Merge 64579 from mainline.

Fix pr3571: If stride is a value defined by an instruction, make sure it dominates the loop preheader. When IV users are strength reduced, the stride is inserted into the preheader. It could create a use before def situation.

llvm-svn: 64789

Author: tlattner

llvm/lib/Transforms/Scalar/LoopStrengthReduce.cpp

@@ -400,7 +400,7 @@ static bool containsAddRecFromDifferentLoop(SCEVHandle S, Loop *L) {
 /// outer loop of the current loop.
 static bool getSCEVStartAndStride(const SCEVHandle &SH, Loop *L,
                                   SCEVHandle &Start, SCEVHandle &Stride,
-                                  ScalarEvolution *SE) {
+                                  ScalarEvolution *SE, DominatorTree *DT) {
   SCEVHandle TheAddRec = Start;   // Initialize to zero.
 
   // If the outer level is an AddExpr, the operands are all start values except
@@ -437,9 +437,21 @@ static bool getSCEVStartAndStride(const SCEVHandle &SH, Loop *L,
 
   Start = SE->getAddExpr(Start, AddRec->getOperand(0));
 
-  if (!isa<SCEVConstant>(AddRec->getOperand(1)))
+  if (!isa<SCEVConstant>(AddRec->getOperand(1))) {
+    // If stride is an instruction, make sure it dominates the loop header.
+    // Otherwise we could end up with a use before def situation.
+    if (SCEVUnknown *SU = dyn_cast<SCEVUnknown>(AddRec->getOperand(1))) {
+      if (Instruction *I = dyn_cast<Instruction>(SU->getValue())) {
+        BasicBlock *StrideBB = I->getParent();
+        BasicBlock *Preheader = L->getLoopPreheader();
+        if (!DT->dominates(StrideBB, Preheader))
+          return false;
+      }
+    }
+
     DOUT << "[" << L->getHeader()->getName()
          << "] Variable stride: " << *AddRec << "\n";
+  }
 
   Stride = AddRec->getOperand(1);
   return true;
@@ -547,7 +559,7 @@ bool LoopStrengthReduce::AddUsersIfInteresting(Instruction *I, Loop *L,
   // Get the start and stride for this expression.
   SCEVHandle Start = SE->getIntegerSCEV(0, ISE->getType());
   SCEVHandle Stride = Start;
-  if (!getSCEVStartAndStride(ISE, L, Start, Stride, SE))
+  if (!getSCEVStartAndStride(ISE, L, Start, Stride, SE, DT))
     return false;  // Non-reducible symbolic expression, bail out.
 
   std::vector<Instruction *> IUsers;

llvm/test/Transforms/LoopStrengthReduce/pr3571.ll

@@ -0,0 +1,27 @@
+; RUN: llvm-as < %s | opt -loop-reduce | llvm-dis
+; PR3571
+
+target triple = "i386-mingw32"
+define void @_ZNK18qdesigner_internal10TreeWidget12drawBranchesEP8QPainterRK5QRectRK11QModelIndex() nounwind {
+entry:
+	br label %_ZNK11QModelIndex7isValidEv.exit.i
+
+bb.i:		; preds = %_ZNK11QModelIndex7isValidEv.exit.i
+	%indvar.next = add i32 %result.0.i, 1		; <i32> [#uses=1]
+	br label %_ZNK11QModelIndex7isValidEv.exit.i
+
+_ZNK11QModelIndex7isValidEv.exit.i:		; preds = %bb.i, %entry
+	%result.0.i = phi i32 [ 0, %entry ], [ %indvar.next, %bb.i ]		; <i32> [#uses=2]
+	%0 = load i32** null, align 4		; <%struct.QAbstractItemDelegate*> [#uses=0]
+	br i1 false, label %_ZN18qdesigner_internalL5levelEP18QAbstractItemModelRK11QModelIndex.exit, label %bb.i
+
+_ZN18qdesigner_internalL5levelEP18QAbstractItemModelRK11QModelIndex.exit:		; preds = %_ZNK11QModelIndex7isValidEv.exit.i
+	%1 = call i32 @_ZNK9QTreeView11indentationEv(i32* null) nounwind		; <i32> [#uses=1]
+	%2 = mul i32 %1, %result.0.i		; <i32> [#uses=1]
+	%3 = add i32 %2, -2		; <i32> [#uses=1]
+	%4 = add i32 %3, 0		; <i32> [#uses=1]
+	store i32 %4, i32* null, align 8
+	unreachable
+}
+
+declare i32 @_ZNK9QTreeView11indentationEv(i32*)