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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - Suboptimal shift+mask code gen with BZHI"
href="https://bugs.llvm.org/show_bug.cgi?id=43381">43381</a>
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<th>Summary</th>
<td>Suboptimal shift+mask code gen with BZHI
</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>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>Backend: X86
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
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<th>Reporter</th>
<td>dave@znu.io
</td>
</tr>
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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>The two C examples at the end of this report should generate the following
assembly when BZHI is available:
```
movb $60, %cl
bzhiq %rcx, (%rdi), %rax
shrq $23, %rax
```
In practice they generate:
```
movq (%rdi), %rax
shrq $23, %rax
movb $37, %cl
bzhiq %rcx, %rax, %rax
```
The example C code:
```
unsigned long example1(unsigned long *mem) {
unsigned long temp = *mem & ((1ul << 60) - 1);
return temp >> 23;
}
unsigned long example2(unsigned long *mem) {
unsigned long temp = *mem >> 23;
return temp & ((1ul << 37) - 1);
}
```
It seems to me that LLVM and/or the backend is biased towards "shift then mask"
but sometimes "mask then shift" generates better code.</pre>
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</p>
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