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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - [x86] forming subo intrinsic thwarts codegen"
href="https://bugs.llvm.org/show_bug.cgi?id=42571">42571</a>
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<th>Summary</th>
<td>[x86] forming subo intrinsic thwarts codegen
</td>
</tr>
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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
</td>
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
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<th>Reporter</th>
<td>spatel+llvm@rotateright.com
</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>Forking this example from <a class="bz_bug_link
bz_status_NEW "
title="NEW - Missed BLSR for x == (x & -x) && x"
href="show_bug.cgi?id=42314">bug 42314</a> because it's not exactly an "isPowerOf2"
pattern:
int foo(int x, int y) {
return x ? (x & (x - 1)) : y;
}
Or as optimized IR (if we should create ctpop from this, then this isn't
strictly an x86 bug):
define i32 @foo(i32 %x, i32 %y) {
%tobool = icmp eq i32 %x, 0
%sub = add nsw i32 %x, -1
%and = and i32 %sub, %x
%cond = select i1 %tobool, i32 %y, i32 %and
ret i32 %cond
}
CGP will transform that to use llvm.usub.with.overflow.i32, and we get:
movl %edi, %eax
subl $1, %eax
andl %edi, %eax
cmpl $1, %edi
cmovbl %esi, %eax
retq
For generic x86, this seems better
leal -1(%rdi), %eax
andl %edi, %eax
testl %edi, %edi
cmove %esi, %eax
And with BMI ('blsr'):
blsr %edi, %eax
testl %edi, %edi
cmove %esi, %eax
<a href="https://godbolt.org/z/Wmgyy9">https://godbolt.org/z/Wmgyy9</a></pre>
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</p>
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