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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - The preserve_all and preserve_most calling conventions on X86 trash the return value."
href="https://bugs.llvm.org/show_bug.cgi?id=42394">42394</a>
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<th>Summary</th>
<td>The preserve_all and preserve_most calling conventions on X86 trash the return value.
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<th>Product</th>
<td>clang
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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>Linux
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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
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<th>Component</th>
<td>-New Bugs
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<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
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<th>Reporter</th>
<td>sunil_srivastava@playstation.sony.com
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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>The preserve_all and preserve_most calling convention attributes are meant to
save and restore more than just the normal callee-save set of registers.
However, at least on x86, they seem to save and restore the return register
also, thus trashing the return value.
//--------------------
#include <stdio.h>
int __attribute__((preserve_most)) foo(int x) { return x+1; }
int main() { printf("%d %d\n", foo(1), foo(2)); }
//--------------------
$ clang -o p.out p.cpp
$ .\p.out
4195616 4195616
Interestingly, with -O2, the result is correct because foo gets inlined. If the
caller and the callee are in two different files, -O2 shows the same problem.
The assembly code shows clearly what is happening:
_Z3fooi: # @_Z3fooi
pushq %rdi
pushq %rax
leal 1(%rdi), %eax
popq %rax <--- return value trashed
popq %rdi
retq</pre>
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