clang++ -fexceptions doesn't enable unwind information generation as documented for all targets

[asb]

At the time of writing, clang/docs/CommandGuide/clang.rst states

Enable generation of unwind information. This allows exceptions to be thrown through Clang compiled stack frames. This is on by default in x86-64.

However, this doesn't seem to actually turn on unwind information for a number of targets (note that the precise triple matters - e.g. a recent patch enabled asynchronous unwind tables by default on RISC-V Linux targets, which hides this issue somewhat):

$ cat test.cpp 
int main(void) {
  return 0;
}
$ for TGT in mips64 riscv64 sparc64 systemz; do echo $TGT; ./llvm-project/build/default/bin/clang++ -target $TGT test.cpp -fexceptions -S -o - | grep cfi; done
mips64
riscv64
sparc64
systemz

And here's what you get when explicitly enabling unwind tables:

$ for TGT in mips64 riscv64 sparc64 systemz; do echo $TGT; ./llvm-project/build/default/bin/clang++ -target $TGT test.cpp -funwind-tables -S -o - | grep cfi; done
mips64
	.cfi_startproc
	.cfi_def_cfa_offset 32
	.cfi_offset 31, -8
	.cfi_offset 30, -16
	.cfi_def_cfa_register 30
	.cfi_endproc
riscv64
	.cfi_startproc
	.cfi_def_cfa_offset 32
	.cfi_offset ra, -8
	.cfi_offset s0, -16
	.cfi_def_cfa s0, 0
	.cfi_endproc
sparc64
	.cfi_startproc
	.cfi_def_cfa_register %fp
	.cfi_window_save
	.cfi_register %o7, %i7
	.cfi_endproc
systemz
	.cfi_startproc
	.cfi_offset %r11, -72
	.cfi_offset %r15, -40
	.cfi_def_cfa_offset 328
	.cfi_def_cfa_register %r11
	.cfi_endproc

Given the quoted documentation, I think I'd expect this same output for -fexceptions. Thanks to @mtvec for raising this issue.

[llvmbot]

@llvm/issue-subscribers-clang-driver

[efriedma-quic]

The point of -fexceptions is, as the documentation suggests, to allow C++ "throw" exceptions to be thrown through clang-compiled functions. This has a few different effects... but it only ensures an unwind table for functions that might have an exception thrown through them. This is the opposite of -fno-exceptions, which disables exception throwing functionality. If clang can prove a function doesn't have an exception thrown through it, -fexceptions is irrelevant.

-funwind-tables means "generate unwind tables even if clang can prove no exception will unwind through this function". This is only useful if you want to generate a stack trace, so it's separate from -fexceptions.

I think this is working as expected; maybe the documentation could be made more clear.

[asb]

I think this is working as expected; maybe the documentation could be made more clear.

I've submitted https://reviews.llvm.org/D145564 as one suggestion for clarifying the documentation.

Thanks for taking a look.

[appujee]

is it okay to close this issue?

[asb]

Marking closed.