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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - Inefficient lowering of constant, negative, anyext return values"
href="https://bugs.llvm.org/show_bug.cgi?id=39092">39092</a>
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<th>Summary</th>
<td>Inefficient lowering of constant, negative, anyext return values
</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
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<th>Status</th>
<td>NEW
</td>
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<th>Severity</th>
<td>enhancement
</td>
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<th>Priority</th>
<td>P
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<th>Component</th>
<td>Common Code Generator Code
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
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<th>Reporter</th>
<td>asb@lowrisc.org
</td>
</tr>
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<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
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<p>
<div>
<pre>Consider a function returning a constant negative value, e.g.
define i16 @neg_const() nounwind {
ret i16 -2047
}
Compile this on a target for which i16 is not a legal type, e.g. PowerPC. For
llc -mtriple=powerpc, the following assembly is generated:
lis 3, 0
ori 3, 3, 65436
blr
We'd rather see:
li 3, -100
blr
The issue is that SelectionDAGBuilder::visitRet will call
SelectionDAG::getCopyToParts with ExtendKind = ISD::ANY_EXTEND. This calls
SelectionDAG::getNode with ISD::ANY_EXTEND, which will recognise it has a
constant argument and choose to zero-extend it. This generates the i32 constant
65436 (i.e. zext of int16(-100)) rather than the preferable -100.
By the time target instruction selection code is reached, there is no way to
know that a sign-extended form of the original constant could have been used.
As a quick experiment I changed the constant-folding code in
SelectionDAG::getNode so it performs a sign-extend for ANY_EXTEND input. That
fixes this particular issue as expected but leads to incorrect codegen
elsewhere, though I haven't yet had a chance to explore in more detail.</pre>
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