Teach ScalarEvolution to sharpen range information.

If x is known to have the range [a, b) in a loop predicated by (icmp
ne x, a), its range can be sharpened to [a + 1, b).  Get
ScalarEvolution and hence IndVars to exploit this fact.
    
This change triggers an optimization to widen-loop-comp.ll, so it had
to be edited to get it to pass.

phabricator: http://reviews.llvm.org/D5639



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@219834 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Analysis/ScalarEvolution.cpp

@@ -6782,6 +6782,44 @@ bool ScalarEvolution::isImpliedCond(ICmpInst::Predicate Pred,
                                    RHS, LHS, FoundLHS, FoundRHS);
   }
 
+  if (FoundPred == ICmpInst::ICMP_NE) {
+    // If we are predicated on something with range [a, b) known not
+    // equal to a, the range can be sharpened to [a + 1, b).  Use this
+    // fact.
+    auto CheckRange = [this, Pred, LHS, RHS](const SCEV *C, const SCEV *V) {
+      if (!isa<SCEVConstant>(C)) return false;
+
+      ConstantInt *CI = cast<SCEVConstant>(C)->getValue();
+      APInt Min = ICmpInst::isSigned(Pred) ?
+        getSignedRange(V).getSignedMin() : getUnsignedRange(V).getUnsignedMin();
+
+      if (Min!= CI->getValue()) return false; // nothing to sharpen
+      Min++;
+
+      // We know V >= Min, in the same signedness as in Pred.  We can
+      // use this to simplify slt and ult.  If the range had a single
+      // value, Min, we now know that FoundValue can never be true;
+      // and any answer is a correct answer.
+
+      switch (Pred) {
+        case ICmpInst::ICMP_SGT:
+        case ICmpInst::ICMP_UGT:
+          return isImpliedCondOperands(Pred, LHS, RHS, V, getConstant(Min));
+
+        default:
+          llvm_unreachable("don't call with predicates other than ICMP_SGT "
+                           "and ICMP_UGT");
+      }
+    };
+
+    if ((Pred == ICmpInst::ICMP_SGT) || (Pred == ICmpInst::ICMP_UGT)) {
+      // Inequality is reflexive -- check both the combinations.
+      if (CheckRange(FoundLHS, FoundRHS) || CheckRange(FoundRHS, FoundLHS)) {
+        return true;
+      }
+    }
+  }
+
   // Check whether the actual condition is beyond sufficient.
   if (FoundPred == ICmpInst::ICMP_EQ)
     if (ICmpInst::isTrueWhenEqual(Pred))