| Issue |
163470
|
| Summary |
[clang] No-zero result from `a*a - b*b` when a == b
|
| Labels |
clang
|
| Assignees |
|
| Reporter |
zhqrbitee
|
Hi,
For below codes,
```cpp
#include <iostream>
#include <cmath>
double test1(double a, double b) {
return (a*a - b*b) ;
}
int main() {
std::cout << "---------------------------------" << std::endl;
std::cout << test1(1.1, 1.1) << std::endl;
std::cout << "---------------------------------" << std::endl;
return 0;
}
```
If `a == b`, then I would expect the `test1` function return me exactly `0.0`, as `a * a` should be exactly equal to `b * b`. However, it always return me a small value in my MacBook (with M2 chips).
I compared the assembly codes from latest clang and GCC with `-O3` in compiler explorer (in arm64): both of them use fused multiply-add instructions, but the GCC one can produce 0.0 as expected, while the clang one produce `8.88178e-18`.
<img width="2559" height="882" alt="Image" src="" />
The compiler explorer link is: https://godbolt.org/z/93Tej6d66
I know floating-point issues can be subtle, but it seems returning 0.0 in this case makes more sense to me.
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