rjmccall added a comment.
In D96418#2780687 <https://reviews.llvm.org/D96418#2780687>, @leonardchan wrote:
> Hi all, it looks like this commit led to some unexpected behavior in our
> build. When compiling something like:
>
> // clang++ -c -S -o - /tmp/test.cc -std=c++17 -O1
> static int do_nothing(void* unused) {
> for (;;) {
> }
> return 0;
> }
>
> typedef int (*func_t)(void *);
> void call(func_t);
>
> void func() {
> call(&do_nothing);
> }
>
> we get the following assembly for `do_nothing`:
>
> .p2align 4, 0x90 # -- Begin function
> _ZL10do_nothingPv
> .type _ZL10do_nothingPv,@function
> _ZL10do_nothingPv: # @_ZL10do_nothingPv
> .cfi_startproc
> # %bb.0:
> .Lfunc_end1:
> .size _ZL10do_nothingPv, .Lfunc_end1-_ZL10do_nothingPv
> .cfi_endproc
> # -- End function
> .ident "Fuchsia clang version 13.0.0
> (https://llvm.googlesource.com/a/llvm-project
> 6555e53ab0f2bca3dc30f5ab3a2d6872d50b6ff8)"
> .section ".note.GNU-stack","",@progbits
> .addrsig
> .addrsig_sym _ZL10do_nothingPv
>
> It seems that the function doesn't have a return statement or halting
> instruction and it would just jump into the next function. While I do follow
> what this patch is trying to do, this behavior seems pretty unexpected from a
> C++ user perspective. I could be wrong, but it doesn't seem clear in this
> case that the infinite loop results in UB which would justify this assembly.
Well, no, I'm afraid it is actually clear that that code does have UB according
to the C++ standard. Perhaps you mean that it *shouldn't* have UB, or that
Clang should define its behavior despite the standard.
I might agree with you that I don't see the value in using this stronger rule
in C++, but I think it would help to understand the problem a little better. I
assume this is causing problems for a less trivial test case? Or do you really
have code that's relying on that loop not terminating?
Repository:
rG LLVM Github Monorepo
CHANGES SINCE LAST ACTION
https://reviews.llvm.org/D96418/new/
https://reviews.llvm.org/D96418
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