Because the clang front end reexecs itself, breakpoints on the interesting parts of the clang front end don’t get hit by default. Here’s an example

$ cat g2
b llvm::Module::setDataLayout
b BackendConsumer::BackendConsumer
b llvm::TargetMachine::TargetMachine
b llvm::TargetMachine::createDataLayout
run -mbig-endian -m64 -c bytes.c -emit-llvm -o big.bc

$ gdb `which clang`
GNU gdb (GDB) Red Hat Enterprise Linux 7.9.1-19.lz.el7
(gdb) source g2
Breakpoint 1 at 0x2c04c3d: llvm::Module::setDataLayout. (2 locations)
Breakpoint 2 at 0x3d08870: file /source/llvm/lib/Target/TargetMachine.cpp, line 47.
Breakpoint 3 at 0x33108ca: file /source/llvm/include/llvm/Target/TargetMachine.h, line 133.
Detaching after vfork from child process 15795.
[Inferior 1 (process 15789) exited normally]

(The debugger finishes and exits, hitting none of the breakpoints)

One way to deal with this is to set the fork mode to child:

(gdb) set follow-fork-mode child

An alternate way of dealing with this is to use strace to collect the command line that clang invokes itself with. For example:

$ strace -f -s 1024 -v clang -mbig-endian -m64 big.bc -c 2>&1 | grep exec | tail -2 | head -1

This provides the command line options for the self invocation of clang

[pid  4650] execve("/usr/local/bin/clang-3.9", ["/usr/local/bin/clang-3.9", "-cc1", "-triple", "aarch64_be-unknown-linux-gnu", "-emit-obj", "-mrelax-all", "-disable-free", "-main-file-name", "big.bc", "-mrelocation-model", "static", "-mthread-model", "posix", "-mdisable-fp-elim", "-fmath-errno", "-masm-verbose", "-mconstructor-aliases", "-fuse-init-array", "-target-cpu", "generic", "-target-feature", "+neon", "-target-abi", "aapcs", "-dwarf-column-info", "-debugger-tuning=gdb", "-coverage-file", "/workspace/pass/run/big.bc", "-resource-dir", "/usr/local/bin/../lib/clang/3.9.0", "-fdebug-compilation-dir", "/workspace/pass/run", "-ferror-limit", "19", "-fmessage-length", "0", "-fallow-half-arguments-and-returns", "-fno-signed-char", "-fobjc-runtime=gcc", "-fdiagnostics-show-option", "-o", "big.o", "-x", "ir", "big.bc"],

With a bit of vim tweaking you can turn this into a command line that can be executed (or debugged) directly

/usr/local/bin/clang-3.9 -cc1 -triple aarch64_be-unknown-linux-gnu -emit-obj -mrelax-all -disable-free -main-file-name big.bc -mrelocation-model static -mthread-model posix -mdisable-fp-elim -fmath-errno -masm-verbose -mconstructor-aliases -fuse-init-array -target-cpu generic -target-feature +neon -target-abi aapcs -dwarf-column-info -debugger-tuning=gdb -coverage-file /workspace/pass/run/big.bc -resource-dir /usr/local/bin/../lib/clang/3.9.0 -fdebug-compilation-dir /workspace/pass/run -ferror-limit 19 -fmessage-length 0 -fallow-half-arguments-and-returns -fno-signed-char -fobjc-runtime=gcc -fdiagnostics-show-option -o big.o -x ir big.bc

Note that doing this also provides a mechanism to change the compiler triple manually, which is something that I wondered how to do (since clang documents -triple as an option, but seems to ignore it). For example, I’m able to able to change -triple aarch64_be to aarch64 and get little endian object code from bytecode prepared with -mbig-endian.