[SCEV] Fix wrong Equal predicate created in getAddRecForPhiWithCasts

CreateAddRecFromPHIWithCastsImpl() adds an IncrementNUSW overflow predicate
which allows the PSCEV rewriter to rewrite this scev expression:
 (zext i8 {0, + , (trunc i32 step to i8)} to i32)
into
 {0, +, (sext i8 (trunc i32 step to i8) to i32)}

But then it adds the wrong Equal predicate:
 %step == (zext i8 (trunc i32 %step to i8) to i32).
instead of:
 %step == (sext i8 (trunc i32 %step to i8) to i32)

This is fixed here.

Differential Revision: https://reviews.llvm.org/D40641

llvm-svn: 320298

1 files changed, 9 insertions, 6 deletions

diff --git a/llvm/lib/Analysis/ScalarEvolution.cpp b/llvm/lib/Analysis/ScalarEvolution.cpp
index 3ed7dde47bb..960bd64830c 100644
--- a/llvm/lib/Analysis/ScalarEvolution.cpp
+++ b/llvm/lib/Analysis/ScalarEvolution.cpp
@@ -4636,18 +4636,19 @@ ScalarEvolution::createAddRecFromPHIWithCastsImpl(const SCEVUnknown *SymbolicPHI
 
   // Construct the extended SCEV: (Ext ix (Trunc iy (Expr) to ix) to iy)
   // for each of StartVal and Accum
-  auto GetExtendedExpr = [&](const SCEV *Expr) -> const SCEV * {
+  auto getExtendedExpr = [&](const SCEV *Expr, 
+                             bool CreateSignExtend) -> const SCEV * {
     assert(isLoopInvariant(Expr, L) && "Expr is expected to be invariant");
     const SCEV *TruncatedExpr = getTruncateExpr(Expr, TruncTy);
     const SCEV *ExtendedExpr =
-        Signed ? getSignExtendExpr(TruncatedExpr, Expr->getType())
-               : getZeroExtendExpr(TruncatedExpr, Expr->getType());
+        CreateSignExtend ? getSignExtendExpr(TruncatedExpr, Expr->getType())
+                         : getZeroExtendExpr(TruncatedExpr, Expr->getType());
     return ExtendedExpr;
   };
 
   // Given:
   //  ExtendedExpr = (Ext ix (Trunc iy (Expr) to ix) to iy
-  //               = GetExtendedExpr(Expr)
+  //               = getExtendedExpr(Expr)
   // Determine whether the predicate P: Expr == ExtendedExpr
   // is known to be false at compile time
   auto PredIsKnownFalse = [&](const SCEV *Expr,
@@ -4656,13 +4657,15 @@ ScalarEvolution::createAddRecFromPHIWithCastsImpl(const SCEVUnknown *SymbolicPHI
            isKnownPredicate(ICmpInst::ICMP_NE, Expr, ExtendedExpr);
   };
 
-  const SCEV *StartExtended = GetExtendedExpr(StartVal);
+  const SCEV *StartExtended = getExtendedExpr(StartVal, Signed);
   if (PredIsKnownFalse(StartVal, StartExtended)) {
     DEBUG(dbgs() << "P2 is compile-time false\n";);
     return None;
   }
 
-  const SCEV *AccumExtended = GetExtendedExpr(Accum);
+  // The Step is always Signed (because the overflow checks are either
+  // NSSW or NUSW)
+  const SCEV *AccumExtended = getExtendedExpr(Accum, /*CreateSignExtend=*/true);
   if (PredIsKnownFalse(Accum, AccumExtended)) {
     DEBUG(dbgs() << "P3 is compile-time false\n";);
     return None;