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[DA] control compile-time spent by MIV tests
Function exploreDirections() in DependenceAnalysis implements a recursive algorithm for refining direction vectors. This algorithm has worst-case complexity of O(3^(n+1)) where n is the number of common loop levels. In this patch I'm adding a threshold to control the amount of time we spend in doing MIV tests (which most of the time end up resulting in over pessimistic direction vectors anyway). Reviewed By: Meinersbur Differential Revision: https://reviews.llvm.org/D107159
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‎llvm/lib/Analysis/DependenceAnalysis.cpp

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@@ -119,6 +119,11 @@ static cl::opt<bool> DisableDelinearizationChecks(
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"dependence vectors for languages that allow the subscript of one "
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"dimension to underflow or overflow into another dimension."));
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static cl::opt<unsigned> MIVMaxLevelThreshold(
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"da-miv-max-level-threshold", cl::init(7), cl::Hidden, cl::ZeroOrMore,
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cl::desc("Maximum depth allowed for the recursive algorithm used to "
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"explore MIV direction vectors."));
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//===----------------------------------------------------------------------===//
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// basics
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@@ -2602,6 +2607,19 @@ unsigned DependenceInfo::exploreDirections(unsigned Level, CoefficientInfo *A,
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const SmallBitVector &Loops,
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unsigned &DepthExpanded,
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const SCEV *Delta) const {
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// This algorithm has worst case complexity of O(3^n), where 'n' is the number
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// of common loop levels. To avoid excessive compile-time, pessimize all the
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// results and immediately return when the number of common levels is beyond
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// the given threshold.
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if (CommonLevels > MIVMaxLevelThreshold) {
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LLVM_DEBUG(dbgs() << "Number of common levels exceeded the threshold. MIV "
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"direction exploration is terminated.\n");
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for (unsigned K = 1; K <= CommonLevels; ++K)
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if (Loops[K])
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Bound[K].DirSet = Dependence::DVEntry::ALL;
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return 1;
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}
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if (Level > CommonLevels) {
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// record result
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LLVM_DEBUG(dbgs() << "\t[");

‎llvm/test/Analysis/DependenceAnalysis/MIVMaxLevelThreshold.ll

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; RUN: opt < %s -aa-pipeline=default -passes='print<da>' -da-miv-max-level-threshold=2
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;; Check to make sure when MIV tests reach a maximum depth level
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;; threshold, the resulting dependence is conservatively correct.
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;; for (int i = 0; i < n; i++)
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;; for (int j = 0; j < m; j++)
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;; for (int k = 0; k < o; k++)
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;; A[i+j+k] = A[i-j+k] + B[i+j+k];
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; CHECK: Src: %2 = load float, float* %arrayidx, align 4 --> Dst: %5 = load float, float* %arrayidx12, align 4
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; CHECK-NEXT: da analyze - none!
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; CHECK: Src: %2 = load float, float* %arrayidx, align 4 --> Dst: store float %add13, float* %arrayidx17, align 4
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; CHECK-NEXT: da analyze - anti [* * *|<]!
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; CHECK: Src: %5 = load float, float* %arrayidx12, align 4 --> Dst: store float %add13, float* %arrayidx17, align 4
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; CHECK-NEXT: da analyze - none!
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define void @foo(float* noalias %A, float* noalias %B, i32 signext %m, i32 signext %n, i32 signext %o) {
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entry:
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%cmp5 = icmp sgt i32 %n, 0
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br i1 %cmp5, label %for.body.preheader, label %for.end23
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for.body.preheader: ; preds = %entry
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%wide.trip.count21 = zext i32 %n to i64
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br label %for.body
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for.body: ; preds = %for.body.preheader, %for.inc21
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%indvars.iv19 = phi i64 [ 0, %for.body.preheader ], [ %indvars.iv.next20, %for.inc21 ]
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%cmp23 = icmp sgt i32 %m, 0
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br i1 %cmp23, label %for.body4.preheader, label %for.inc21
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for.body4.preheader: ; preds = %for.body
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%wide.trip.count17 = zext i32 %m to i64
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br label %for.body4
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for.body4: ; preds = %for.body4.preheader, %for.inc18
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%indvars.iv12 = phi i64 [ 0, %for.body4.preheader ], [ %indvars.iv.next13, %for.inc18 ]
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%cmp61 = icmp sgt i32 %o, 0
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br i1 %cmp61, label %for.body8.preheader, label %for.inc18
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for.body8.preheader: ; preds = %for.body4
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%wide.trip.count = zext i32 %o to i64
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br label %for.body8
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for.body8: ; preds = %for.body8.preheader, %for.body8
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%indvars.iv = phi i64 [ 0, %for.body8.preheader ], [ %indvars.iv.next, %for.body8 ]
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%0 = sub nsw i64 %indvars.iv19, %indvars.iv12
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%1 = add nsw i64 %0, %indvars.iv
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%arrayidx = getelementptr inbounds float, float* %A, i64 %1
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%2 = load float, float* %arrayidx, align 4
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%3 = add nuw nsw i64 %indvars.iv19, %indvars.iv12
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%4 = add nuw nsw i64 %3, %indvars.iv
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%arrayidx12 = getelementptr inbounds float, float* %B, i64 %4
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%5 = load float, float* %arrayidx12, align 4
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%add13 = fadd fast float %2, %5
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%6 = add nuw nsw i64 %indvars.iv19, %indvars.iv12
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%7 = add nuw nsw i64 %6, %indvars.iv
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%arrayidx17 = getelementptr inbounds float, float* %A, i64 %7
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store float %add13, float* %arrayidx17, align 4
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%indvars.iv.next = add nuw nsw i64 %indvars.iv, 1
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%exitcond = icmp ne i64 %indvars.iv.next, %wide.trip.count
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br i1 %exitcond, label %for.body8, label %for.inc18.loopexit
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for.inc18.loopexit: ; preds = %for.body8
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br label %for.inc18
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for.inc18: ; preds = %for.inc18.loopexit, %for.body4
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%indvars.iv.next13 = add nuw nsw i64 %indvars.iv12, 1
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%exitcond18 = icmp ne i64 %indvars.iv.next13, %wide.trip.count17
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br i1 %exitcond18, label %for.body4, label %for.inc21.loopexit
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for.inc21.loopexit: ; preds = %for.inc18
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br label %for.inc21
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for.inc21: ; preds = %for.inc21.loopexit, %for.body
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%indvars.iv.next20 = add nuw nsw i64 %indvars.iv19, 1
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%exitcond22 = icmp ne i64 %indvars.iv.next20, %wide.trip.count21
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br i1 %exitcond22, label %for.body, label %for.end23.loopexit
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for.end23.loopexit: ; preds = %for.inc21
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br label %for.end23
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for.end23: ; preds = %for.end23.loopexit, %entry
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ret void
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}

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