[ARM] Fix computeKnownBits for ARMISD::CMOV

Summary:
The true and false operands for the CMOV are operands 0 and 1.
ARMISelLowering.cpp::computeKnownBits was looking at operands 1 and 2
instead.  This can cause CMOV instructions to be incorrectly folded into
BFI if value set by the CMOV is another CMOV, whose known bits are
computed incorrectly.

This patch fixes the issue and adds a test case.

Reviewers: kristof.beyls, jmolloy

Subscribers: llvm-commits, aemerson, srhines, rengolin

Differential Revision: https://reviews.llvm.org/D31265

llvm-svn: 298624

llvm/lib/Target/ARM/ARMISelLowering.cpp

@@ -11695,8 +11695,8 @@ static void computeKnownBits(SelectionDAG &DAG, SDValue Op, APInt &KnownZero,
   if (Op.getOpcode() == ARMISD::CMOV) {
     APInt KZ2(KnownZero.getBitWidth(), 0);
     APInt KO2(KnownOne.getBitWidth(), 0);
-    computeKnownBits(DAG, Op.getOperand(1), KnownZero, KnownOne);
-    computeKnownBits(DAG, Op.getOperand(2), KZ2, KO2);
+    computeKnownBits(DAG, Op.getOperand(0), KnownZero, KnownOne);
+    computeKnownBits(DAG, Op.getOperand(1), KZ2, KO2);
 
     KnownZero &= KZ2;
     KnownOne &= KO2;

llvm/test/CodeGen/ARM/no-cmov2bfi.ll

@@ -0,0 +1,19 @@
+; RUN: llc < %s -mtriple=thumbv7 | FileCheck --check-prefix=CHECK-NOBFI %s
+
+declare zeroext i1 @dummy()
+
+define i8 @test(i8 %a1, i1 %c) {
+; CHECK-NOBFI-NOT: bfi
+; CHECK-NOBFI: bl      dummy
+; CHECK-NOBFI: cmp     r0, #0
+; CHECK-NOBFI: it      ne
+; CHECK-NOBFI: orrne   [[REG:r[0-9]+]], [[REG]], #8
+; CHECK-NOBFI: mov     r0, [[REG]]
+
+  %1 = and i8 %a1, -9
+  %2 = select i1 %c, i8 %1, i8 %a1
+  %3 = tail call zeroext i1 @dummy()
+  %4 = or i8 %2, 8
+  %ret = select i1 %3, i8 %4, i8 %2
+  ret i8 %ret
+}