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<base href="https://bugs.llvm.org/">
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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - [InstCombine] icmp with sdiv operand transformed strangely"
href="https://bugs.llvm.org/show_bug.cgi?id=36240">36240</a>
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<th>Summary</th>
<td>[InstCombine] icmp with sdiv operand transformed strangely
</td>
</tr>
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<th>Product</th>
<td>libraries
</td>
</tr>
<tr>
<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>All
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<th>Status</th>
<td>NEW
</td>
</tr>
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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>Scalar Optimizations
</td>
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
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</tr>
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<th>Reporter</th>
<td>spatel+llvm@rotateright.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr></table>
<p>
<div>
<pre>define i1 @cmpdiv(i32 %x) {
%d = sdiv i32 1, %x
%c = icmp slt i32 %d, 0
ret i1 %c
}
$ opt -instcombine cmpsignbit.ll -S
define i1 @cmpdiv(i32 %x) {
%1 = add i32 %x, 1
%2 = icmp ult i32 %1, 3
%c1 = icmp slt i32 %x, 0
%c = and i1 %2, %c1
ret i1 %c
That's a weird way to write:
%c = icmp eq i32 %x, -1
-------------------------------------------------------------------------------
Note that we don't do anything with this case:
define i1 @cmpdiv400(i32 %x) {
%d = sdiv i32 400, %x
%c = icmp slt i32 %d, 0
ret i1 %c
}
But we could turn this into a range check:
Name: sdiv400
%d = sdiv i32 400, %x
%c = icmp slt i32 %d, 0
=>
%c1 = icmp sge i32 %x, -400
%c2 = icmp slt i32 %x, 0
%c = and i1 %c1, %c2
<a href="https://rise4fun.com/Alive/cDM">https://rise4fun.com/Alive/cDM</a>
...so whatever is trying to improve the 1st case could probably use
generalizing and fixing.</pre>
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