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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - Sub-optimial code generation for masking off top bit"
href="https://bugs.llvm.org/show_bug.cgi?id=49377">49377</a>
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<th>Summary</th>
<td>Sub-optimial code generation for masking off top bit
</td>
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<th>Product</th>
<td>new-bugs
</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>enhancement
</td>
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<th>Priority</th>
<td>P
</td>
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<th>Component</th>
<td>new bugs
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
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<th>Reporter</th>
<td>arichardson.kde@gmail.com
</td>
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<th>CC</th>
<td>htmldeveloper@gmail.com, llvm-bugs@lists.llvm.org
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<p>
<div>
<pre>I was looking at the code generated for __builtin_fabs() in soft-float mode and
noticed that Clang generates worse code than GCC:
```
double
fabs(double x)
{
return (__builtin_fabs(x));
}
```
<a href="https://godbolt.org/z/33q7Eq">https://godbolt.org/z/33q7Eq</a>
For RV64 (but this also affects e.g. MIPS) Clang:
addi a1, zero, -1
slli a1, a1, 63
addi a1, a1, -1
and a0, a0, a1
ret
GCC:
slli a0,a0,1
srli a0,a0,1
ret
This integer version could also be folded to the shift sequence to avoid
materializing the awkward constant:
```
unsigned long
fabs_int(unsigned long x)
{
return x & ~(1ul << 63);
}
```
I think this fold could be beneficial to all targets since it reduces code size
(GCC emits 9 bytes on x86, clang emits 15).</pre>
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