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<base href="https://bugs.llvm.org/">
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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - __attribute__((force_align_arg_pointer)) ignored by non-virtual thunk on x86"
href="https://bugs.llvm.org/show_bug.cgi?id=47405">47405</a>
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<th>Summary</th>
<td>__attribute__((force_align_arg_pointer)) ignored by non-virtual thunk on x86
</td>
</tr>
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<th>Product</th>
<td>clang
</td>
</tr>
<tr>
<th>Version</th>
<td>trunk
</td>
</tr>
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<th>Hardware</th>
<td>PC
</td>
</tr>
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<th>OS</th>
<td>All
</td>
</tr>
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<th>Status</th>
<td>NEW
</td>
</tr>
<tr>
<th>Severity</th>
<td>normal
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
</tr>
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<th>Component</th>
<td>LLVM Codegen
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>shadow_mosk@mail.ru
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org, neeilans@live.com, richard-llvm@metafoo.co.uk
</td>
</tr></table>
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<pre>Compiler generates incorrect non-virtual thunk in case of multiple inheritance.
It seems that clang ignores __attribute__((force_align_arg_pointer)) for thunk
and accepts it only for original function.
GCC (at least 7.3.0) handles the alignment correctly for original function and
for thunk.
As workaround we use an option -mstackrealign for clang.
But it causes unnecessary alternative prologue/epilogue for functions without
problematic thunks.
Minimal test case (<a href="https://godbolt.org/z/a847vd">https://godbolt.org/z/a847vd</a>):
Options: -target i386-pc-linux-gnu -O2
#include <stdio.h>
#include <memory.h>
struct A
{
virtual void TestA() { printf("TestA\n"); }
};
struct B
{
//__attribute__((force_align_arg_pointer))
virtual void TestB() { printf("TestB\n"); }
};
struct C : public A, public B
{
__attribute__((force_align_arg_pointer))
virtual void TestB() { printf("TestB by C\n"); }
virtual void TestC() { printf("TestC\n"); }
};
----------------------------------------------------------
CLANG 8, 9, 10
C::TestB(): # @C::TestB()
push ebp
mov ebp, esp
and esp, -16 <--- Stack alignment
sub esp, 16
mov dword ptr [esp], offset .Lstr.5
call puts
mov esp, ebp
pop ebp
ret
non-virtual thunk to C::TestB(): # @non-virtual thunk to C::TestB()
sub esp, 12 <--- No Stack alignment
mov dword ptr [esp], offset .Lstr.5
call puts
add esp, 12
ret
-----------------------------------------
GCC 7.3.0:
C::TestB():
lea ecx, [esp+4]
and esp, -16
push DWORD PTR [ecx-4]
push ebp
mov ebp, esp
push ecx
sub esp, 16
push OFFSET FLAT:.LC2
call puts
mov ecx, DWORD PTR [ebp-4]
add esp, 16
leave
lea esp, [ecx-4]
ret
non-virtual thunk to C::TestB():
lea ecx, [esp+4]
and esp, -16
push DWORD PTR [ecx-4]
push ebp
mov ebp, esp
push ecx
sub esp, 16
push OFFSET FLAT:.LC2
call puts
mov ecx, DWORD PTR [ebp-4]
add esp, 16
leave
lea esp, [ecx-4]
ret</pre>
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