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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - Clobbered Callee-Save Registers Not Saved"
href="http://llvm.org/bugs/show_bug.cgi?id=16102">16102</a>
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<th>Summary</th>
<td>Clobbered Callee-Save Registers Not Saved
</td>
</tr>
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<th>Product</th>
<td>new-bugs
</td>
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<th>Version</th>
<td>3.2
</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>unassignedbugs@nondot.org
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<th>Reporter</th>
<td>bugzilla@uhsure.com
</td>
</tr>
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<th>CC</th>
<td>llvmbugs@cs.uiuc.edu
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<th>Classification</th>
<td>Unclassified
</td>
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<p>
<div>
<pre>I had guessed that mark a callee-saved register as clobbered would cause LLVM
to save the register in the prologue, but it turns out not to. Specifically, I
would expect that on the x86-64, this function:
define i32 @testfun(i32 %arg)
{
tail call void asm sideeffect "" , "~{%r12},~{%r13}"()
ret i32 %arg
}
Would cause both r12 and r13 to be pushed in the prologue and popped before
returning, but it actually just compiles into this:
_testfun: ## @testfun
Ltmp69:
## BB#0:
## InlineAsm Start
## InlineAsm End
movl>---%edi, %eax
ret
Ltmp70:
Just in case it was being very clever, I changed the assembly to actually
clobber r12 and r13 with no effect.
Unless I'm misunderstanding clobbering -- always a possibility -- the above
leads to unsafe code generation, as the calling function of "tempfun" will
assume r12 and r13 are safe across the call, but this function destroys them.
If I hadn't marked them as clobbered I wouldn't have been surprised at the
behavior, but the fact that I did mark them as clobbered makes this seem like a
bug.</pre>
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</p>
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