[Bug tree-optimization/83348] Missed optimization in math expression: can be used std::pow function

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=83348

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |missed-optimization
             Target|                            |x86_64-*-* i?86-*-*
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2017-12-11
                 CC|                            |rguenth at gcc dot gnu.org
     Ever confirmed|0                           |1

--- Comment #1 from Richard Biener <rguenth at gcc dot gnu.org> ---
We recognize this as an integer power of 'a' and unconditionally expand that to
an (optimal) power series.  We are not considering using libgccs powi or libms
pow.  Actually we do, but it's

/* Provide a default value for POWI_MAX_MULTS, the maximum number of
   multiplications to inline before calling the system library's pow
   function.  powi(x,n) requires at worst 2*bits(n)-2 multiplications,
   so this default never requires calling pow, powf or powl.  */

#ifndef POWI_MAX_MULTS
#define POWI_MAX_MULTS  (2*HOST_BITS_PER_WIDE_INT-2)
#endif

and no target defines this.  These days it should probably be a --param.
Note that for -Os you get a call to powidf2.

@defmac POWI_MAX_MULTS
If defined, this macro is interpreted as a signed integer C expression
that specifies the maximum number of floating point multiplications
that should be emitted when expanding exponentiation by an integer
constant inline.  When this value is defined, exponentiation requiring
more than this number of multiplications is implemented by calling the
system library's @code{pow}, @code{powf} or @code{powl} routines.
The default value places no upper bound on the multiplication count.
@end defmac


I guess we should simply use a more reasonable default and eventually
also decide for a cut-off calling pow () instead of libgcc provided
powi.  powi does

TYPE
NAME (TYPE x, int m)
{
  unsigned int n = m < 0 ? -m : m;
  TYPE y = n % 2 ? x : 1;
  while (n >>= 1)
    {
      x = x * x;
      if (n % 2)
        y = y * x;
    }
  return m < 0 ? 1/y : y;
}