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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_ASSIGNED "
title="ASSIGNED --- - [Reassociation w/fast-math] Assert when calling getNeg() on undef of type float"
href="http://llvm.org/bugs/show_bug.cgi?id=21205">21205</a>
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<th>Summary</th>
<td>[Reassociation w/fast-math] Assert when calling getNeg() on undef of type float
</td>
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<th>Product</th>
<td>libraries
</td>
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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>
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<th>Status</th>
<td>ASSIGNED
</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>Transformation Utilities
</td>
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<th>Assignee</th>
<td>mcrosier@codeaurora.org
</td>
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<th>Reporter</th>
<td>mcrosier@codeaurora.org
</td>
</tr>
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<th>CC</th>
<td>chandlerc@gmail.com, llvmbugs@cs.uiuc.edu, resistor@mac.com
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<th>Classification</th>
<td>Unclassified
</td>
</tr></table>
<p>
<div>
<pre>An assertion is hit in the Reassociation pass on x86 at -O2 -ffast-math with
the following code:
a, b;
fn1() {
switch (0)
case 0: {
float c;
b = a - c - 1;
}
}
The SROA introduces the following instruction:
%sub = fsub fast float %conv, undef
Complete function:
define i32 @fn1() {
entry:
%0 = load i32* @a, align 4, !tbaa !1
%conv = sitofp i32 %0 to float
%sub = fsub fast float %conv, undef
%sub1 = fsub fast float %sub, 1.000000e+00
%conv2 = fptosi float %sub1 to i32
store i32 %conv2, i32* @b, align 4, !tbaa !1
ret i32 undef
}
The reassociation pass is trying to canonicalize X-Y into X+(0-Y) to expose
more opportunities for reassociation. In this case, Y is undef with type
float, but getNeg() is being called, as opposed to getFNeg().
The logic in the pass looks like this:
static Value *NegateValue(Value *V, Instruction *BI) {
if (ConstantFP *C = dyn_cast<ConstantFP>(V))
return ConstantExpr::getFNeg(C);
if (Constant *C = dyn_cast<Constant>(V))
return ConstantExpr::getNeg(C);
...
I guess an undef of type float isn't a ConstantFP... ..continuing to
investigate a solution.</pre>
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</p>
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