[Bug tree-optimization/114363] New: inconsistent optimization of pow(x,2)+pow(y,2)

[Bug tree-optimization/114363] New: inconsistent optimization of pow(x,2)+pow(y,2)

[vincenzo.innocente at cern dot ch]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114363

            Bug ID: 114363
           Summary: inconsistent optimization of pow(x,2)+pow(y,2)
           Product: gcc
           Version: 14.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: vincenzo.innocente at cern dot ch
  Target Milestone: ---

while pow(x,2) is optimized in x*x   (float x)
in  pow(x,2)+pow(y,2) x and y are first promoted to double 
which I find inconsistent

see
https://godbolt.org/z/rYfoaxr89

[Bug tree-optimization/114363] inconsistent optimization of pow(x,2)+pow(y,2)

[harald at gigawatt dot nl]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114363

Harald van Dijk <harald at gigawatt dot nl> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |harald at gigawatt dot nl

--- Comment #1 from Harald van Dijk <harald at gigawatt dot nl> ---
This is, I believe, correct. Before C++11, calling std::pow with float and int
arguments, it returned a float. As of C++11, it returns a double.

If the result of pow(x,2) is immediately converted to float, then it is a valid
optimisation to convert it to x*x: that is guaranteed to produce the exact same
result. But if it isn't, then converting to x*x loses accuracy and alters the
result.

You can call std::powf instead of std::pow to avoid the promotion to double.

[Bug tree-optimization/114363] inconsistent optimization of pow(x,2)+pow(y,2)

[xry111 at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114363

Xi Ruoyao <xry111 at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
         Resolution|---                         |INVALID
                 CC|                            |xry111 at gcc dot gnu.org
             Status|UNCONFIRMED                 |RESOLVED

--- Comment #2 from Xi Ruoyao <xry111 at gcc dot gnu.org> ---
(In reply to Harald van Dijk from comment #1)
> This is, I believe, correct. Before C++11, calling std::pow with float and
> int arguments, it returned a float. As of C++11, it returns a double.
> 
> If the result of pow(x,2) is immediately converted to float, then it is a
> valid optimisation to convert it to x*x: that is guaranteed to produce the
> exact same result. But if it isn't, then converting to x*x loses accuracy
> and alters the result.

Thus invalid.

> You can call std::powf instead of std::pow to avoid the promotion to double.

Or add -std=c++98.

[Bug tree-optimization/114363] inconsistent optimization of pow(x,2)+pow(y,2)

[jakub at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114363

Jakub Jelinek <jakub at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |jakub at gcc dot gnu.org

--- Comment #3 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
Or use pow(x,2.0f)+pow(y,2.0f).
Anyway, see https://eel.is/c++draft/c.math#cmath.syn-3

[Bug tree-optimization/114363] inconsistent optimization of pow(x,2)+pow(y,2)

[vincenzo.innocente at cern dot ch]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114363

--- Comment #4 from vincenzo Innocente <vincenzo.innocente at cern dot ch> ---
Thanks Harald, I missed the point that float z = pow(double(x),2) and
float z = x*x would indeed produce exactly the same result, while in all other
cases of course not.