Fix an overly general check in SimplifyIndvar to handle useless phi cycles.

The right way to check for a binary operation is
cast<BinaryOperator>. The original check: cast<Instruction> &&
numOperands() == 2 would match phi "instructions", leading to an
infinite loop in extreme corner case: a useless phi with operands
[self, constant] that prior optimization passes failed to remove,
being used in the loop by another useless phi, in turn being used by an
lshr or udiv.

Fixes PR11350: runaway iteration assertion.

llvm-svn: 144935

llvm/lib/Transforms/Utils/SimplifyIndVar.cpp

@@ -107,8 +107,8 @@ Value *SimplifyIndvar::foldIVUser(Instruction *UseInst, Instruction *IVOperand)
 
     // Attempt to fold a binary operator with constant operand.
     // e.g. ((I + 1) >> 2) => I >> 2
-    if (IVOperand->getNumOperands() != 2 ||
+    if (!isa<BinaryOperator>(IVOperand)
-        !isa<ConstantInt>(IVOperand->getOperand(1)))
+        || !isa<ConstantInt>(IVOperand->getOperand(1)))
       return 0;
 
     IVSrc = IVOperand->getOperand(0);

llvm/test/Transforms/IndVarSimplify/2011-11-17-selfphi.ll

@@ -0,0 +1,29 @@
+; RUN: opt < %s -indvars -S | FileCheck %s
+; PR11350: Check that SimplifyIndvar handles a cycle of useless self-phis.
+
+; CHECK: @test
+; CHECK-NOT: lcssa = phi
+define void @test() nounwind {
+entry:
+  br label %for.cond.preheader
+
+for.cond.preheader:                               ; preds = %entry
+  br label %for.cond.outer
+
+for.cond.outer:                                   ; preds = %for.cond.preheader, %for.end
+  %p_41.addr.0.ph = phi i32 [ %p_41.addr.1.lcssa, %for.end ], [ 1, %for.cond.preheader ]
+  br label %for.cond
+
+for.cond:
+  br i1 true, label %for.end, label %for.ph
+
+for.ph:                                   ; preds = %for.cond4.preheader
+  br label %for.end
+
+for.end:
+  %p_41.addr.1.lcssa = phi i32 [ undef, %for.ph ], [ %p_41.addr.0.ph, %for.cond ]
+  %p_68.lobit.i = lshr i32 %p_41.addr.1.lcssa, 31
+  %cmp7 = icmp eq i32 %p_41.addr.1.lcssa, 0
+  %conv8 = zext i1 %cmp7 to i32
+  br label %for.cond.outer
+}