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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - LLVM disregards windows’ x64 vararg ABI"
href="https://llvm.org/bugs/show_bug.cgi?id=26159">26159</a>
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<th>Summary</th>
<td>LLVM disregards windows’ x64 vararg ABI
</td>
</tr>
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<th>Product</th>
<td>new-bugs
</td>
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<th>Version</th>
<td>3.7
</td>
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<th>Hardware</th>
<td>PC
</td>
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<th>OS</th>
<td>other
</td>
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<th>Status</th>
<td>NEW
</td>
</tr>
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<th>Severity</th>
<td>normal
</td>
</tr>
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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>unassignedbugs@nondot.org
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</tr>
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<th>Reporter</th>
<td>simonas+llvm.org@kazlauskas.me
</td>
</tr>
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<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
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<th>Classification</th>
<td>Unclassified
</td>
</tr></table>
<p>
<div>
<pre>LLVM will reliably mis-translate code calling vararg-using functions which
targets Windows x64.
x64 ABI requires no special handling to varargs as compared to non-vararg
functions and wants some varargs to be placed into registers, not always stack.
LLVM, though, always places all of the varargs onto stack.
Here’s a few C programs that, as compiled with clang 3.7.1 on Windows x64,
produce unexpected results:
#include<stdio.h>
extern int wsprintfA(char *out, const char* in, ...);
int main() {
char out[100];
wsprintfA(out, "%d %f %c", 42, 42.25, 'a');
printf("%s", out); // prints “42 f”
return 0;
}
---
extern int wsprintfA(char *out, const char* in, ...);
int main() {
char out[100];
wsprintfA(out, "%d %f %c %s", 42, 42.25, 'a', "hello world"); //segfaults
return 0;
}
(you can think of wsprintfA as of alias to sprintf, because it [essentially is
one](<a href="https://msdn.microsoft.com/en-us/library/windows/desktop/ms647550(v=vs.85).aspx">https://msdn.microsoft.com/en-us/library/windows/desktop/ms647550(v=vs.85).aspx</a>))
I have verified that LLVM generated code correctly places the non-vararg
arguments into the correct registers, however it fails to do so for the
vararg-arguments.
---
I also have a minimal LLVM-IR file which reproduces the issue, but is not as
trivial to build.</pre>
</div>
</p>
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