Make ScalarEvolution::isKnownPredicate a little smarter

Here we make ScalarEvolution::isKnownPredicate, indirectly, a little smarter.
Given some relational comparison operator OP, and two AddRec SCEVs, {I,+,S} OP
{J,+,T}, we can reduce this to the comparison I OP J when S == T, both AddRecs
are for the same loop, and both are known not to wrap.

As it turns out, because of the way that backedge-guard expressions can be
leveraged when computing known predicates, this allows indvars to simplify the
if-statement comparison in this loop:

  void foo (int *a, int *b, int n) {
    for (int i = 0; i < n; ++i) {
      if (i > n)
        a[i] = b[i] + 1;
    }
  }

which, somewhat surprisingly, we were not previously optimizing away.

llvm-svn: 245400

1 files changed, 38 insertions, 1 deletions

diff --git a/llvm/lib/Analysis/ScalarEvolution.cpp b/llvm/lib/Analysis/ScalarEvolution.cpp
index 7cf08bd7156..363731a0b6c 100644
--- a/llvm/lib/Analysis/ScalarEvolution.cpp
+++ b/llvm/lib/Analysis/ScalarEvolution.cpp
@@ -7326,6 +7326,42 @@ static bool IsMinConsistingOf(ScalarEvolution &SE,
   return IsMaxConsistingOf<MaxExprType>(MaybeMaxExpr, SE.getNotSCEV(Candidate));
 }
 
+static bool IsKnownPredicateViaAddRecStart(ScalarEvolution &SE,
+                                           ICmpInst::Predicate Pred,
+                                           const SCEV *LHS, const SCEV *RHS) {
+
+  // If both sides are affine addrecs for the same loop, with equal
+  // steps, and we know the recurrences don't wrap, then we only
+  // need to check the predicate on the starting values.
+
+  if (!ICmpInst::isRelational(Pred))
+    return false;
+
+  const SCEVAddRecExpr *LAR = dyn_cast<SCEVAddRecExpr>(LHS);
+  if (!LAR)
+    return false;
+  const SCEVAddRecExpr *RAR = dyn_cast<SCEVAddRecExpr>(RHS);
+  if (!RAR)
+    return false;
+  if (LAR->getLoop() != RAR->getLoop())
+    return false;
+  if (!LAR->isAffine() || !RAR->isAffine())
+    return false;
+
+  if (LAR->getStepRecurrence(SE) != RAR->getStepRecurrence(SE))
+    return false;
+
+  auto CheckWrap = [Pred](const SCEVAddRecExpr *AR) -> bool {
+    if (ICmpInst::isSigned(Pred))
+      return AR->getNoWrapFlags(SCEV::FlagNSW);
+    return AR->getNoWrapFlags(SCEV::FlagNUW);
+  };
+
+  if (!CheckWrap(LAR) || !CheckWrap(RAR))
+    return false;
+
+  return SE.isKnownPredicate(Pred, LAR->getStart(), RAR->getStart());
+}
 
 /// Is LHS `Pred` RHS true on the virtue of LHS or RHS being a Min or Max
 /// expression?
@@ -7371,7 +7407,8 @@ ScalarEvolution::isImpliedCondOperandsHelper(ICmpInst::Predicate Pred,
   auto IsKnownPredicateFull =
       [this](ICmpInst::Predicate Pred, const SCEV *LHS, const SCEV *RHS) {
     return isKnownPredicateWithRanges(Pred, LHS, RHS) ||
-        IsKnownPredicateViaMinOrMax(*this, Pred, LHS, RHS);
+        IsKnownPredicateViaMinOrMax(*this, Pred, LHS, RHS) ||
+        IsKnownPredicateViaAddRecStart(*this, Pred, LHS, RHS);
   };
 
   switch (Pred) {