[Bug tree-optimization/70840] New: revisit reassoc handling of pow / powi, amend match.pd for powi

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70840

            Bug ID: 70840
           Summary: revisit reassoc handling of pow / powi, amend match.pd
                    for powi
           Product: gcc
           Version: 6.0
            Status: UNCONFIRMED
          Keywords: missed-optimization
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: rguenth at gcc dot gnu.org
  Target Milestone: ---

Currently reassoc builds powi from repeated multiplications guarded with
-funsafe-math-optimizations where later expansion of powi is safe for
fp-contract != off.

The acceptable_pow_call cases are globally guarded with
flag_unsafe_math_optimizations as well but POWI is always safe
and only POW has to be guarded with flag_unsafe_math_optimizations.

match.pd has quite some patterns involving POW but none involving POWI.
At least for flag_unsafe_math_optimizations POWI can be treated as POW
and whether we emit POW or POWI does not matter for
flag_unsafe_math_optimziations.