[Thumb] Teach ISel how to lower compares of AND bitmasks efficiently

For the common pattern (CMPZ (AND x, #bitmask), #0), we can do some more efficient instruction selection if the bitmask is one consecutive sequence of set bits (32 - clz(bm) - ctz(bm) == popcount(bm)).

1) If the bitmask touches the LSB, then we can remove all the upper bits and set the flags by doing one LSLS.
2) If the bitmask touches the MSB, then we can remove all the lower bits and set the flags with one LSRS.
3) If the bitmask has popcount == 1 (only one set bit), we can shift that bit into the sign bit with one LSLS and change the condition query from NE/EQ to MI/PL (we could also implement this by shifting into the carry bit and branching on BCC/BCS).
4) Otherwise, we can emit a sequence of LSLS+LSRS to remove the upper and lower zero bits of the mask.

1-3 require only one 16-bit instruction and can elide the CMP. 4 requires two 16-bit instructions but can elide the CMP and doesn't require materializing a complex immediate, so is also a win.

llvm-svn: 281323

1 files changed, 133 insertions, 4 deletions

diff --git a/llvm/lib/Target/ARM/ARMISelDAGToDAG.cpp b/llvm/lib/Target/ARM/ARMISelDAGToDAG.cpp
index 30586aa2539..4d8df27f21f 100644
--- a/llvm/lib/Target/ARM/ARMISelDAGToDAG.cpp
+++ b/llvm/lib/Target/ARM/ARMISelDAGToDAG.cpp
@@ -244,7 +244,8 @@ private:
   bool tryInlineAsm(SDNode *N);
 
   void SelectConcatVector(SDNode *N);
-
+  void SelectCMPZ(SDNode *N, bool &SwitchEQNEToPLMI);
+  
   bool trySMLAWSMULW(SDNode *N);
 
   void SelectCMP_SWAP(SDNode *N);
@@ -2693,6 +2694,83 @@ void ARMDAGToDAGISel::SelectConcatVector(SDNode *N) {
   ReplaceNode(N, createDRegPairNode(VT, N->getOperand(0), N->getOperand(1)));
 }
 
+static Optional<std::pair<unsigned, unsigned>>
+getContiguousRangeOfSetBits(const APInt &A) {
+  unsigned FirstOne = A.getBitWidth() - A.countLeadingZeros() - 1;
+  unsigned LastOne = A.countTrailingZeros();
+  if (A.countPopulation() != (FirstOne - LastOne + 1))
+    return Optional<std::pair<unsigned,unsigned>>();
+  return std::make_pair(FirstOne, LastOne);
+}
+
+void ARMDAGToDAGISel::SelectCMPZ(SDNode *N, bool &SwitchEQNEToPLMI) {
+  assert(N->getOpcode() == ARMISD::CMPZ);
+  SwitchEQNEToPLMI = false;
+  
+  if (!Subtarget->isThumb())
+    // FIXME: Work out whether it is profitable to do this in A32 mode - LSL and
+    // LSR don't exist as standalone instructions - they need the barrel shifter.
+    return;
+  // select (cmpz (and X, C), #0) -> (LSLS X) or (LSRS X) or (LSRS (LSLS X))
+  SDValue And = N->getOperand(0);
+  SDValue Zero = N->getOperand(1);
+  if (!isa<ConstantSDNode>(Zero) || !cast<ConstantSDNode>(Zero)->isNullValue() ||
+      And->getOpcode() != ISD::AND)
+    return;
+  SDValue X = And.getOperand(0);
+  auto C = dyn_cast<ConstantSDNode>(And.getOperand(1));
+
+  if (!C || !X->hasOneUse())
+    return;
+  auto Range = getContiguousRangeOfSetBits(C->getAPIntValue());
+  if (!Range)
+    return;
+
+  // There are several ways to lower this:
+  SDNode *NewN;
+  SDLoc dl(N);
+
+  auto EmitShift = [&](unsigned Opc, SDValue Src, unsigned Imm) -> SDNode* {
+    if (Subtarget->isThumb2()) {
+      Opc = (Opc == ARM::tLSLri) ? ARM::t2LSLri : ARM::t2LSRri;
+      SDValue Ops[] = { Src, CurDAG->getTargetConstant(Imm, dl, MVT::i32),
+                        getAL(CurDAG, dl), CurDAG->getRegister(0, MVT::i32),
+                        CurDAG->getRegister(0, MVT::i32) };
+      return CurDAG->getMachineNode(Opc, dl, MVT::i32, Ops);
+    } else {
+      SDValue Ops[] = {CurDAG->getRegister(ARM::CPSR, MVT::i32), Src,
+                       CurDAG->getTargetConstant(Imm, dl, MVT::i32),
+                       getAL(CurDAG, dl), CurDAG->getRegister(0, MVT::i32)};
+      return CurDAG->getMachineNode(Opc, dl, MVT::i32, Ops);
+    }
+  };
+  
+  if (Range->second == 0) {
+    //  1. Mask includes the LSB -> Simply shift the top N bits off
+    NewN = EmitShift(ARM::tLSLri, X, 31 - Range->first);
+    ReplaceNode(And.getNode(), NewN);
+  } else if (Range->first == 31) {
+    //  2. Mask includes the MSB -> Simply shift the bottom N bits off
+    NewN = EmitShift(ARM::tLSRri, X, Range->second);
+    ReplaceNode(And.getNode(), NewN);
+  } else if (Range->first == Range->second) {
+    //  3. Only one bit is set. We can shift this into the sign bit and use a
+    //     PL/MI comparison.
+    NewN = EmitShift(ARM::tLSLri, X, 31 - Range->first);
+    ReplaceNode(And.getNode(), NewN);
+
+    SwitchEQNEToPLMI = true;
+  } else if (!Subtarget->hasV6T2Ops()) {
+    //  4. Do a double shift to clear bottom and top bits, but only in
+    //     thumb-1 mode as in thumb-2 we can use UBFX.
+    NewN = EmitShift(ARM::tLSLri, X, 31 - Range->first);
+    NewN = EmitShift(ARM::tLSRri, SDValue(NewN, 0),
+                     Range->second + (31 - Range->first));
+    ReplaceNode(And.getNode(), NewN);
+  }
+
+}
+
 void ARMDAGToDAGISel::Select(SDNode *N) {
   SDLoc dl(N);
 
