[Bug libstdc++/11706] New: std::pow(T, int) implementation pessimizes code

[rguenth at tat dot physik dot uni-tuebingen dot de]

PLEASE REPLY TO gcc-bugzilla@gcc.gnu.org ONLY, *NOT* gcc-bugs@gcc.gnu.org.

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=11706

           Summary: std::pow(T, int) implementation pessimizes code
           Product: gcc
           Version: 3.4
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: libstdc++
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: rguenth at tat dot physik dot uni-tuebingen dot de
                CC: gcc-bugs at gcc dot gnu dot org
  GCC host triplet: i686-pc-linux-gnu

The std::__pow_helper() implementation ends up emitting a call to
std::__cmath_power() for all constant powers as f.i. in the call to
std::pow(x, 2). This severly pessimizes code and can be cured by teaching
optimal computation to std::__pow_helper() via the __builtin_constant_p
mechanism of gcc.

See http://gcc.gnu.org/ml/gcc/2003-07/msg01950.html for an example improved
implementation.

The same problem exists on the 3.3 branch.