Public bug reported:
g++ 9.3.0-17ubuntu1~20.04 miscompiles the following example for x86
(bar_stdcall should pop 4 bytes fewer than an bar_cdecl, as stdcall is
callee-cleanup for the argument). To see it in this minimized example
one must use `-O0` as it will otherwise be inlined and the calling-
convention will no longer be meaningful. Larger examples it can hit with
optimizations on.
>>>>>>>>>>>>>>>
// g++-9 -c -O0 simple.cpp -m32
//
// objdump -S simple.o
template<bool bIsStdcall, typename T> struct func_ptr_t;
template<typename T> struct func_ptr_t<false,T> {
using type = void(*)(T);
};
template<typename T> struct func_ptr_t<true,T> {
#if 1
using type = void(__attribute__((__stdcall__))*)(T);
#else
using type = void(__attribute__((__stdcall__))*)(int); // this works,
using T is important somehow
#endif
};
#if 1
using foo_stdcall_ptr = func_ptr_t<true,int>::type;
using foo_cdecl_ptr = func_ptr_t<false,int>::type;
#else
using foo_stdcall_ptr = void(__attribute__((__stdcall__))*)(int,int);
using foo_cdecl_ptr = void(*)(int,int);
#endif
foo_stdcall_ptr foo_stdcall;
foo_cdecl_ptr foo_cdecl;
void bar_stdcall() {
foo_stdcall(1);
}
void bar_cdecl() {
foo_cdecl(1);
}
>>>>>>>>>>>>>>>
--- bar_cdecl.S 2021-03-09 00:54:58.404022904 +0000
+++ bar_stdcall.S 2021-03-09 00:52:07.900015002 +0000
@@ -1,15 +1,15 @@
-<bar_cdecl()>:
+<bar_stdcall()>:
0: f3 0f 1e fb endbr32
4: 55 push %ebp
5: 89 e5 mov %esp,%ebp
7: 83 ec 08 sub $0x8,%esp
- a: e8 fc ff ff ff call 32 <bar_cdecl()+0xb>
+ a: e8 fc ff ff ff call b <bar_stdcall()+0xb>
f: 05 01 00 00 00 add $0x1,%eax
14: 8b 80 00 00 00 00 mov 0x0(%eax),%eax
1a: 83 ec 0c sub $0xc,%esp
1d: 6a 01 push $0x1
1f: ff d0 call *%eax
- 21: 83 c4 10 add $0x10,%esp
+ 21: 83 c4 0c add $0xc,%esp
24: 90 nop
25: c9 leave
26: c3 ret
```
Upstream gcc-9.3.0 does not have this bug (See
https://godbolt.org/z/vK3TdM318), but 9.4.0, 10.0, and 10.1 do). The
regression seems to have happened upstream in
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=90750 -
https://gcc.gnu.org/git/?p=gcc.git;a=commit;h=aa988998be8f85334665a6b049d5d9139408c250
- which ubuntu has backported into git-updates.patch (it's in in
gcc-9_9.3.0-17ubuntu1~20.04.debian.tar.xz, I have not pinned down when
ubuntu picked this up). The regression was fixed in 10.2 by
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95222 -
https://gcc.gnu.org/git/?p=gcc.git;a=commit;efdbd4fbea08005091e490ec3f9972aa9c946374
and
https://gcc.gnu.org/git/?p=gcc.git;a=commit;0c473d8f32510fcc96d584ee5099b856cfd3d8d6).
I don't think it was realized (when the was found/fixed upstream) that
it could also cause wrong-code, so I'm just posting this to make you
aware that ubuntu has backported a regression and not the fix. I assume
you'll want to align with upstream on whether gcc-9 branch ends up
getting this fixed or not.
** Affects: gcc-9 (Ubuntu)
Importance: Undecided
Status: New
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https://bugs.launchpad.net/bugs/1956236
Title:
wrong-code if stdcall is applied via template parameters
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