regression in the llvm17 branch: C backend fails "align(N) on functions" behavior test

[andrewrk]

zig version: 0.12.0-dev.116+4cc2544cd (llvm17 branch)

$ zig build test-behavior
run test behavior-native-Debug-libc-cbe: error: 'test.align(N) on functions' failed
run test behavior-native-Debug-libc-cbe: error: Illegal instruction at address 0x49d1b2
/home/andy/misc/zig/zig-cache/o/095b4cdbe22fa6116fcdfeb6662f9b6f/test.c:56772:2: 0x49d1b2 in behavior_cast_test_cast_20function_20with_20an_20opaque_20parameter__2424 (/home/andy/misc/zig/zig-cache/o/095b4cdbe22fa6116fcdfeb6662f9b6f/test.c)
 t7(t5);
 ^
/home/andy/misc/zig/zig-cache/o/095b4cdbe22fa6116fcdfeb6662f9b6f/test.c:25836:10: 0x4336d4 in test_runner_mainServer__158 (/home/andy/misc/zig/zig-cache/o/095b4cdbe22fa6116fcdfeb6662f9b6f/test.c)
    t6 = t43();
         ^
/home/andy/misc/zig/zig-cache/o/095b4cdbe22fa6116fcdfeb6662f9b6f/test.c:26014:9: 0x2a4beb in test_runner_main__157 (/home/andy/misc/zig/zig-cache/o/095b4cdbe22fa6116fcdfeb6662f9b6f/test.c)
  t17 = test_runner_mainServer__158();
        ^
/home/andy/misc/zig/zig-cache/o/095b4cdbe22fa6116fcdfeb6662f9b6f/test.c:23664:2: 0x2a3a07 in main (/home/andy/misc/zig/zig-cache/o/095b4cdbe22fa6116fcdfeb6662f9b6f/test.c)
 test_runner_main__157();
 ^
???:?:?: 0x7fc26b8de24d in ??? (libc.so.6)
Unwind information for `libc.so.6:0x7fc26b8de24d` was not available, trace may be incomplete

???:?:?: 0x7ffcc8b29884 in ??? (???)

Some possible reasons this regressed with the llvm17 branch:

• UBSAN in clang 17 is now catching a true positive
• a regression in clang 17 when compiling the C code
• changes in clang 17 that have a side-effect of uncovering a latent bug in zig's C backend output
• a miscompilation in the compiler, which was compiled with the LLVM (17) backend

I suspect this has the same root cause for the failure of the behavior test "cast function with an opaque parameter" as well.

[jacobly0]

This regressed because llvm is inserting cfi information at the aligned function address requested and using an offset address for the function pointer.

To quote the LangRef

Prefix data is laid out as if it were an initializer for a global variable of the prefix data’s type. The function will be placed such that the beginning of the prefix data is aligned. This means that if the size of the prefix data is not a multiple of the alignment size, the function’s entrypoint will not be aligned. If alignment of the function’s entrypoint is desired, padding must be added to the prefix data.