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.
