[LLVMdev] determining the number of iteration of a loop

[ether zhhb]

hi,

you could try the getBackedgeTakenCount function of ScalarEvolution.

opt -scalar-evolution -analyze
define void @f(i64* nocapture %a, i64 %N) nounwind {
entry:
  %0 = icmp sgt i64 %N, 0                         ; <i1> [#uses=1]
  br i1 %0, label %bb, label %return

bb:                                               ; preds = %bb, %entry
  %1 = phi i64 [ 0, %entry ], [ %2, %bb ]         ; <i64> [#uses=3]
  %scevgep = getelementptr i64* %a, i64 %1        ; <i64*> [#uses=1]
  store i64 %1, i64* %scevgep, align 8
  %2 = add nsw i64 %1, 1                          ; <i64> [#uses=2]
  %exitcond = icmp eq i64 %2, %N                  ; <i1> [#uses=1]
  br i1 %exitcond, label %return, label %bb

return:                                           ; preds = %bb, %entry
  ret void
}
Printing analysis 'Scalar Evolution Analysis' for function 'f':
Classifying expressions for: @f
  %0 = icmp sgt i64 %N, 0                         ; <i1> [#uses=1]
  -->  %0
  %1 = phi i64 [ 0, %entry ], [ %2, %bb ]         ; <i64> [#uses=3]
  -->  {0,+,1}<%bb>             Exits: (-1 + %N)
  %scevgep = getelementptr i64* %a, i64 %1        ; <i64*> [#uses=1]
  -->  {%a,+,sizeof(i64)}<%bb>          Exits: (((-1 + %N) * sizeof(i64)) +
%a)
  %2 = add nsw i64 %1, 1                          ; <i64> [#uses=2]
  -->  {1,+,1}<%bb>             Exits: %N
  %exitcond = icmp eq i64 %2, %N                  ; <i1> [#uses=1]
  -->  %exitcond                Exits: <<Unknown>>
Determining loop execution counts for: @f
Loop %bb: backedge-taken count is (-1 + %N)
Loop %bb: max backedge-taken count is (-1 + %N)

--best regares
ether
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