<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/163470>163470</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            [clang] No-zero result from `a*a - b*b` when a == b
        </td>
    </tr>

    <tr>
      <th>Labels</th>
      <td>
            clang
      </td>
    </tr>

    <tr>
      <th>Assignees</th>
      <td>
      </td>
    </tr>

    <tr>
      <th>Reporter</th>
      <td>
          zhqrbitee
      </td>
    </tr>
</table>

<pre>
    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="https://github.com/user-attachments/assets/bffbe404-01dd-4e8d-a49a-0e7a794a7644" />

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. 





</pre>
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