[Bug fortran/85531] New: Implement some loop fusion in the Fortran front end

[tkoenig at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85531

            Bug ID: 85531
           Summary: Implement some loop fusion in the Fortran front end
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: enhancement
          Priority: P3
         Component: fortran
          Assignee: unassigned at gcc dot gnu.org
          Reporter: tkoenig at gcc dot gnu.org
  Target Milestone: ---

See the discussion starting at https://gcc.gnu.org/ml/gcc/2018-04/msg00128.html
.

Basically, for code like

    ZQICE(1:NPROMA,1:NFLEVG) = PGFL(1:NPROMA,1:NFLEVG,YI%MP)
    ZQLI(1:NPROMA,1:NFLEVG) = PGFL(1:NPROMA,1:NFLEVG,YL%MP)
    ZQRAIN(1:NPROMA,1:NFLEVG) = PGFL(1:NPROMA,1:NFLEVG,YR%MP)
    ZQSNOW(1:NPROMA,1:NFLEVG) = PGFL(1:NPROMA,1:NFLEVG,YS%MP)

the front end could notice that the right-hand sides always agree
and translate this to


  DO J=1,NFLEVG
    DO I=1,NPROMA
      ZQICE(I,J) = PGFL(I,J,YI%MP)
      ZQLI(I,J) = PGFL(I,J,YI%MP)
      ZQRAIN(I,J) = PGFL(I,J,YI%MP)
      ZQSNOW(I,J) = PGFL(I,J,YI%MP)
    END DO
  END DO

This would require some data flow analysis (are we sure that PGFL
is not changed in the meantime?) and some basic block analysis.