| Issue |
128545
|
| Summary |
-ffp-contract difference for runtime / compile-time operations
|
| Labels |
new issue
|
| Assignees |
|
| Reporter |
jcarreira
|
I have a piece of code that computes different floating point results depending on whether the computation happens at compilation time or at runtime (see [https://godbolt.org/z/q65q4f7c3](url) ).
When the values are known at compile time the compiler contracts the operation and computes the result. When they are not known, the _expression_ is computed at runtime without contraction.
```
#include <iostream>
#include <cmath>
#include <iomanip>
int main() {
double x = 30.508474576271183309;
double y = 6.1016949152542370172;
double r = (x-std::trunc(x/y)*y);
std::cout << std::setprecision(17);
std::cout << "compile time (w/ fma): " << r << std::endl;
volatile double *xx = &x;
volatile double *yy = &y;
r = (*xx-std::trunc(*xx/(*yy))*(*yy));
std::cout << "runtime (w/o fma): " << r << std::endl;
r = std::fma(std::trunc(*xx/(*yy)),-(*yy),*xx);
std::cout << "runtime (w/ explicit fma): " << r << std::endl;
return 0;
}
```
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