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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - [powerpc] missed CSE opportunity in DAG?"
href="https://llvm.org/bugs/show_bug.cgi?id=24363">24363</a>
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<th>Summary</th>
<td>[powerpc] missed CSE opportunity in DAG?
</td>
</tr>
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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
</td>
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<th>Status</th>
<td>NEW
</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>Backend: PowerPC
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
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<th>Reporter</th>
<td>spatel+llvm@rotateright.com
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</tr>
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<th>CC</th>
<td>llvm-bugs@lists.llvm.org
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<th>Classification</th>
<td>Unclassified
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<p>
<div>
<pre>While investigating fast-math-flags propagation in the DAG (see r244053), I
noticed that PPC ends up with different code than AArch64 or x86 for this
example:
define float @fmf(float %a, float %b) {
%mul1 = fmul fast float %a, %b
%nega = fsub fast float 0.0, %a
%mul2 = fmul fast float %nega, %b
%abx2 = fsub fast float %mul1, %mul2
ret float %abx2
}
$ ./llc -o - badflags.ll -march=ppc64 -enable-unsafe-fp-math
...
fmuls f0, f1, f2
fmadds f1, f1, f2, f0
blr
--------------------------------------------------------------------------------
I was expecting to see an 'fadds f1, f0, f0' instead of the 'fmadds' (less
register usage; potentially faster to execute an 'add' than an 'fma').
AArch64 does this:
$ ./llc -o - badflags.ll -march=aarch64 -enable-unsafe-fp-math
...
fmul s0, s0, s1
fadd s0, s0, s0
ret</pre>
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