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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - x86_64: force_align_arg_pointer fails to realign stack before spilling SSE registers"
href="https://bugs.llvm.org/show_bug.cgi?id=48080">48080</a>
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<th>Summary</th>
<td>x86_64: force_align_arg_pointer fails to realign stack before spilling SSE registers
</td>
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<th>Product</th>
<td>clang
</td>
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<th>Version</th>
<td>trunk
</td>
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<th>Hardware</th>
<td>PC
</td>
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<th>OS</th>
<td>All
</td>
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<th>Status</th>
<td>NEW
</td>
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<th>Severity</th>
<td>normal
</td>
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<th>Priority</th>
<td>P
</td>
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<th>Component</th>
<td>-New Bugs
</td>
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<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
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<th>Reporter</th>
<td>bshanks@codeweavers.com
</td>
</tr>
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<th>CC</th>
<td>htmldeveloper@gmail.com, llvm-bugs@lists.llvm.org, neeilans@live.com, richard-llvm@metafoo.co.uk
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<p>
<div>
<pre>In the prologue of a function marked with ms_abi and force_align_arg_pointer,
which calls a sysv_abi function, Clang aligns %rsp but then uses the unaligned
%rbp to spill %xmm6-15 to the stack. This causes a crash.
I've seen this cause a real-world game to crash under Wine, where
force_align_arg_pointer is used on all ms_abi functions called by Windows
application code, since only 4 byte alignment is guaranteed.
Here's a test app that misaligns the stack and then calls the ms_abi function.
Crashes with clang 11.0 but succeeds with gcc.
Godbolt: <a href="https://godbolt.org/z/rY7nhq">https://godbolt.org/z/rY7nhq</a>
#include <stdio.h>
int b(int num) {
printf("%d", num*num);
return num * num;
}
extern int __attribute__((ms_abi)) __attribute__((force_align_arg_pointer))
a(int num) {
return b(num);
}
int main()
{
//a(4);
asm( "subq $4, %%rsp\n"
"movl %0, %%ecx\n"
"call %P1\n"
"addq $4, %%rsp\n"
:
: "r" (4), "i"(a)
);
return 0;
}
Compiled by Clang into:
a(int): # @a(int)
pushq %rbp
movq %rsp, %rbp
pushq %rsi
pushq %rdi
andq $-16, %rsp <== %rsp aligned
subq $176, %rsp
movaps %xmm15, -32(%rbp) # 16-byte Spill <== but %rbp
used for spill
movaps %xmm14, -48(%rbp) # 16-byte Spill
movaps %xmm13, -64(%rbp) # 16-byte Spill
...</pre>
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</p>
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