Commit 10357e1c authored by Evgeniy Brevnov's avatar Evgeniy Brevnov
Browse files

[LoopUtils] Better accuracy for getLoopEstimatedTripCount. 

Summary: Current implementation of getLoopEstimatedTripCount returns 1 iteration less than it should. The reason is that in bottom tested loop first iteration is executed before first back branch is taken. For example for loop with !{!"branch_weights", i32 1 // taken, i32 1 // exit} metadata getLoopEstimatedTripCount gives 1 while actual number of iterations is 2.

Reviewers: Ayal, fhahn

Reviewed By: Ayal

Subscribers: mgorny, hiraditya, zzheng, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D71990
parent 84c4c87e

llvm/lib/Transforms/Utils/LoopUtils.cpp

+8 −5
Original line number Diff line number Diff line
@@ -723,12 +723,15 @@ Optional<unsigned> llvm::getLoopEstimatedTripCount(Loop *L) { 
  if (LatchBR->getSuccessor(0) != L->getHeader())

    std::swap(BackedgeTakenWeight, LatchExitWeight);



  if (!BackedgeTakenWeight || !LatchExitWeight)

    return 0;

  if (!LatchExitWeight)

    return None;



  // Divide the count of the backedge by the count of the edge exiting the loop,

  // rounding to nearest.

  return llvm::divideNearest(BackedgeTakenWeight, LatchExitWeight);

  // Estimated backedge taken count is a ratio of the backedge taken weight by

  // the the edge exiting weight, rounded to nearest.

  uint64_t BackedgeTakenCount =

      llvm::divideNearest(BackedgeTakenWeight, LatchExitWeight);

  // Estimated trip count is one plus estimated backedge taken count.

  return BackedgeTakenCount + 1;

}



bool llvm::hasIterationCountInvariantInParent(Loop *InnerLoop,

llvm/test/Transforms/LoopUnroll/peel-loop-conditions-pgo-1.ll

+1 −1
Original line number Diff line number Diff line
@@ -11,7 +11,7 @@ declare void @f2() 
define void @test1(i32 %k) !prof !4 {

; CHECK: Loop Unroll: F[test1] Loop %for.body

; CHECK: PEELING loop %for.body with iteration count 2!

; CHECK: PEELING loop %for.body with iteration count 4!

; CHECK: PEELING loop %for.body with iteration count 5!

; CHECK: llvm.loop.unroll.disable

for.body.lr.ph:

  br label %for.body

llvm/test/Transforms/LoopUnroll/peel-loop-pgo-deopt-idom-2.ll

+1 −1
Original line number Diff line number Diff line
@@ -5,7 +5,7 @@ 
; Regression test for setting the correct idom for exit blocks.



; CHECK: Loop Unroll: F[basic]

; CHECK: PEELING loop %for.body with iteration count 1!

; CHECK: PEELING loop %for.body with iteration count 2!



define i32 @basic(i32* %p, i32 %k, i1 %c1, i1 %c2) #0 !prof !3 {

entry:

llvm/test/Transforms/LoopUnroll/peel-loop-pgo-deopt-idom.ll

+1 −1
Original line number Diff line number Diff line
@@ -5,7 +5,7 @@ 
; Regression test for setting the correct idom for exit blocks.



; CHECK: Loop Unroll: F[basic]

; CHECK: PEELING loop %for.body with iteration count 1!

; CHECK: PEELING loop %for.body with iteration count 2!



define i32 @basic(i32* %p, i32 %k, i1 %c1, i1 %c2) #0 !prof !3 {

entry:

llvm/test/Transforms/LoopUnroll/peel-loop-pgo-deopt.ll

+1 −1
Original line number Diff line number Diff line
@@ -8,7 +8,7 @@ 
; All side exits to deopt does not change weigths.



; CHECK: Loop Unroll: F[basic]

; CHECK: PEELING loop %for.body with iteration count 3!

; CHECK: PEELING loop %for.body with iteration count 4!

; CHECK-NO-PEEL-NOT: PEELING loop %for.body

; CHECK-LABEL: @basic

; CHECK: br i1 %c, label %{{.*}}, label %side_exit, !prof !15