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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - [InstCombine] functionally-equivalent comparison code produces different IR"
href="https://llvm.org/bugs/show_bug.cgi?id=28476">28476</a>
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<th>Summary</th>
<td>[InstCombine] functionally-equivalent comparison code produces different IR
</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>normal
</td>
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<th>Priority</th>
<td>P
</td>
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<th>Component</th>
<td>Scalar Optimizations
</td>
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
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<th>Reporter</th>
<td>spatel+llvm@rotateright.com
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</tr>
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<th>CC</th>
<td>llvm-bugs@lists.llvm.org
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<th>Classification</th>
<td>Unclassified
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</tr></table>
<p>
<div>
<pre>int good(int x, int y) {
return (x == 0 || y == 0) ? 1 : 0;
}
int bad(int x, int y) {
return (x != 0 && y != 0) ? 0 : 1;
}
These are equivalent (DeMorgan's Law), but they generate different IR (and
ultimately different asm):
$ ./clang -O1 xy.c -S -o - -emit-llvm
define i32 @good(i32 %x, i32 %y) local_unnamed_addr #0 {
entry:
%cmp = icmp eq i32 %x, 0
%cmp1 = icmp eq i32 %y, 0
%0 = or i1 %cmp, %cmp1
%cond = zext i1 %0 to i32
ret i32 %cond
}
define i32 @bad(i32 %x, i32 %y) local_unnamed_addr #0 {
entry:
%cmp = icmp ne i32 %x, 0
%cmp1 = icmp ne i32 %y, 0
%0 = and i1 %cmp, %cmp1
%1 = zext i1 %0 to i32
%cond = xor i32 %1, 1
ret i32 %cond
}
--------------------------------------------------------------------------
I think this can be fixed by removing a canonicalization in
InstCombiner::visitZExt() that causes us to miss 'not' logic:
// zext (xor i1 X, true) to i32 --> xor (zext i1 X to i32), 1
This issue also came up in:
<a href="http://reviews.llvm.org/D21899">http://reviews.llvm.org/D21899</a></pre>
</div>
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