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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 - Assembler operand constraints are ineffective with global register variables"
href="https://bugs.llvm.org/show_bug.cgi?id=45585">45585</a>
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<th>Summary</th>
<td>Assembler operand constraints are ineffective with global register variables
</td>
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<th>Product</th>
<td>clang
</td>
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<th>Version</th>
<td>9.0
</td>
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<th>Hardware</th>
<td>PC
</td>
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<th>OS</th>
<td>Linux
</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>C
</td>
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<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
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<th>Reporter</th>
<td>ubizjak@gmail.com
</td>
</tr>
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<th>CC</th>
<td>blitzrakete@gmail.com, dgregor@apple.com, erik.pilkington@gmail.com, llvm-bugs@lists.llvm.org, richard-llvm@metafoo.co.uk
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<p>
<div>
<pre>Following testcase:
--cut here--
register unsigned long current_stack_pointer asm("rsp");
void
foo (void)
{
asm volatile ("# %0" :: "a" (current_stack_pointer));
}
--cut here--
compiles on x86 with -O2 to:
#APP
# %rsp
#NO_APP
Please note that the asm uses input operand constraint "a" that would allow
only %rax register.
GCC gets this right and reloads %rsp to %rax:
movq %rsp, %rax
#APP
# %rax
#NO_APP
If the constraint is changed to "r", which allows all general registers,
including %rsp, GCC generates
#APP
# %rsp
#NO_APP
I'd like to argue that the compiler should reload the input argument, so that
the asm input operand constraint is always respected.</pre>
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</p>
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