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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - [DAGCombine] enableAggressiveFMAFusion incorrectly uses associativity of addition"
href="https://llvm.org/bugs/show_bug.cgi?id=31626">31626</a>
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<th>Summary</th>
<td>[DAGCombine] enableAggressiveFMAFusion incorrectly uses associativity of addition
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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>Linux
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<th>Status</th>
<td>NEW
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<th>Severity</th>
<td>normal
</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>nhaehnle@gmail.com
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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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</tr></table>
<p>
<div>
<pre>The following transforms:
// fold (fadd (fma x, y, (fmul u, v)), z) -> (fma x, y (fma u, v, z))
// fold (fadd x, (fma y, z, (fmul u, v)) -> (fma y, z (fma u, v, x))
// fold (fsub (fma x, y, (fmul u, v)), z)
// -> (fma x, y (fma u, v, (fneg z)))
// fold (fsub x, (fma y, z, (fmul u, v)))
// -> (fma (fneg y), z, (fma (fneg u), v, x))
make use of associativity of addition (and the last one probably even more).
Other parts of the compiler take pains not to use associativity unless
UnsafeFPMath is enabled.
However, these transforms are only guarded by: ((AllowFusion || HasFMAD) &&
Aggressive).
Aggressive is defined by
/// Return true if target always beneficiates from combining into FMA for a
/// given value type. This must typically return false on targets where FMA
/// takes more cycles to execute than FADD.
i.e. it is only about the relative performance of different code sequences
which produce the same result.
I think the guard for the transforms above should instead be: ((AllowFusion ||
HasFMAD) && Options.UnsafeFPMath && Aggressive)</pre>
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