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From: "tkoenig at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug tree-optimization/108839] New: Option for rerolling loops
Date: Fri, 17 Feb 2023 17:16:12 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D108839

            Bug ID: 108839
           Summary: Option for rerolling loops
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: enhancement
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: tkoenig at gcc dot gnu.org
  Target Milestone: ---

Code sometimes contains manual unrolling.  For example, the BLAS
reference implementation, subroutine DSCAL, has

      IF (INCX.EQ.1) THEN
*
*        code for increment equal to 1
*
*
*        clean-up loop
*
         M =3D MOD(N,5)
         IF (M.NE.0) THEN
            DO I =3D 1,M
               DX(I) =3D DA*DX(I)
            END DO
            IF (N.LT.5) RETURN
         END IF
         MP1 =3D M + 1
         DO I =3D MP1,N,5
            DX(I) =3D DA*DX(I)
            DX(I+1) =3D DA*DX(I+1)
            DX(I+2) =3D DA*DX(I+2)
            DX(I+3) =3D DA*DX(I+3)
            DX(I+4) =3D DA*DX(I+4)
         END DO
      ELSE

While such code may have been beneficial on old architectures, by
now this disturbs the compiler's own unrolling and vectorization,
and it increases code size.

It could be beneficial to have a -freroll-loops option, which
undid the manual unrolling of the code above. This could be
stand-alone, or included in options such as -Os.=