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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 - Failure to exploit assume to eliminate use of load"
href="https://bugs.llvm.org/show_bug.cgi?id=43158">43158</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>Failure to exploit assume to eliminate use of load
</td>
</tr>
<tr>
<th>Product</th>
<td>libraries
</td>
</tr>
<tr>
<th>Version</th>
<td>trunk
</td>
</tr>
<tr>
<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>
</tr>
<tr>
<th>Severity</th>
<td>enhancement
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
</tr>
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<th>Component</th>
<td>Scalar Optimizations
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>listmail@philipreames.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr></table>
<p>
<div>
<pre>Test case:
define i32 @test(i32* %p, i32 %v) {
%load = load i32, i32* %p
%c = icmp eq i32 %load, %v
call void @llvm.assume(i1 %c)
ret i32 %load
}
define i32 @reverse(i32* %p, i32 %v) {
%load = load i32, i32* %p
%c = icmp eq i32 %load, %v
call void @llvm.assume(i1 %c)
ret i32 %v
}
declare void @llvm.assume(i1)
Both of these examples are stable under O3; we really should be canonicalizing
to one or the other. In this particular example, reverse is the better of the
two, but a general heuristic requires more thought.
GVN's propagateEquality has a "prefer early valno" heuristic which appears
reasonable. GVN has a separate constant replacement in operands heuristic
which also seems reasonable. I suspect these two need to be unified into
something of a broader scope.</pre>
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</p>
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