ScalarEvolution: Fix handling of AddRecs in isKnownPredicate

ScalarEvolution::isKnownPredicate() can wrongly reduce a comparison
when both the LHS and RHS are SCEVAddRecExprs. This checks that both
LHS and RHS are guarded in the case when both are SCEVAddRecExprs.

The test case is against indvars because I could not find a way to
directly test SCEV.

Patch by Sanjay Patel!

llvm-svn: 209487

Author: bogner

llvm/lib/Analysis/ScalarEvolution.cpp

@@ -6135,18 +6135,30 @@ bool ScalarEvolution::isKnownPredicate(ICmpInst::Predicate Pred,
 
   // If LHS or RHS is an addrec, check to see if the condition is true in
   // every iteration of the loop.
-  if (const SCEVAddRecExpr *AR = dyn_cast<SCEVAddRecExpr>(LHS))
-    if (isLoopEntryGuardedByCond(
-          AR->getLoop(), Pred, AR->getStart(), RHS) &&
-        isLoopBackedgeGuardedByCond(
-          AR->getLoop(), Pred, AR->getPostIncExpr(*this), RHS))
-      return true;
-  if (const SCEVAddRecExpr *AR = dyn_cast<SCEVAddRecExpr>(RHS))
-    if (isLoopEntryGuardedByCond(
-          AR->getLoop(), Pred, LHS, AR->getStart()) &&
-        isLoopBackedgeGuardedByCond(
-          AR->getLoop(), Pred, LHS, AR->getPostIncExpr(*this)))
-      return true;
+  // If LHS and RHS are both addrec, both conditions must be true in
+  // every iteration of the loop.
+  const SCEVAddRecExpr *LAR = dyn_cast<SCEVAddRecExpr>(LHS);
+  const SCEVAddRecExpr *RAR = dyn_cast<SCEVAddRecExpr>(RHS);
+  bool LeftGuarded = false;
+  bool RightGuarded = false;
+  if (LAR) {
+    const Loop *L = LAR->getLoop();
+    if (isLoopEntryGuardedByCond(L, Pred, LAR->getStart(), RHS) &&
+        isLoopBackedgeGuardedByCond(L, Pred, LAR->getPostIncExpr(*this), RHS)) {
+      if (!RAR) return true;
+      LeftGuarded = true;
+    }
+  }
+  if (RAR) {
+    const Loop *L = RAR->getLoop();
+    if (isLoopEntryGuardedByCond(L, Pred, LHS, RAR->getStart()) &&
+        isLoopBackedgeGuardedByCond(L, Pred, LHS, RAR->getPostIncExpr(*this))) {
+      if (!LAR) return true;
+      RightGuarded = true;
+    }
+  }
+  if (LeftGuarded && RightGuarded)
+    return true;
 
   // Otherwise see what can be done with known constant ranges.
   return isKnownPredicateWithRanges(Pred, LHS, RHS);

llvm/test/Transforms/IndVarSimplify/pr18223.ll

@@ -0,0 +1,30 @@
+; RUN: opt -indvars -S < %s | FileCheck %s
+
+; indvars should transform the phi node pair from the for-loop
+; CHECK-LABEL: @main(
+; CHECK: ret = phi i32 [ 0, %entry ], [ 0, {{.*}} ]
+
+@c = common global i32 0, align 4
+
+define i32 @main() #0 {
+entry:
+  %0 = load i32* @c, align 4
+  %tobool = icmp eq i32 %0, 0
+  br i1 %tobool, label %for.body, label %exit
+
+for.body:
+  %inc2 = phi i32 [ 0, %entry ], [ %inc, %for.inc ]
+  %sub = add i32 %inc2, -1
+  %cmp1 = icmp uge i32 %sub, %inc2
+  %conv = zext i1 %cmp1 to i32
+  br label %for.inc
+
+for.inc:
+  %inc = add nsw i32 %inc2, 1
+  %cmp = icmp slt i32 %inc, 5
+  br i1 %cmp, label %for.body, label %exit
+
+exit:
+  %ret = phi i32 [ 0, %entry ], [ %conv, %for.inc ]
+  ret i32 %ret
+}