[PATCH] D22377: [SCEV] trip count calculation for loops with unknown stride

[Pankaj Chawla via llvm-commits]

pankajchawla created this revision.
pankajchawla added a reviewer: sanjoy.
pankajchawla added a subscriber: llvm-commits.
Herald added subscribers: mzolotukhin, sanjoy.

Hello,

The patch teaches ScalarEvolution to compute trip count for loops such as these-

  for(int i=0; i<n; i+=s) {
    A[i] = i;
  }

The basic idea is that if the IV has a nsw/nuw flag and the loop has an entry guard then we can assume the stride to be positive.



https://reviews.llvm.org/D22377

Files:
  llvm/lib/Analysis/ScalarEvolution.cpp
  llvm/test/Analysis/ScalarEvolution/trip-count-assumed-positive-stride.ll

Index: llvm/lib/Analysis/ScalarEvolution.cpp
===================================================================
--- llvm/lib/Analysis/ScalarEvolution.cpp
+++ llvm/lib/Analysis/ScalarEvolution.cpp
@@ -8659,8 +8659,12 @@
 
   const SCEV *Stride = IV->getStepRecurrence(*this);
 
+  bool PositiveStride = isKnownPositive(Stride);
+
   // Avoid negative or zero stride values
-  if (!isKnownPositive(Stride))
+  // If NoWrap is true, we can assume stride to be positive if loop has an entry
+  // guard.
+  if (!NoWrap && !PositiveStride)
     return getCouldNotCompute();
 
   // Avoid proven overflow cases: this will ensure that the backedge taken count
@@ -8674,8 +8678,12 @@
                                       : ICmpInst::ICMP_ULT;
   const SCEV *Start = IV->getStart();
   const SCEV *End = RHS;
-  if (!isLoopEntryGuardedByCond(L, Cond, getMinusSCEV(Start, Stride), RHS))
+  if (!isLoopEntryGuardedByCond(L, Cond, getMinusSCEV(Start, Stride), RHS)) {
+    if (!PositiveStride) 
+      return getCouldNotCompute();
+    
     End = IsSigned ? getSMaxExpr(RHS, Start) : getUMaxExpr(RHS, Start);
+  }
 
   const SCEV *BECount = computeBECount(getMinusSCEV(End, Start), Stride, false);
 
Index: llvm/test/Analysis/ScalarEvolution/trip-count-assumed-positive-stride.ll
===================================================================
--- llvm/test/Analysis/ScalarEvolution/trip-count-assumed-positive-stride.ll
+++ llvm/test/Analysis/ScalarEvolution/trip-count-assumed-positive-stride.ll
@@ -0,0 +1,55 @@
+; RUN: opt < %s -analyze -scalar-evolution | FileCheck %s
+
+; ScalarEvolution should be able to compute trip count by assuming stride to be
+; positive by checking the nsw flag on the IV and the presence of a loop entry
+; guard.
+
+; CHECK: Determining loop execution counts for: @foo1
+; CHECK: backedge-taken count is ((-1 + %n) /u %s)
+
+target datalayout = "e-m:e-p:32:32-f64:32:64-f80:32-n8:16:32-S128"
+
+; Function Attrs: norecurse nounwind
+define void @foo1(i32* nocapture %A, i32 %n, i32 %s) #0 {
+entry:
+  %cmp4 = icmp sgt i32 %n, 0
+  br i1 %cmp4, label %for.body, label %for.end
+
+for.body:                                         ; preds = %entry, %for.body
+  %i.05 = phi i32 [ %add, %for.body ], [ 0, %entry ]
+  %arrayidx = getelementptr inbounds i32, i32* %A, i32 %i.05
+  %0 = load i32, i32* %arrayidx, align 4
+  %inc = add nsw i32 %0, 1
+  store i32 %inc, i32* %arrayidx, align 4
+  %add = add nsw i32 %i.05, %s
+  %cmp = icmp slt i32 %add, %n
+  br i1 %cmp, label %for.body, label %for.end
+
+for.end:                                          ; preds = %for.body, %entry
+  ret void
+}
+
+
+; Loop should have unpredictable trip count without the loop entry guard.
+; CHECK: Determining loop execution counts for: @foo2
+; CHECK: Unpredictable backedge-taken count
+
+; Function Attrs: norecurse nounwind
+define void @foo2(i32* nocapture %A, i32 %n, i32 %s) #0 {
+entry:
+  br label %for.body
+
+for.body:                                         ; preds = %entry, %for.body
+  %i.05 = phi i32 [ %add, %for.body ], [ 0, %entry ]
+  %arrayidx = getelementptr inbounds i32, i32* %A, i32 %i.05
+  %0 = load i32, i32* %arrayidx, align 4
+  %inc = add nsw i32 %0, 1
+  store i32 %inc, i32* %arrayidx, align 4
+  %add = add nsw i32 %i.05, %s
+  %cmp = icmp slt i32 %add, %n
+  br i1 %cmp, label %for.body, label %for.end
+
+for.end:                                          ; preds = %for.body, %entry
+  ret void
+}
+


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