Reapply "[ARM] Combine CMOV into BFI where possible"

[oota-llvm.git] / lib / Target / ARM / ARMISelLowering.cpp

diff --git a/lib/Target/ARM/ARMISelLowering.cpp b/lib/Target/ARM/ARMISelLowering.cpp
index f6a382b44fcbdf2e116bf2921dbd9df62e3e7393..b83d3f78a3428c0374cb8f0ab0632ba4a5ad8619 100644 (file)
--- a/lib/Target/ARM/ARMISelLowering.cpp
+++ b/lib/Target/ARM/ARMISelLowering.cpp
@@ -10241,6 +10241,111 @@ static SDValue PerformExtendCombine(SDNode *N, SelectionDAG &DAG,
   return SDValue();
 }
 
+static void computeKnownBits(SelectionDAG &DAG, SDValue Op, APInt &KnownZero,
+                             APInt &KnownOne) {
+  if (Op.getOpcode() == ARMISD::BFI) {
+    // Conservatively, we can recurse down the first operand
+    // and just mask out all affected bits.
+    computeKnownBits(DAG, Op.getOperand(0), KnownZero, KnownOne);
+
+    // The operand to BFI is already a mask suitable for removing the bits it
+    // sets.
+    ConstantSDNode *CI = cast<ConstantSDNode>(Op.getOperand(2));
+    APInt Mask = CI->getAPIntValue();
+    KnownZero &= Mask;
+    KnownOne &= Mask;
+    return;
+  }
+  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);
+
+    KnownZero &= KZ2;
+    KnownOne &= KO2;
+    return;
+  }
+  return DAG.computeKnownBits(Op, KnownZero, KnownOne);
+}
+
+SDValue ARMTargetLowering::PerformCMOVToBFICombine(SDNode *CMOV, SelectionDAG &DAG) const {
+  // If we have a CMOV, OR and AND combination such as:
+  //   if (x & CN)
+  //     y |= CM;
+  //
+  // And:
+  //   * CN is a single bit;
+  //   * All bits covered by CM are known zero in y
+  //
+  // Then we can convert this into a sequence of BFI instructions. This will
+  // always be a win if CM is a single bit, will always be no worse than the
+  // TST&OR sequence if CM is two bits, and for thumb will be no worse if CM is
+  // three bits (due to the extra IT instruction).
+
+  SDValue Op0 = CMOV->getOperand(0);
+  SDValue Op1 = CMOV->getOperand(1);
+  SDValue CmpZ = CMOV->getOperand(4);
+
+  assert(CmpZ->getOpcode() == ARMISD::CMPZ);
+  SDValue And = CmpZ->getOperand(0);
+  if (And->getOpcode() != ISD::AND)
+    return SDValue();
+  ConstantSDNode *AndC = dyn_cast<ConstantSDNode>(And->getOperand(1));
+  if (!AndC || !AndC->getAPIntValue().isPowerOf2())
+    return SDValue();
+  SDValue X = And->getOperand(0);
+
+  if (Op1->getOpcode() != ISD::OR)
+    return SDValue();
+
+  ConstantSDNode *OrC = dyn_cast<ConstantSDNode>(Op1->getOperand(1));
+  if (!OrC)
+    return SDValue();
+  SDValue Y = Op1->getOperand(0);
+
+  if (Op0 != Y)
+    return SDValue();
+
+  // Now, is it profitable to continue?
+  APInt OrCI = OrC->getAPIntValue();
+  unsigned Heuristic = Subtarget->isThumb() ? 3 : 2;
+  if (OrCI.countPopulation() > Heuristic)
+    return SDValue();
+
+  // Lastly, can we determine that the bits defined by OrCI
+  // are zero in Y?
+  APInt KnownZero, KnownOne;
+  computeKnownBits(DAG, Y, KnownZero, KnownOne);
+  if ((OrCI & KnownZero) != OrCI)
+    return SDValue();
+
+  // OK, we can do the combine.
+  SDValue V = Y;
+  SDLoc dl(X);
+  EVT VT = X.getValueType();
+  unsigned BitInX = AndC->getAPIntValue().logBase2();
+  
+  if (BitInX != 0) {
+    // We must shift X first.
+    X = DAG.getNode(ISD::SRL, dl, VT, X,
+                    DAG.getConstant(BitInX, dl, VT));
+  }
+
+  for (unsigned BitInY = 0, NumActiveBits = OrCI.getActiveBits();
+       BitInY < NumActiveBits; ++BitInY) {
+    if (OrCI[BitInY] == 0)
+      continue;
+    APInt Mask(VT.getSizeInBits(), 0);
+    Mask.setBit(BitInY);
+    V = DAG.getNode(ARMISD::BFI, dl, VT, V, X,
+                    // Confusingly, the operand is an *inverted* mask.
+                    DAG.getConstant(~Mask, dl, VT));
+  }
+
+  return V;
+}
+
 /// PerformCMOVCombine - Target-specific DAG combining for ARMISD::CMOV.
 SDValue
 ARMTargetLowering::PerformCMOVCombine(SDNode *N, SelectionDAG &DAG) const {
@@ -10259,6 +10364,13 @@ ARMTargetLowering::PerformCMOVCombine(SDNode *N, SelectionDAG &DAG) const {
   ARMCC::CondCodes CC =
     (ARMCC::CondCodes)cast<ConstantSDNode>(ARMcc)->getZExtValue();
 
+  // BFI is only available on V6T2+.
+  if (!Subtarget->isThumb1Only() && Subtarget->hasV6T2Ops()) {
+    SDValue R = PerformCMOVToBFICombine(N, DAG);
+    if (R)
+      return R;
+  }
+
   // Simplify
   //   mov     r1, r0
   //   cmp     r1, x