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<base href="https://llvm.org/bugs/" />
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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - Optimizer doesn't simplify obvious contradiction"
href="https://llvm.org/bugs/show_bug.cgi?id=27869">27869</a>
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<th>Summary</th>
<td>Optimizer doesn't simplify obvious contradiction
</td>
</tr>
<tr>
<th>Product</th>
<td>libraries
</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>
</tr>
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<th>Status</th>
<td>NEW
</td>
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<th>Severity</th>
<td>normal
</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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<th>Reporter</th>
<td>me@manueljacob.de
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr>
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<th>Classification</th>
<td>Unclassified
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</tr></table>
<p>
<div>
<pre>The following C code contains an "obvious" contradiction. An integer can't
both be 0 and negative, so this should be simplifies to false:
_Bool test(int i) {
return (i == 0) & (i < 0);
}
The LLVM IR with SROA run on it:
define zeroext i1 @test(i32 %i) #0 {
entry:
%cmp = icmp eq i32 %i, 0
%conv = zext i1 %cmp to i32
%cmp1 = icmp slt i32 %i, 0
%conv2 = zext i1 %cmp1 to i32
%and = and i32 %conv, %conv2
%tobool = icmp ne i32 %and, 0
ret i1 %tobool
}
However running all optimizations on it will result in the following code:
define zeroext i1 @test(i32 %i) #0 {
entry:
%cmp = icmp eq i32 %i, 0
%conv = zext i1 %cmp to i32
%i.lobit = lshr i32 %i, 31
%and = and i32 %conv, %i.lobit
%tobool = icmp ne i32 %and, 0
ret i1 %tobool
}
The problem is that InstCombine transforms the (zext (< i 0)) pattern into
clever bit shifting: (lshr i 31). At first this saves one operation, but then
the obvious contradiction is obfuscated.
I see two options how to solve this:
1) Deferring the (zext (< i 0)) -> (lshr i 31) transformation, allowing
InstCombine to simplify the contradiction.
2) Making subsequent transformations more clever, so they can "look through"
the lshr instruction, seeing that it's actually a "less than" predicate.</pre>
</div>
</p>
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