@@ -2920,6 +2998,7 @@ void ARMDAGToDAGISel::Select(SDNode *N) {
         return;
       }
     }
+
     break;
   }
   case ARMISD::VMOVRRD:
@@ -3110,9 +3189,27 @@ void ARMDAGToDAGISel::Select(SDNode *N) {
     assert(N2.getOpcode() == ISD::Constant);
     assert(N3.getOpcode() == ISD::Register);
 
-    SDValue Tmp2 = CurDAG->getTargetConstant(((unsigned)
-                               cast<ConstantSDNode>(N2)->getZExtValue()), dl,
-                               MVT::i32);
+    unsigned CC = (unsigned) cast<ConstantSDNode>(N2)->getZExtValue();
+    
+    if (InFlag.getOpcode() == ARMISD::CMPZ) {
+      bool SwitchEQNEToPLMI;
+      SelectCMPZ(InFlag.getNode(), SwitchEQNEToPLMI);
+      InFlag = N->getOperand(4);
+
+      if (SwitchEQNEToPLMI) {
+        switch ((ARMCC::CondCodes)CC) {
+        default: llvm_unreachable("CMPZ must be either NE or EQ!");
+        case ARMCC::NE:
+          CC = (unsigned)ARMCC::MI;
+          break;
+        case ARMCC::EQ:
+          CC = (unsigned)ARMCC::PL;
+          break;
+        }
+      }
+    }
+
+    SDValue Tmp2 = CurDAG->getTargetConstant(CC, dl, MVT::i32);
     SDValue Ops[] = { N1, Tmp2, N3, Chain, InFlag };
     SDNode *ResNode = CurDAG->getMachineNode(Opc, dl, MVT::Other,
                                              MVT::Glue, Ops);
@@ -3167,6 +3264,38 @@ void ARMDAGToDAGISel::Select(SDNode *N) {
     // Other cases are autogenerated.
     break;
   }
+
+  case ARMISD::CMOV: {
+    SDValue InFlag = N->getOperand(4);
+
+    if (InFlag.getOpcode() == ARMISD::CMPZ) {
+      bool SwitchEQNEToPLMI;
+      SelectCMPZ(InFlag.getNode(), SwitchEQNEToPLMI);
+
+      if (SwitchEQNEToPLMI) {
+        SDValue ARMcc = N->getOperand(2);
+        ARMCC::CondCodes CC =
+          (ARMCC::CondCodes)cast<ConstantSDNode>(ARMcc)->getZExtValue();
+
+        switch (CC) {
+        default: llvm_unreachable("CMPZ must be either NE or EQ!");
+        case ARMCC::NE:
+          CC = ARMCC::MI;
+          break;
+        case ARMCC::EQ:
+          CC = ARMCC::PL;
+          break;
+        }
+        SDValue NewARMcc = CurDAG->getConstant((unsigned)CC, dl, MVT::i32);
+        SDValue Ops[] = {N->getOperand(0), N->getOperand(1), NewARMcc,
+                         N->getOperand(3), N->getOperand(4)};
+        CurDAG->MorphNodeTo(N, ARMISD::CMOV, N->getVTList(), Ops);
+      }
+
+    }
+    // Other cases are autogenerated.
+    break;
+  }
     
   case ARMISD::VZIP: {
     unsigned Opc = 0;