RE: [Patch for suggestions]: How do we know a loop is the peeled version?

["Fang, Changpeng" <Changpeng dot Fang at amd dot com>]

Hi,

An additional problem is that after a loop is completely-unrolled, the loop structure
seems not destroyed. And thus later optimization passes may still be performed 
on these non-existence loops.


Changpeng

________________________________________
From: Fang, Changpeng
Sent: Thursday, July 01, 2010 1:41 PM
To: Richard Guenther; Sebastian Pop
Cc: Zdenek Dvorak; Christian Borntraeger; gcc-patches@gcc.gnu.org; uweigand@de.ibm.com
Subject: [Patch for suggestions]: How do we know a loop is the peeled version?

Hi,

Just found that many optimizations (prefetch, loop unrolling) are performed on the peeled loops.
This causes code size and compilation time increase without benefit.

MODULE kinds
   INTEGER, PARAMETER :: RK8 = SELECTED_REAL_KIND(15, 300)
END MODULE kinds
! --------------------------------------------------------------------
PROGRAM TEST_FPU  ! A number-crunching benchmark using matrix inversion.
USE kinds         ! Implemented by:    David Frank  Dave_Frank@hotmail.com
IMPLICIT NONE     ! Gauss  routine by: Tim Prince   N8TM@aol.com
                  ! Crout  routine by: James Van Buskirk  torsop@ix.netcom.com
                  ! Lapack routine by: Jos Bergervoet bergervo@IAEhv.nl

REAL(RK8) :: pool(101, 101,1000), a(101, 101)
INTEGER :: i

      DO i = 1,1000
         a = pool(:,:,i)         ! get next matrix to invert
      END DO

END PROGRAM TEST_FPU

For this example (-O3 -fprefetch-loop-arrays -funroll-loops), the vectorizer peels the loop.
And the prefetching and loop unrolling are performed on the peeled loops.

In the attached patch, the vectorizer marked the loop as peeled, and the prefetching
gives up. However, the RTL unroller could not get this information and still unroll the peeled
loop.

I need suggestion: How the optimizer recognizes that the loop is the peeled version (preloop or postloop)?

Thanks,

Changpeng