Commit 5a6dfb27 authored by Max Kazantsev's avatar Max Kazantsev
Browse files

[IndVars] Replace PHIs if loop exits on 1st iteration 

This is a part of D108910.
We replace all loop PHIs with values coming from the loop preheader if
we proved that backedge is never taken.

Patch by Dmitry Makogon!

Differential Revision: https://reviews.llvm.org/D109596
Reviewed By: lebedev.ri
parent d48a3f9f

llvm/lib/Transforms/Scalar/IndVarSimplify.cpp

+14 −2
Original line number Diff line number Diff line
@@ -1309,6 +1309,17 @@ static void foldExit(const Loop *L, BasicBlock *ExitingBB, bool IsTaken, 
  replaceExitCond(BI, NewCond, DeadInsts);

}



static void replaceLoopPHINodesWithPreheaderValues(

    Loop *L, SmallVectorImpl<WeakTrackingVH> &DeadInsts) {

  auto *LoopPreheader = L->getLoopPreheader();

  auto *LoopHeader = L->getHeader();

  for (auto &PN : LoopHeader->phis()) {

    auto *PreheaderIncoming = PN.getIncomingValueForBlock(LoopPreheader);

    PN.replaceAllUsesWith(PreheaderIncoming);

    DeadInsts.emplace_back(&PN);

  }

}



static void replaceWithInvariantCond(

    const Loop *L, BasicBlock *ExitingBB, ICmpInst::Predicate InvariantPred,

    const SCEV *InvariantLHS, const SCEV *InvariantRHS, SCEVExpander &Rewriter,
@@ -1499,10 +1510,11 @@ bool IndVarSimplify::optimizeLoopExits(Loop *L, SCEVExpander &Rewriter) { 
    // If we know we'd exit on the first iteration, rewrite the exit to

    // reflect this.  This does not imply the loop must exit through this

    // exit; there may be an earlier one taken on the first iteration.

    // TODO: Given we know the backedge can't be taken, we should go ahead

    // and break it.  Or at least, kill all the header phis and simplify.

    // We know that the backedge can't be taken, so we replace all

    // the header PHIs with values coming from the preheader.

    if (ExitCount->isZero()) {

      foldExit(L, ExitingBB, true, DeadInsts);

      replaceLoopPHINodesWithPreheaderValues(L, DeadInsts);

      Changed = true;

      continue;

    }

llvm/test/Transforms/IndVarSimplify/eliminate-backedge.ll

+4 −7
Original line number Diff line number Diff line
@@ -15,16 +15,13 @@ define i1 @kill_backedge_and_phis(i8* align 1 %lhs, i8* align 1 %rhs, i32 %len) 
; CHECK:       loop_preheader:

; CHECK-NEXT:    br label %loop

; CHECK:       loop:

; CHECK-NEXT:    %iv = phi i32 [ 0, %loop_preheader ], [ %iv.next, %exiting_3 ]

; CHECK-NEXT:    %iv.wide = phi i64 [ 0, %loop_preheader ], [ %iv.wide.next, %exiting_3 ]

; CHECK-NEXT:    %iv.next = add nuw nsw i32 %iv, 1

; CHECK-NEXT:    %iv.wide.next = add nuw nsw i64 %iv.wide, 1

; CHECK-NEXT:    %left_ptr = getelementptr inbounds i8, i8* %lhs, i32 %iv

; CHECK-NEXT:    %right_ptr = getelementptr inbounds i8, i8* %rhs, i32 %iv

; CHECK-NEXT:    %iv.next = add nuw nsw i32 0, 1

; CHECK-NEXT:    %left_ptr = getelementptr inbounds i8, i8* %lhs, i32 0

; CHECK-NEXT:    %right_ptr = getelementptr inbounds i8, i8* %rhs, i32 0

; CHECK-NEXT:    %result = call i1 @foo(i8* %left_ptr, i8* %right_ptr)

; CHECK-NEXT:    br i1 %result, label %exiting_1, label %exit.loopexit

; CHECK:       exiting_1:

; CHECK-NEXT:    %iv.wide.is_not_zero = icmp ne i64 %iv.wide, 0

; CHECK-NEXT:    %iv.wide.is_not_zero = icmp ne i64 0, 0

; CHECK-NEXT:    br i1 false, label %exiting_2, label %exit.loopexit

; CHECK:       exiting_2:

; CHECK-NEXT:    %bar_ret = call i1 @bar()

llvm/test/Transforms/IndVarSimplify/eliminate-exit-no-dl.ll

+2 −3
Original line number Diff line number Diff line
@@ -14,10 +14,9 @@ define void @foo() { 
; CHECK-NEXT:  bb:

; CHECK-NEXT:    br label [[BB3:%.*]]

; CHECK:       bb3:

; CHECK-NEXT:    [[TMP:%.*]] = phi i8* [ [[TMP4:%.*]], [[BB7:%.*]] ], [ getelementptr inbounds ([0 x i8], [0 x i8]* @global, i64 0, i64 2), [[BB:%.*]] ]

; CHECK-NEXT:    [[TMP4]] = getelementptr inbounds i8, i8* [[TMP]], i64 -1

; CHECK-NEXT:    [[TMP4:%.*]] = getelementptr inbounds i8, i8* getelementptr inbounds ([0 x i8], [0 x i8]* @global, i64 0, i64 2), i64 -1

; CHECK-NEXT:    [[TMP6:%.*]] = load i8, i8* [[TMP4]], align 1

; CHECK-NEXT:    br i1 false, label [[BB7]], label [[BB11:%.*]]

; CHECK-NEXT:    br i1 false, label [[BB7:%.*]], label [[BB11:%.*]]

; CHECK:       bb7:

; CHECK-NEXT:    [[TMP8:%.*]] = zext i8 [[TMP6]] to i64

; CHECK-NEXT:    br i1 true, label [[BB11]], label [[BB3]]

llvm/test/Transforms/IndVarSimplify/floating-point-iv.ll

+1 −3
Original line number Diff line number Diff line
@@ -65,9 +65,7 @@ define void @test3() nounwind { 
; CHECK-NEXT:  entry:

; CHECK-NEXT:    br label [[BB:%.*]]

; CHECK:       bb:

; CHECK-NEXT:    [[IV:%.*]] = phi double [ 0.000000e+00, [[ENTRY:%.*]] ], [ [[TMP1:%.*]], [[BB]] ]

; CHECK-NEXT:    [[TMP0:%.*]] = tail call i32 @foo(double [[IV]]) #[[ATTR0]]

; CHECK-NEXT:    [[TMP1]] = fadd double [[IV]], 1.000000e+00

; CHECK-NEXT:    [[TMP0:%.*]] = tail call i32 @foo(double 0.000000e+00) #[[ATTR0]]

; CHECK-NEXT:    br i1 false, label [[BB]], label [[RETURN:%.*]]

; CHECK:       return:

; CHECK-NEXT:    ret void