Fix PR3874 by restoring a condition I removed, but making it more

precise than it used to be.

llvm-svn: 67662

Author: lattner

llvm/lib/Transforms/Scalar/InstructionCombining.cpp

@@ -6582,7 +6582,8 @@ Instruction *InstCombiner::visitICmpInstWithInstAndIntCst(ICmpInst &ICI,
       // preferable because it allows the C<<Y expression to be hoisted out
       // of a loop if Y is invariant and X is not.
       if (Shift && Shift->hasOneUse() && RHSV == 0 &&
-          ICI.isEquality() && !Shift->isArithmeticShift()) {
+          ICI.isEquality() && !Shift->isArithmeticShift() &&
+          !isa<Constant>(Shift->getOperand(0))) {
         // Compute C << Y.
         Value *NS;
         if (Shift->getOpcode() == Instruction::LShr) {

llvm/test/Transforms/InstCombine/2009-03-24-InfLoop.ll

@@ -0,0 +1,9 @@
+; PR3874
+; RUN: llvm-as < %s | opt -instcombine | llvm-dis
+  define i1 @test(i32 %x) {
+    %A = lshr i32 3968, %x
+    %B = and i32 %A, 1
+    %C = icmp eq i32 %B, 0
+    ret i1 %C
+  }
+