[PATCH] D38494: [ScalarEvolution] Handling for ICmp occuring in the evolution chain.

[Jun Ryung Ju via Phabricator via llvm-commits]

junryoungju added a comment.

In https://reviews.llvm.org/D38494#899255, @jbhateja wrote:

> In https://reviews.llvm.org/D38494#888590, @sanjoy wrote:
>
> > With this IR:
> >
> >   define void @f() {
> >   entry:
> >     br label %loop
> >  
> >   loop:
> >     %iv = phi i32 [ 0, %entry ], [ %iv.inc, %loop ]
> >     %iv.inc = add i32 %iv, 1
> >     %cmp = icmp ne i32 %iv.inc, 10
> >     %cmp.zext = zext i1 %cmp to i32
> >     br i1 %cmp, label %loop, label %leave
> >  
> >   leave:
> >     ret void
> >   }
> >
> >
> > and `opt -analyze -scalar-evolution` , on master:
> >
> >   Printing analysis 'Scalar Evolution Analysis' for function 'f':
> >   Classifying expressions for: @f
> >     %iv = phi i32 [ 0, %entry ], [ %iv.inc, %loop ]
> >     -->  {0,+,1}<%loop> U: [0,10) S: [0,10)               Exits: 9                LoopDispositions: { %loop: Computable }
> >     %iv.inc = add i32 %iv, 1
> >     -->  {1,+,1}<%loop> U: [1,11) S: [1,11)               Exits: 10               LoopDispositions: { %loop: Computable }
> >     %cmp.zext = zext i1 %cmp to i32
> >     -->  (zext i1 %cmp to i32) U: [0,2) S: [0,2)          Exits: 0                LoopDispositions: { %loop: Variant }
> >   Determining loop execution counts for: @f
> >   Loop %loop: backedge-taken count is 9
> >   Loop %loop: max backedge-taken count is 9
> >   Loop %loop: Predicated backedge-taken count is 9
> >    Predicates:
> >  
> >   Loop %loop: Trip multiple is 10
> >
> >
> > and with your patch:
> >
> >   Printing analysis 'Scalar Evolution Analysis' for function 'f':
> >   Classifying expressions for: @f
> >     %iv = phi i32 [ 0, %entry ], [ %iv.inc, %loop ]
> >     -->  {0,+,1}<%loop> U: [0,10) S: [0,10)               Exits: 9                LoopDispositions: { %loop: Computable }
> >     %iv.inc = add i32 %iv, 1
> >     -->  {1,+,1}<%loop> U: [1,11) S: [1,11)               Exits: 10               LoopDispositions: { %loop: Computable }
> >     %cmp.zext = zext i1 %cmp to i32
> >     -->  1 U: [1,2) S: [1,2)              Exits: 1                LoopDispositions: { %loop: Invariant }
> >   Determining loop execution counts for: @f
> >   Loop %loop: backedge-taken count is 9
> >   Loop %loop: max backedge-taken count is 9
> >   Loop %loop: Predicated backedge-taken count is 9
> >    Predicates:
> >  
> >   Loop %loop: Trip multiple is 10
> >
> >
> > Note that the SCEV for `%cmp.zext` is different.  I'm not sure how you got a
> >  different answer on your end, since this should basically be the same as the IR
> >  from Hal's example.  Perhaps in the example you tried the icmp feeding in to the
> >  latch was a different llvm::Instruction than the icmp feeding into the zext?  If
> >  so, that's a missing-CSE bug.
>
>
> @sanjoy , with the latest revision changes following is the result of scalar evolution analysis for above example.
>
> PROMPT>opt -analyze -scalar-evolution sdas.ll
>  Printing analysis 'Scalar Evolution Analysis' for function 'f':
>  Classifying expressions for: @f
>
>   %iv = phi i32 [ 0, %entry ], [ %iv.inc, %loop ]
>   -->  {0,+,1}<%loop> U: [0,10) S: [0,10)               Exits: 9                LoopDispositions: { %loop: Computable }
>   %iv.inc = add i32 %iv, 1
>   -->  {1,+,1}<%loop> U: [1,11) S: [1,11)               Exits: 10               LoopDispositions: { %loop: Computable }
>   %cmp.zext = zext i1 %cmp to i32
>   -->  (zext i1 %cmp to i32) U: [0,2) S: [0,2)          Exits: 0                LoopDispositions: { %loop: Variant }
>
> Determining loop execution counts for: @f
>  Loop %loop: backedge-taken count is 9
>  Loop %loop: max backedge-taken count is 9
>  Loop %loop: Predicated backedge-taken count is 9
>
>   Predicates:
>   
>
> Loop %loop: Trip multiple is 10


I don't think this is correct way to solve problem.
because it doesn't know that how much AddExpr increasing to target.

try this code.

  int foo() {
      int start = 0;
      int end = 10000;
      int tag = 0;
      do {
          tag = 0;
          if (start < end) start+=2, tag = 1;
      } while (tag);
      return 0;
  }

This will not optimized ( I have tested this code )


https://reviews.llvm.org/D38494