[Bug tree-optimization/108839] New: Option for rerolling loops

[Bug tree-optimization/108839] New: Option for rerolling loops

[tkoenig at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108839

            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 = MOD(N,5)
         IF (M.NE.0) THEN
            DO I = 1,M
               DX(I) = DA*DX(I)
            END DO
            IF (N.LT.5) RETURN
         END IF
         MP1 = M + 1
         DO I = MP1,N,5
            DX(I) = DA*DX(I)
            DX(I+1) = DA*DX(I+1)
            DX(I+2) = DA*DX(I+2)
            DX(I+3) = DA*DX(I+3)
            DX(I+4) = 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.

[Bug tree-optimization/108839] Option for rerolling loops

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108839

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Note the SLP vectorizer should kick in for most cases of manually unrolled
loops.

[Bug tree-optimization/108839] Option for rerolling loops

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108839

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
     Ever confirmed|0                           |1
             Status|UNCONFIRMED                 |NEW
             Blocks|                            |53947
   Last reconfirmed|                            |2023-02-20

--- Comment #2 from Richard Biener <rguenth at gcc dot gnu.org> ---
Note the SLP loop vectorizer could be used to re-roll loops (for -Os?) by
vectorizing with one element vectors and a "fractional" VF.  I just never got
around playing with this idea (even when actually vectorizing but facing
highly manually unrolled code vectorizing with a single vector and a
"fractional" VF might be worth it for example for register pressure reasons).


Referenced Bugs:

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=53947
[Bug 53947] [meta-bug] vectorizer missed-optimizations