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Fix ScalarEvolution's backedge-taken count computations to check for
overflow when computing a integer division to round up. Thanks to Nick Lewycky for noticing this! git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@73862 91177308-0d34-0410-b5e6-96231b3b80d8
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@ -332,6 +332,13 @@ namespace llvm {
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const SCEVHandle &SymName,
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const SCEVHandle &NewVal);
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/// getBECount - Subtract the end and start values and divide by the step,
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/// rounding up, to get the number of times the backedge is executed. Return
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/// CouldNotCompute if an intermediate computation overflows.
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SCEVHandle getBECount(const SCEVHandle &Start,
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const SCEVHandle &End,
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const SCEVHandle &Step);
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/// getBackedgeTakenInfo - Return the BackedgeTakenInfo for the given
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/// loop, lazily computing new values if the loop hasn't been analyzed
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/// yet.
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@ -3788,6 +3788,33 @@ bool ScalarEvolution::isLoopGuardedByCond(const Loop *L,
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return false;
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}
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/// getBECount - Subtract the end and start values and divide by the step,
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/// rounding up, to get the number of times the backedge is executed. Return
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/// CouldNotCompute if an intermediate computation overflows.
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SCEVHandle ScalarEvolution::getBECount(const SCEVHandle &Start,
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const SCEVHandle &End,
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const SCEVHandle &Step) {
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const Type *Ty = Start->getType();
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SCEVHandle NegOne = getIntegerSCEV(-1, Ty);
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SCEVHandle Diff = getMinusSCEV(End, Start);
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SCEVHandle RoundUp = getAddExpr(Step, NegOne);
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// Add an adjustment to the difference between End and Start so that
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// the division will effectively round up.
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SCEVHandle Add = getAddExpr(Diff, RoundUp);
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// Check Add for unsigned overflow.
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// TODO: More sophisticated things could be done here.
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const Type *WideTy = IntegerType::get(getTypeSizeInBits(Ty) + 1);
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SCEVHandle OperandExtendedAdd =
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getAddExpr(getZeroExtendExpr(Diff, WideTy),
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getZeroExtendExpr(RoundUp, WideTy));
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if (getZeroExtendExpr(Add, WideTy) != OperandExtendedAdd)
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return CouldNotCompute;
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return getUDivExpr(Add, Step);
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}
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/// HowManyLessThans - Return the number of times a backedge containing the
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/// specified less-than comparison will execute. If not computable, return
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/// CouldNotCompute.
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@ -3869,16 +3896,11 @@ HowManyLessThans(const SCEV *LHS, const SCEV *RHS,
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// Finally, we subtract these two values and divide, rounding up, to get
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// the number of times the backedge is executed.
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SCEVHandle BECount = getUDivExpr(getAddExpr(getMinusSCEV(End, Start),
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getAddExpr(Step, NegOne)),
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Step);
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SCEVHandle BECount = getBECount(Start, End, Step);
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// The maximum backedge count is similar, except using the minimum start
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// value and the maximum end value.
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SCEVHandle MaxBECount = getUDivExpr(getAddExpr(getMinusSCEV(MaxEnd,
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MinStart),
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getAddExpr(Step, NegOne)),
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Step);
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SCEVHandle MaxBECount = getBECount(MinStart, MaxEnd, Step);;
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return BackedgeTakenInfo(BECount, MaxBECount);
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}
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@ -1,4 +1,9 @@
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; RUN: llvm-as < %s | opt -analyze -scalar-evolution -disable-output |& grep {/u 3}
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; RUN: llvm-as < %s | opt -analyze -scalar-evolution -disable-output \
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; RUN: | grep {Loop bb: Unpredictable backedge-taken count\\.}
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; ScalarEvolution can't compute a trip count because it doesn't know if
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; dividing by the stride will have a remainder. This could theoretically
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; be teaching it how to use a more elaborate trip count computation.
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define i32 @f(i32 %x) nounwind readnone {
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entry:
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@ -1,5 +1,9 @@
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; RUN: llvm-as < %s | opt -scalar-evolution -analyze -disable-output \
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; RUN: | grep {backedge-taken count is ((64 + (-64 smax (-1 + (-1 \\* %0))) + %0) /u 64)}
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; RUN: | grep {Loop bb3\\.i: Unpredictable backedge-taken count\\.}
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; ScalarEvolution can't compute a trip count because it doesn't know if
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; dividing by the stride will have a remainder. This could theoretically
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; be teaching it how to use a more elaborate trip count computation.
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target datalayout = "e-p:64:64:64-i1:8:8-i8:8:8-i16:16:16-i32:32:32-i64:64:64-f32:32:32-f64:64:64-v64:64:64-v128:128:128-a0:0:64-s0:64:64-f80:128:128"
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target triple = "x86_64-unknown-linux-gnu"
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@ -1,7 +1,9 @@
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; RUN: llvm-as < %s | opt -indvars -loop-deletion | llvm-dis | grep phi | count 1
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; XFAIL: *
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; Indvars should be able to evaluate this loop, allowing loop deletion
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; to delete it.
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; Indvars can't evaluate this loop, because ScalarEvolution can't compute
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; an exact trip count, because it doesn't know if dividing by the stride will
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; have a remainder. It could be done with more aggressive VRP though.
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define i32 @test(i32 %x_offs) nounwind readnone {
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entry:
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