http://llvm.org/bugs/show_bug.cgi?id=21492
Bug ID: 21492
Summary: Fused Multiply-Add (FMA) yields wrong results when
inlined
Product: new-bugs
Version: trunk
Hardware: PC
OS: Linux
Status: NEW
Severity: normal
Priority: P
Component: new bugs
Assignee: [email protected]
Reporter: [email protected]
CC: [email protected]
Classification: Unclassified
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));
}
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