[PR88579, patch] Calculating power of powers of two

[Harald Anlauf <anlauf at gmx dot de>]

Dear all,

now that my copyright assignment is on file, here's my first attempt
at a non-trivial patch.

I have slightly rearranged Thomas' code, removed some, and added
generalizations for (2**e)**n and (-2**e)**n.  It works with the
testcase below.

I'd like some feedback how to properly handle the case when
(-2**e) = -HUGE()-1, i.e. the number that falls outside the
symmetric range.  Should one skip that one?  Does anyone have
a template how to detect that case easily?  Regtesting otherwise
would fail for the last test (which most likely should be removed
later).

Thanks,
Harald

P.S.: Changelogs will follow with the final submission.

! { dg-do run }
! { dg-additional-options "-fdump-tree-original" }
!
! PR88579 - Test optimizations for bases that are powers of 2 or -2.
program p
  implicit none
  integer(4) :: i, u
  integer(8) :: v
   u = 1
   do i=1,5
      u = u * 64_4
      if (u /= 64_4 ** i) stop 1
   end do
   v = 1
   do i=1,7
      v = v * 256_8
      if (v /= 256_8 ** i) stop 2
   end do
   v = 1
   do i=1,3
      v = v * 65536_8
      if (v /= 65536_8 ** i) stop 3
   end do
   v = 1
   do i=1,7
      v = v * (-256_8)
      if (v /= (-256_8) ** i) stop 4
   end do
   u = 1
   do i=1,1
      u = u * (-HUGE(1_4)-1)
      if (u /= (-HUGE(1_4)-1) ** i) stop 5
   end do
 end program p
! { dg-final { scan-tree-dump-not "_gfortran_pow" "original" } }

Attachment: pr88579.patch-v0
Description: Text document