Properly check if a CMPZ node is in fact comparing against zero

This was left implicit and never ever checked, which means we could have a CMPZ against some non-zero value and we were carrying on with BFI conversion regardless.

Caught by Oliver Stannard using csmith; regression test added.

llvm-svn: 253195

llvm/lib/Target/ARM/ARMISelLowering.cpp

@@ -10398,6 +10398,12 @@ SDValue ARMTargetLowering::PerformCMOVToBFICombine(SDNode *CMOV, SelectionDAG &D
   auto CC = CCNode->getAPIntValue().getLimitedValue();
   SDValue CmpZ = CMOV->getOperand(4);
 
+  // The compare must be against zero.
+  SDValue Zero = CmpZ->getOperand(1);
+  if (!isa<ConstantSDNode>(Zero.getNode()) ||
+      !cast<ConstantSDNode>(Zero.getNode())->isNullValue())
+    return SDValue();
+
   assert(CmpZ->getOpcode() == ARMISD::CMPZ);
   SDValue And = CmpZ->getOperand(0);
   if (And->getOpcode() != ISD::AND)

llvm/test/CodeGen/ARM/bfi.ll

@@ -158,3 +158,14 @@ define i32 @f12(i32 %x, i32 %y) {
   %sel = select i1 %cmp, i32 %y2, i32 %or
   ret i32 %sel
 }
+
+define i32 @f13(i32 %x, i32 %y) {
+; CHECK-LABEL: f13:
+; CHECK-NOT: bfi
+  %y2 = and i32 %y, 4294967040 ; 0xFFFFFF00
+  %and = and i32 %x, 4
+  %or = or i32 %y2, 16
+  %cmp = icmp eq i32 %and, 42 ; Not comparing against zero!
+  %sel = select i1 %cmp, i32 %y2, i32 %or
+  ret i32 %sel
+}