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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - Bad code generation for __builtin_clrsbl"
href="https://bugs.llvm.org/show_bug.cgi?id=42358">42358</a>
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<th>Summary</th>
<td>Bad code generation for __builtin_clrsbl
</td>
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<th>Product</th>
<td>libraries
</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>Linux
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<th>Status</th>
<td>NEW
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<th>Severity</th>
<td>enhancement
</td>
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<th>Priority</th>
<td>P
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<th>Component</th>
<td>Backend: X86
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
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<th>Reporter</th>
<td>gabravier@gmail.com
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<th>CC</th>
<td>craig.topper@gmail.com, llvm-bugs@lists.llvm.org, llvm-dev@redking.me.uk, spatel+llvm@rotateright.com
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<p>
<div>
<pre>When compiling this code :
int clrsbl(unsigned long x)
{
return __builtin_clrsbl(x);
}
Clang generates really bad code on several backends (x64, x86, RISCV-32,
RISCV-64), such as (on x64)
mov eax, 64
add eax, -1
or (on RISCV-32)
addi a0, zero, 32
addi a0, a0, -1
This bug, however, is not present on certain backends such as WebAssembly or
PowerPC 64 (le and not le)
I looked at the emitted llvm code, and I think this is probably an optimization
problem related specifically to @llvm.ctlz. It looks like it's expanding ctlz
after the passes that would eliminate the bad code.
Link to see examples of this across backends : <a href="https://godbolt.org/z/pbBKgD">https://godbolt.org/z/pbBKgD</a>
I filed this as an x86 backend bug because I do not know how to file a bug for
several backends at the same time</pre>
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