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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - Missed opportunity to simplify bitselect pattern"
href="https://bugs.llvm.org/show_bug.cgi?id=50816">50816</a>
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<th>Summary</th>
<td>Missed opportunity to simplify bitselect pattern
</td>
</tr>
<tr>
<th>Product</th>
<td>libraries
</td>
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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>Windows NT
</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>Scalar Optimizations
</td>
</tr>
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
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</tr>
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<th>Reporter</th>
<td>llvm-dev@redking.me.uk
</td>
</tr>
<tr>
<th>CC</th>
<td>lebedev.ri@gmail.com, llvm-bugs@lists.llvm.org, spatel+llvm@rotateright.com
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<pre>Pulled from bullet source code:
unsigned btSelect(unsigned condition, unsigned valueIfConditionNonZero,
unsigned valueIfConditionZero)
{
// Set testNz to 0xFFFFFFFF if condition is nonzero, 0x00000000 if
condition is zero
// Rely on positive value or'ed with its negative having sign bit on
// and zero value or'ed with its negative (which is still zero) having sign
bit off
// Use arithmetic shift right, shifting the sign bit through all 32 bits
unsigned testNz = (unsigned)(((int)condition | -(int)condition) >> 31);
unsigned testEqz = ~testNz;
return ((valueIfConditionNonZero & testNz) | (valueIfConditionZero &
testEqz));
}
<a href="https://c.godbolt.org/z/rMf6Ta613">https://c.godbolt.org/z/rMf6Ta613</a>
The following code doesn't get simplified:
define i8 @src(i8 %a0, i8 %a1, i8 %a2) {
%add = add i8 %a0, -1
%xor = xor i8 %a0, -1
%and = and i8 %add, %xor
%cmp = icmp sgt i8 %and, -1
%sel = select i1 %cmp, i8 %a1, i8 %a2
ret i8 %sel
}
define i8 @tgt(i8 %a0, i8 %a1, i8 %a2) {
%cmp = icmp ne i8 %a0, 0
%sel = select i1 %cmp, i8 %a1, i8 %a2
ret i8 %sel
}
Transformation seems to be correct!</pre>
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