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From: "tnfchris at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug tree-optimization/114932] Improvement in CHREC can give large
 performance gains
Date: Fri, 03 May 2024 08:41:18 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D114932

--- Comment #4 from Tamar Christina <tnfchris at gcc dot gnu.org> ---
reduced more:

---
  module brute_force
    integer, parameter :: r=3D9
     integer  block(r, r, 0)
    contains
  subroutine brute
     do
      do
          do
           do
                do
                     do
                         do i7 =3D l0, 1
                       select case(1 )
                       case(1)
                           block(:2, 7:, 1) =3D block(:2, 7:, i7) - 1
                       end select
                            do i8 =3D 1, 1
                               do i9 =3D 1, 1
                            if(1 =3D=3D 1) then
                                    call digits_20
                                end if
                                end do
                          end do
                    end do
                    end do
              end do
              end do
           end do
     end do
  end do
 end
  end
---

I'll have to stop now till I'm back, but the main difference seems to be in:

good:

<Induction Vars>:
IV struct:
  SSA_NAME:     _1
  Type: integer(kind=3D8)
  Base: (integer(kind=3D8)) ((unsigned long) l0_19(D) * 81)
  Step: 81
  Biv:  N
  Overflowness wrto loop niter: Overflow
IV struct:
  SSA_NAME:     _20
  Type: integer(kind=3D8)
  Base: (integer(kind=3D8)) l0_19(D)
  Step: 1
  Biv:  N
  Overflowness wrto loop niter: No-overflow
IV struct:
  SSA_NAME:     i7_28
  Type: integer(kind=3D4)
  Base: l0_19(D) + 1
  Step: 1
  Biv:  Y
  Overflowness wrto loop niter: No-overflow
IV struct:
  SSA_NAME:     vectp.22_46
  Type: integer(kind=3D4) *
  Base: (integer(kind=3D4) *) &block + ((sizetype) ((unsigned long) l0_19(D=
) *
324) + 36)
  Step: 324
  Object:       (void *) &block
  Biv:  N
  Overflowness wrto loop niter: No-overflow

bad:

<Induction Vars>:
IV struct:
  SSA_NAME:     _1
  Type: integer(kind=3D8)
  Base: (integer(kind=3D8)) l0_19(D) * 81
  Step: 81
  Biv:  N
  Overflowness wrto loop niter: No-overflow
IV struct:
  SSA_NAME:     _20
  Type: integer(kind=3D8)
  Base: (integer(kind=3D8)) l0_19(D)
  Step: 1
  Biv:  N
  Overflowness wrto loop niter: No-overflow
IV struct:
  SSA_NAME:     i7_28
  Type: integer(kind=3D4)
  Base: l0_19(D) + 1
  Step: 1
  Biv:  Y
  Overflowness wrto loop niter: No-overflow
IV struct:
  SSA_NAME:     vectp.22_46
  Type: integer(kind=3D4) *
  Base: (integer(kind=3D4) *) &block + ((sizetype) ((integer(kind=3D8)) l0_=
19(D) *
81) + 9) * 4
  Step: 324
  Object:       (void *) &block
  Biv:  N
  Overflowness wrto loop niter: No-overflow=