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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - [DAGCombine] Missed A + -2.0*B*C -> A - (B+B)*C"
href="https://bugs.llvm.org/show_bug.cgi?id=32939">32939</a>
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<th>Summary</th>
<td>[DAGCombine] Missed A + -2.0*B*C -> A - (B+B)*C
</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>
</tr>
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<th>OS</th>
<td>Windows NT
</td>
</tr>
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<th>Status</th>
<td>NEW
</td>
</tr>
<tr>
<th>Severity</th>
<td>enhancement
</td>
</tr>
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<th>Priority</th>
<td>P
</td>
</tr>
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<th>Component</th>
<td>Common Code Generator Code
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
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<th>Reporter</th>
<td>mcrosier@codeaurora.org
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr></table>
<p>
<div>
<pre>Given the following C code
double test1(double a, double b, double c) {
return a + 2.0*b*c;
}
Clang removes a multiply by transforming '2.0*b' to 'b+b' during DAGCombine.
For example, we generate the following when targeting AArch64:
test1: // @test1
fadd d1, d1, d1
fmul d1, d1, d2
fadd d0, d1, d0
ret
Given the below code
double test2(double a, double b, double c) {
return a + -2.0*b*c;
}
we should be able to perform a similar transform if we can remove the negative
constant by converting the addition into subtraction.
Currently we generate the below when targeting AArch64:
test2: // @test2
fmov d3, #-2.00000000
fmul d1, d1, d3
fmul d1, d1, d2
fadd d0, d1, d0
ret
but I'd expect
test2: // @test2
fadd d1, d1, d1
fmul d1, d1, d2
fsub d0, d1, d0
ret
AFAIK, this doesn't require fast-math.</pre>
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