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

[Jatin Bhateja via Phabricator via llvm-commits]

jbhateja added a comment.

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


https://reviews.llvm.org/D38494