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<base href="http://llvm.org/bugs/" />
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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - Fused Multiply-Add (FMA) yields wrong results when inlined"
href="http://llvm.org/bugs/show_bug.cgi?id=21492">21492</a>
</td>
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<tr>
<th>Summary</th>
<td>Fused Multiply-Add (FMA) yields wrong results when inlined
</td>
</tr>
<tr>
<th>Product</th>
<td>new-bugs
</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>Linux
</td>
</tr>
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<th>Status</th>
<td>NEW
</td>
</tr>
<tr>
<th>Severity</th>
<td>normal
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
</tr>
<tr>
<th>Component</th>
<td>new bugs
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>dan.spencer.np@gmail.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvmbugs@cs.uiuc.edu
</td>
</tr>
<tr>
<th>Classification</th>
<td>Unclassified
</td>
</tr></table>
<p>
<div>
<pre>Reproducible in:
* x86-64 Ubuntu 14.04, clang++ 3.4 and 3.5.1
* x86-64 Ubuntu 14.04, Rust nightly
If the third argument to std::fma() in C++11 or Float::mul_add() in Rust is 0,
it always returns the first argument. If the third argument is != 0, it returns
the correct result. This happens for both the float and double types.
The issue occurs only when all arguments are constants and compiler
optimizations are turned on. This is why I believe this is an issue with LLVM
inlining the call.
gcc does not produce this issue.
clang++ -O2 --std=c++11 fmatest.cpp
#include <cmath>
#include <iostream>
int main() {
// prints "10"
std::cout << std::fma(10.0f, 20.0f, 0.0f) << std::endl;
// prints "200.001" (10*20 + 0.001)
std::cout << std::fma(10.0f, 20.0f, 0.001f) << std::endl;
return 0;
}
rustc -O fmatest.rs
fn main() {
// prints "10"
println!("{}", Float::mul_add(10.0f32, 20.0, 0.0));
// prints "200.001007" (10*20 + 0.001)
println!("{}", Float::mul_add(10.0f32, 20.0, 0.001));
}</pre>
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