Commit 9364e3bf authored by Hans Wennborg's avatar Hans Wennborg
Browse files

Merging r309758: 

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r309758 | sanjoy | 2017-08-01 15:37:58 -0700 (Tue, 01 Aug 2017) | 6 lines

[SCEV/IndVars] Always compute loop exiting values if the backedge count is 0

If SCEV can prove that the backedge taken count for a loop is zero, it does not
need to "understand" a recursive PHI to compute its exiting value.

This should fix PR33885.
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llvm-svn: 310538
parent 92f3b69a

llvm/lib/Analysis/ScalarEvolution.cpp

+19 −0
Original line number Diff line number Diff line
@@ -7582,6 +7582,25 @@ const SCEV *ScalarEvolution::computeSCEVAtScope(const SCEV *V, const Loop *L) { 
            const SCEV *BackedgeTakenCount = getBackedgeTakenCount(LI);

            if (const SCEVConstant *BTCC =

                  dyn_cast<SCEVConstant>(BackedgeTakenCount)) {



              // This trivial case can show up in some degenerate cases where

              // the incoming IR has not yet been fully simplified.

              if (BTCC->getValue()->isZero()) {

                Value *InitValue = nullptr;

                bool MultipleInitValues = false;

                for (unsigned i = 0; i < PN->getNumIncomingValues(); i++) {

                  if (!LI->contains(PN->getIncomingBlock(i))) {

                    if (!InitValue)

                      InitValue = PN->getIncomingValue(i);

                    else if (InitValue != PN->getIncomingValue(i)) {

                      MultipleInitValues = true;

                      break;

                    }

                  }

                  if (!MultipleInitValues && InitValue)

                    return getSCEV(InitValue);

                }

              }

              // Okay, we know how many times the containing loop executes.  If

              // this is a constant evolving PHI node, get the final value at

              // the specified iteration number.

llvm/test/Transforms/IndVarSimplify/exit_value_test2.ll

+25 −3
Original line number Diff line number Diff line
@@ -3,15 +3,14 @@ 


; Check IndVarSimplify should not replace exit value because or else

; udiv will be introduced by expand and the cost will be high.

;

; CHECK-LABEL: @_Z3fooPKcjj(

; CHECK-NOT: udiv



declare void @_Z3mixRjj(i32* dereferenceable(4), i32)

declare void @llvm.lifetime.start.p0i8(i64, i8* nocapture)

declare void @llvm.lifetime.end.p0i8(i64, i8* nocapture)



define i32 @_Z3fooPKcjj(i8* nocapture readonly %s, i32 %len, i32 %c) {

; CHECK-LABEL: @_Z3fooPKcjj(

; CHECK-NOT: udiv

entry:

  %a = alloca i32, align 4

  %tmp = bitcast i32* %a to i8*
@@ -50,3 +49,26 @@ while.end: ; preds = %while.cond.while.en 
  call void @llvm.lifetime.end.p0i8(i64 4, i8* %tmp)

  ret i32 %tmp4

}



define i32 @zero_backedge_count_test(i32 %unknown_init, i32* %unknown_mem) {

; CHECK-LABEL: @zero_backedge_count_test(

entry:

  br label %loop



loop:

  %iv = phi i32 [ 0, %entry], [ %iv.inc, %loop ]

  %unknown_phi = phi i32 [ %unknown_init, %entry ], [ %unknown_next, %loop ]

  %iv.inc = add i32 %iv, 1

  %be_taken = icmp ne i32 %iv.inc, 1

  %unknown_next = load volatile i32, i32* %unknown_mem

  br i1 %be_taken, label %loop, label %leave



leave:

; We can fold %unknown_phi even though the backedge value for it is completely

; unknown, since we can prove that the loop's backedge taken count is 0.



; CHECK: leave:

; CHECK: ret i32 %unknown_init

  %exit_val = phi i32 [ %unknown_phi, %loop ]

  ret i32 %exit_val

}