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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - does not eliminate a branch on shifts"
href="https://bugs.llvm.org/show_bug.cgi?id=41647">41647</a>
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<th>Summary</th>
<td>does not eliminate a branch on shifts
</td>
</tr>
<tr>
<th>Product</th>
<td>new-bugs
</td>
</tr>
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<th>Version</th>
<td>trunk
</td>
</tr>
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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>enhancement
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
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<th>Component</th>
<td>new bugs
</td>
</tr>
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
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<th>Reporter</th>
<td>slandden@gmail.com
</td>
</tr>
<tr>
<th>CC</th>
<td>htmldeveloper@gmail.com, llvm-bugs@lists.llvm.org
</td>
</tr></table>
<p>
<div>
<pre>When creating a mask, with unsigned n (I'm doing this in Zig because underflow
is undefined in Zig)
if (n == 0) {
return 0;
} else {
return u64(math.maxInt(u64)) >> @intCast(u6, ((64 - @intCast(u7, n))));
}
mask:
test dil, dil
je .LBB0_1
neg dil
mov rax, -1
shrx rax, rax, rdi
ret
.LBB0_1:
xor eax, eax
ret
But the branch is unneeded.
<a href="https://www.felixcloutier.com/x86/sarx:shlx:shrx">https://www.felixcloutier.com/x86/sarx:shlx:shrx</a>
If the value specified in the first source operand exceeds OperandSize -1, the
COUNT value is masked.</pre>
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</p>
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