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Date: Sun, 01 Jan 2006 12:29:00 -0000
Subject: [Bug tree-optimization/25620]  New: Missed optimisation with power
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From: "jv244 at cam dot ac dot uk" <gcc-bugzilla@gcc.gnu.org>
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The following is not optimised (I tested current 4.2) with -ffast-math -O2 (or
any of the other options I tried):

SUBROUTINE S41(a,b,c,N)
 IMPLICIT NONE
 integer :: N
 real*8  :: a(N),b(N),c(N)
 integer :: i
 c=0.0D0
 DO i=1,N
   b(i)=b(i)+a(i)**(4.0D0/3.0D0)
   c(i)=c(i)+a(i)**(2.0D0/3.0D0)
 ENDDO
END SUBROUTINE

This could be written as 

SUBROUTINE S42(a,b,c,N)
 IMPLICIT NONE
 integer :: N
 real*8  :: a(N),b(N),c(N),tmp,tmp2,tmp4
 real*8, parameter :: p=1.0D0/3.0D0
 integer :: i
 c=0.0D0
 DO i=1,N
   tmp=a(i)**p ! could even be done with a cube root
   tmp2=tmp*tmp
   tmp4=tmp2*tmp2
   b(i)=b(i)+tmp4
   c(i)=c(i)+tmp2
 ENDDO
END SUBROUTINE

saving at least one expensive power computation. Also replacing the cube root
with specific code would be nice.


-- 
           Summary: Missed optimisation with power
           Product: gcc
           Version: 4.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: jv244 at cam dot ac dot uk


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=25620