[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

2 files changed, 104 insertions, 0 deletions

diff --git a/llvm/lib/Analysis/DependenceAnalysis.cpp b/llvm/lib/Analysis/DependenceAnalysis.cpp
index 9564cfb2a..9c09894 100644
--- a/llvm/lib/Analysis/DependenceAnalysis.cpp
+++ b/llvm/lib/Analysis/DependenceAnalysis.cpp
@@ -119,6 +119,11 @@ static cl::opt<bool> DisableDelinearizationChecks(
         "dependence vectors for languages that allow the subscript of one "
         "dimension to underflow or overflow into another dimension."));
 
+static cl::opt<unsigned> MIVMaxLevelThreshold(
+    "da-miv-max-level-threshold", cl::init(7), cl::Hidden, cl::ZeroOrMore,
+    cl::desc("Maximum depth allowed for the recursive algorithm used to "
+             "explore MIV direction vectors."));
+
 //===----------------------------------------------------------------------===//
 // basics
 
@@ -2602,6 +2607,19 @@ unsigned DependenceInfo::exploreDirections(unsigned Level, CoefficientInfo *A,
                                            const SmallBitVector &Loops,
                                            unsigned &DepthExpanded,
                                            const SCEV *Delta) const {
+  // This algorithm has worst case complexity of O(3^n), where 'n' is the number
+  // of common loop levels. To avoid excessive compile-time, pessimize all the
+  // results and immediately return when the number of common levels is beyond
+  // the given threshold.
+  if (CommonLevels > MIVMaxLevelThreshold) {
+    LLVM_DEBUG(dbgs() << "Number of common levels exceeded the threshold. MIV "
+                         "direction exploration is terminated.\n");
+    for (unsigned K = 1; K <= CommonLevels; ++K)
+      if (Loops[K])
+        Bound[K].DirSet = Dependence::DVEntry::ALL;
+    return 1;
+  }
+
   if (Level > CommonLevels) {
     // record result
     LLVM_DEBUG(dbgs() << "\t[");
diff --git a/llvm/test/Analysis/DependenceAnalysis/MIVMaxLevelThreshold.ll b/llvm/test/Analysis/DependenceAnalysis/MIVMaxLevelThreshold.ll
new file mode 100644
index 0000000..80d5dbd
--- /dev/null
+++ b/llvm/test/Analysis/DependenceAnalysis/MIVMaxLevelThreshold.ll
@@ -0,0 +1,86 @@
+; RUN: opt < %s -aa-pipeline=default -passes='print<da>' -da-miv-max-level-threshold=2
+
+;; Check to make sure when MIV tests reach a maximum depth level
+;; threshold, the resulting dependence is conservatively correct.
+
+;; for (int i = 0; i < n; i++)
+;;   for (int j = 0; j < m; j++)
+;;     for (int k = 0; k < o; k++)
+;;       A[i+j+k] = A[i-j+k] + B[i+j+k];
+
+
+; CHECK:       Src:  %2 = load float, float* %arrayidx, align 4 --> Dst:  %5 = load float, float* %arrayidx12, align 4
+; CHECK-NEXT:  da analyze - none!
+; CHECK:       Src:  %2 = load float, float* %arrayidx, align 4 --> Dst:  store float %add13, float* %arrayidx17, align 4
+; CHECK-NEXT:  da analyze - anti [* * *|<]!
+; CHECK:       Src:  %5 = load float, float* %arrayidx12, align 4 --> Dst:  store float %add13, float* %arrayidx17, align 4
+; CHECK-NEXT:  da analyze - none!
+
+define void @foo(float* noalias %A, float* noalias %B, i32 signext %m, i32 signext %n, i32 signext %o) {
+entry:
+  %cmp5 = icmp sgt i32 %n, 0
+  br i1 %cmp5, label %for.body.preheader, label %for.end23
+
+for.body.preheader:                               ; preds = %entry
+  %wide.trip.count21 = zext i32 %n to i64
+  br label %for.body
+
+for.body:                                         ; preds = %for.body.preheader, %for.inc21
+  %indvars.iv19 = phi i64 [ 0, %for.body.preheader ], [ %indvars.iv.next20, %for.inc21 ]
+  %cmp23 = icmp sgt i32 %m, 0
+  br i1 %cmp23, label %for.body4.preheader, label %for.inc21
+
+for.body4.preheader:                              ; preds = %for.body
+  %wide.trip.count17 = zext i32 %m to i64
+  br label %for.body4
+
+for.body4:                                        ; preds = %for.body4.preheader, %for.inc18
+  %indvars.iv12 = phi i64 [ 0, %for.body4.preheader ], [ %indvars.iv.next13, %for.inc18 ]
+  %cmp61 = icmp sgt i32 %o, 0
+  br i1 %cmp61, label %for.body8.preheader, label %for.inc18
+
+for.body8.preheader:                              ; preds = %for.body4
+  %wide.trip.count = zext i32 %o to i64
+  br label %for.body8
+
+for.body8:                                        ; preds = %for.body8.preheader, %for.body8
+  %indvars.iv = phi i64 [ 0, %for.body8.preheader ], [ %indvars.iv.next, %for.body8 ]
+  %0 = sub nsw i64 %indvars.iv19, %indvars.iv12
+  %1 = add nsw i64 %0, %indvars.iv
+  %arrayidx = getelementptr inbounds float, float* %A, i64 %1
+  %2 = load float, float* %arrayidx, align 4
+  %3 = add nuw nsw i64 %indvars.iv19, %indvars.iv12
+  %4 = add nuw nsw i64 %3, %indvars.iv
+  %arrayidx12 = getelementptr inbounds float, float* %B, i64 %4
+  %5 = load float, float* %arrayidx12, align 4
+  %add13 = fadd fast float %2, %5
+  %6 = add nuw nsw i64 %indvars.iv19, %indvars.iv12
+  %7 = add nuw nsw i64 %6, %indvars.iv
+  %arrayidx17 = getelementptr inbounds float, float* %A, i64 %7
+  store float %add13, float* %arrayidx17, align 4
+  %indvars.iv.next = add nuw nsw i64 %indvars.iv, 1
+  %exitcond = icmp ne i64 %indvars.iv.next, %wide.trip.count
+  br i1 %exitcond, label %for.body8, label %for.inc18.loopexit
+
+for.inc18.loopexit:                               ; preds = %for.body8
+  br label %for.inc18
+
+for.inc18:                                        ; preds = %for.inc18.loopexit, %for.body4
+  %indvars.iv.next13 = add nuw nsw i64 %indvars.iv12, 1
+  %exitcond18 = icmp ne i64 %indvars.iv.next13, %wide.trip.count17
+  br i1 %exitcond18, label %for.body4, label %for.inc21.loopexit
+
+for.inc21.loopexit:                               ; preds = %for.inc18
+  br label %for.inc21
+
+for.inc21:                                        ; preds = %for.inc21.loopexit, %for.body
+  %indvars.iv.next20 = add nuw nsw i64 %indvars.iv19, 1
+  %exitcond22 = icmp ne i64 %indvars.iv.next20, %wide.trip.count21
+  br i1 %exitcond22, label %for.body, label %for.end23.loopexit
+
+for.end23.loopexit:                               ; preds = %for.inc21
+  br label %for.end23
+
+for.end23:                                        ; preds = %for.end23.loopexit, %entry
+  ret void
+}