[PATCH] D42615: [X86] Generate BT instrutions a bit more agressively

[Craig Topper via Phabricator via llvm-commits]

craig.topper added a comment.

The h-register trick is coming from here in X86ISelDAGToDAG.cpp

  // For example, "testl %eax, $2048" to "testb %ah, $8".
  if (isShiftedUInt<8, 8>(Mask) &&
      (!(Mask & 0x8000) || hasNoSignedComparisonUses(Node))) {
    // Shift the immediate right by 8 bits.
    SDValue ShiftedImm = CurDAG->getTargetConstant(Mask >> 8, dl, MVT::i8);
    SDValue Reg = N0.getOperand(0);
  
    // Extract the h-register.
    SDValue Subreg = CurDAG->getTargetExtractSubreg(X86::sub_8bit_hi, dl,
                                                    MVT::i8, Reg);
  
    // Emit a testb.  The EXTRACT_SUBREG becomes a COPY that can only
    // target GR8_NOREX registers, so make sure the register class is
    // forced.
    SDNode *NewNode = CurDAG->getMachineNode(X86::TEST8ri_NOREX, dl,
                                             MVT::i32, Subreg, ShiftedImm);
    // Replace SUB|CMP with TEST, since SUB has two outputs while TEST has
    // one, do not call ReplaceAllUsesWith.
    ReplaceUses(SDValue(Node, (Opcode == X86ISD::SUB ? 1 : 0)),
                SDValue(NewNode, 0));
    CurDAG->RemoveDeadNode(Node);
    return;
  }


Repository:
  rL LLVM

https://reviews.llvm.org/D42615