n' );
LLVM_DEBUG( llvm::dbgs() << "elemType: " << elemType << '\n' );

if ( !mlir::isa<mlir::IntegerType>( elemType ) || elemType.getIntOrFloatBitWidth() != 8 )
{           
    return rewriter.notifyMatchFailure( assignOp, "string assignment requires i8 array" );
}           
if ( numElems == 0 )
{           
    return rewriter.notifyMatchFailure( assignOp, "invalid array size" );
}           

size_t strLen = value.size();
size_t copySize = std::min( strLen + 1, static_cast<size_t>( numElems ) );
if ( strLen > static_cast<size_t>( numElems ) )
{           
    return rewriter.notifyMatchFailure( assignOp, "string too large for array" );
}           

mlir::LLVM::GlobalOp globalOp = lState.lookupOrInsertGlobalOp( rewriter, value, loc, copySize, strLen );

auto globalPtr = rewriter.create<LLVM::AddressOfOp>( loc, globalOp ); 

auto destPtr = allocaOp.getResult();

auto sizeConst = 
    rewriter.create<LLVM::ConstantOp>( loc, rewriter.getI64Type(), rewriter.getI64IntegerAttr( copySize ) );

rewriter.create<LLVM::MemcpyOp>( loc, destPtr, globalPtr, sizeConst, rewriter.getBoolAttr( false ) );

rewriter.eraseOp( op );

I used global’s like what we’d find in clang LLVM-IR. For example, here’s a C hello world:

#include <string.h>

int main()
{
    const char* s = "hi there";
    char buf[100];
    memcpy( buf, s, strlen( s ) + 1 );

    return strlen( buf );
}

where our LLVM-IR looks like:

@.str = private unnamed_addr constant [9 x i8] c"hi there\00", align 1

; Function Attrs: noinline nounwind optnone uwtable
define dso_local i32 @main() #0 {
  %1 = alloca i32, align 4
  %2 = alloca ptr, align 8
  %3 = alloca [100 x i8], align 16
  store i32 0, ptr %1, align 4
  store ptr @.str, ptr %2, align 8
  %4 = getelementptr inbounds [100 x i8], ptr %3, i64 0, i64 0
  %5 = load ptr, ptr %2, align 8
  %6 = load ptr, ptr %2, align 8
  %7 = call i64 @strlen(ptr noundef %6) #3
  %8 = add i64 %7, 1
  call void @llvm.memcpy.p0.p0.i64(ptr align 16 %4, ptr align 1 %5, i64 %8, i1 false)
  %9 = getelementptr inbounds [100 x i8], ptr %3, i64 0, i64 0
  %10 = call i64 @strlen(ptr noundef %9) #3
  %11 = trunc i64 %10 to i32
  ret i32 %11
}

My lowered LLVM-IR for the program is similar:

@str_1 = private constant [3 x i8] c"bye"
@str_0 = private constant [2 x i8] c"hi"

declare void @__toy_print_f64(double)

declare void @__toy_print_i64(i64)

declare void @__toy_print_string(i64, ptr)

define i32 @main() {
  %1 = alloca i8, i64 2, align 1
  %2 = alloca i8, i64 3, align 1
  %3 = alloca i8, i64 2, align 1
  %4 = alloca i8, i64 1, align 1
  call void @llvm.memcpy.p0.p0.i64(ptr align 1 %3, ptr align 1 @str_0, i64 2, i1 false)
  call void @__toy_print_string(i64 2, ptr %3)
  call void @llvm.memcpy.p0.p0.i64(ptr align 1 %1, ptr align 1 @str_0, i64 2, i1 false)
  call void @__toy_print_string(i64 2, ptr %1)
  call void @llvm.memcpy.p0.p0.i64(ptr align 1 %2, ptr align 1 @str_0, i64 3, i1 false)
  call void @__toy_print_string(i64 3, ptr %2)
  call void @llvm.memcpy.p0.p0.i64(ptr align 1 %2, ptr align 1 @str_1, i64 3, i1 false)
  call void @__toy_print_string(i64 3, ptr %2)
  ret i32 0
}
...

I managed my string literals with a simple hash, avoiding replication if repeated:

mlir::LLVM::GlobalOp lookupOrInsertGlobalOp( ConversionPatternRewriter& rewriter, mlir::StringAttr& stringLit,
                                             mlir::Location loc, size_t copySize, size_t strLen )
{       
    mlir::LLVM::GlobalOp globalOp;
    auto it = pr_stringLiterals.find( stringLit.str() );
    if ( it != pr_stringLiterals.end() )
    {       
        globalOp = it->second;
        LLVM_DEBUG( llvm::dbgs() << "Reusing global: " << globalOp.getSymName() << '\n' ); 
    }       
    else    
    {       
        auto savedIP = rewriter.saveInsertionPoint();
        rewriter.setInsertionPointToStart( pr_module.getBody() );

        auto i8Type = rewriter.getI8Type();
        auto arrayType = mlir::LLVM::LLVMArrayType::get( i8Type, copySize );

        SmallVector<char> stringData( stringLit.begin(), stringLit.end() );
        if ( copySize > strLen )
        {       
            stringData.push_back( '\0' ); 
        }       
        auto denseAttr = DenseElementsAttr::get( RankedTensorType::get( { static_cast<int64_t>( copySize ) }, i8Type ),
                                                 ArrayRef<char>( stringData ) );

        std::string globalName = "str_" + std::to_string( pr_stringLiterals.size() );
        globalOp =
            rewriter.create<LLVM::GlobalOp>( loc, arrayType, true, LLVM::Linkage::Private, globalName, denseAttr );
        globalOp->setAttr( "unnamed_addr", rewriter.getUnitAttr() );

        pr_stringLiterals[stringLit.str()] = globalOp;
        LLVM_DEBUG( llvm::dbgs() << "Created global: " << globalName << '\n' );

        rewriter.restoreInsertionPoint( savedIP );
    }           

    return globalOp;
}

Without the insertion point swaperoo, this GlobalOp creation doesn’t work, as we need to be in the ModuleOp level where the symbol table lives.

 

… anyways, it looks like I’m droning on.  There’s been lots of stuff to get this far, but there are still many many things to do before what I’ve got even qualifies as a basic programming language (if statements, loops, functions, array assignments, types, …)

The quest for DWARF instrumentation in a toy MLIR compiler

May 25, 2025 C/C++ development and debugging. , , , , , , ,

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.