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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - When -ffp-contract=on is effective, fma conversion works even with -O0."
href="https://bugs.llvm.org/show_bug.cgi?id=43223">43223</a>
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<th>Summary</th>
<td>When -ffp-contract=on is effective, fma conversion works even with -O0.
</td>
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<th>Product</th>
<td>new-bugs
</td>
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<th>Version</th>
<td>unspecified
</td>
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<th>Hardware</th>
<td>Other
</td>
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<th>OS</th>
<td>Linux
</td>
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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
</td>
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<th>Component</th>
<td>new bugs
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
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<th>Reporter</th>
<td>ueno.masakazu@jp.fujitsu.com
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<th>CC</th>
<td>htmldeveloper@gmail.com, llvm-bugs@lists.llvm.org
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<pre>When -ffp-contract=on is effective, fma conversion works even with -O0.
I think it is better to stop fma conversion when -O0.
Sample program:
float foo(float b, float c, float d) {
return b * c + d;
}
$ clang -O0 -ffp-contract=on a.c -S -o - | grep fma // OK?
fmadd s0, s0, s1, s2
$ clang -O1 -ffp-contract=on a.c -S -o - | grep fma
fmadd s0, s0, s1, s2
$ clang -O0 -ffp-contract=fast a.c -S -o - | grep fma
$ clang -O1 -ffp-contract=fast a.c -S -o - | grep fma
fmadd s0, s0, s1, s2
It seems that it can be stopped by making the following changes.
What do you think?
diff --git a/clang/lib/CodeGen/CGExprScalar.cpp
b/clang/lib/CodeGen/CGExprScalar.cpp
index 3d082de..f0d9fda 100644
--- a/clang/lib/CodeGen/CGExprScalar.cpp
+++ b/clang/lib/CodeGen/CGExprScalar.cpp
@@ -3308,7 +3308,8 @@ static Value* tryEmitFMulAdd(const BinOpInfo &op,
"Only fadd/fsub can be the root of an fmuladd.");
// Check whether this op is marked as fusable.
- if (!op.FPFeatures.allowFPContractWithinStatement())
+ if (CGF.CGM.getCodeGenOpts().OptimizationLevel == 0 ||
+ !op.FPFeatures.allowFPContractWithinStatement())
return nullptr;
// We have a potentially fusable op. Look for a mul on one of the operands.</pre>
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