[PATCH] D108910: [IndVars] Break backedge and replace PHIs if loop exits on 1st iteration

[Max Kazantsev via Phabricator via llvm-commits]

mkazantsev added a comment.

> ! In D108910#2996649 <https://reviews.llvm.org/D108910#2996649>, @mkazantsev wrote:
> Not sure if IndVars already does it, let me try to wright a simple example...

...Yes it does.

  ; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
  ; RUN: opt -indvars -S < %s | FileCheck %s
  ; RUN: opt -passes=indvars -S < %s | FileCheck %s
  
  declare void @never_called()
  declare void @will_be_called()
  
  define void @test_01(i32 %a, i32 %b) {
  ; CHECK-LABEL: @test_01(
  ; CHECK-NEXT:  entry:
  ; CHECK-NEXT:    [[GUARD_COND:%.*]] = icmp ugt i32 [[A:%.*]], [[B:%.*]]
  ; CHECK-NEXT:    br i1 [[GUARD_COND]], label [[LOOP_PREHEADER:%.*]], label [[FAILURE:%.*]]
  ; CHECK:       loop.preheader:
  ; CHECK-NEXT:    br label [[LOOP:%.*]]
  ; CHECK:       loop:
  ; CHECK-NEXT:    [[IV:%.*]] = phi i32 [ [[IV_NEXT:%.*]], [[BACKEDGE:%.*]] ], [ 0, [[LOOP_PREHEADER]] ]
  ; CHECK-NEXT:    br i1 false, label [[NEVER_CALLED:%.*]], label [[BACKEDGE]]
  ; CHECK:       backedge:
  ; CHECK-NEXT:    [[IV_NEXT]] = add i32 [[IV]], 1
  ; CHECK-NEXT:    [[COND_2:%.*]] = icmp eq i32 [[IV]], [[B]]
  ; CHECK-NEXT:    br i1 [[COND_2]], label [[WILL_BE_CALLED:%.*]], label [[LOOP]]
  ; CHECK:       never_called:
  ; CHECK-NEXT:    call void @never_called()
  ; CHECK-NEXT:    ret void
  ; CHECK:       will_be_called:
  ; CHECK-NEXT:    call void @will_be_called()
  ; CHECK-NEXT:    ret void
  ; CHECK:       failure:
  ; CHECK-NEXT:    ret void
  ;
  entry:
    %guard_cond = icmp ugt i32 %a, %b
    br i1 %guard_cond, label %loop, label %failure
  
  loop:
    %iv = phi i32 [0, %entry], [%iv.next, %backedge]
    %cond_1 = icmp eq i32 %iv, %a
    br i1 %cond_1, label %never_called, label %backedge
  
  backedge:
    %iv.next = add i32 %iv, 1
    %cond_2 = icmp eq i32 %iv, %b
    br i1 %cond_2, label %will_be_called, label %loop
  
  never_called:
    call void @never_called()
    ret void
  
  will_be_called:
    call void @will_be_called()
    ret void
  
  failure:
    ret void
  }


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https://reviews.llvm.org/D108910