X86ISelLowering.cpp

      return Res;
    }

    // 'A' means EAX + EDX.
    if (Constraint == "A") {
      Res.first = X86::EAX;
      Res.second = X86::GR32_ADRegisterClass;
      return Res;
    }
    return Res;
  }

  // Otherwise, check to see if this is a register class of the wrong value
  // type.  For example, we want to map "{ax},i32" -> {eax}, we don't want it to
  // turn into {ax},{dx}.
  if (Res.second->hasType(VT))
    return Res;   // Correct type already, nothing to do.

  // All of the single-register GCC register classes map their values onto
  // 16-bit register pieces "ax","dx","cx","bx","si","di","bp","sp".  If we
  // really want an 8-bit or 32-bit register, map to the appropriate register
  // class and return the appropriate register.
  if (Res.second == X86::GR16RegisterClass) {
    if (VT == MVT::i8) {
      unsigned DestReg = 0;
      switch (Res.first) {
      default: break;
      case X86::AX: DestReg = X86::AL; break;
      case X86::DX: DestReg = X86::DL; break;
      case X86::CX: DestReg = X86::CL; break;
      case X86::BX: DestReg = X86::BL; break;
      }
      if (DestReg) {
        Res.first = DestReg;
        Res.second = X86::GR8RegisterClass;
      }
    } else if (VT == MVT::i32) {
      unsigned DestReg = 0;
      switch (Res.first) {
      default: break;
      case X86::AX: DestReg = X86::EAX; break;
      case X86::DX: DestReg = X86::EDX; break;
      case X86::CX: DestReg = X86::ECX; break;
      case X86::BX: DestReg = X86::EBX; break;
      case X86::SI: DestReg = X86::ESI; break;
      case X86::DI: DestReg = X86::EDI; break;
      case X86::BP: DestReg = X86::EBP; break;
      case X86::SP: DestReg = X86::ESP; break;
      }
      if (DestReg) {
        Res.first = DestReg;
        Res.second = X86::GR32RegisterClass;
      }
    } else if (VT == MVT::i64) {
      unsigned DestReg = 0;
      switch (Res.first) {
      default: break;
      case X86::AX: DestReg = X86::RAX; break;
      case X86::DX: DestReg = X86::RDX; break;
      case X86::CX: DestReg = X86::RCX; break;
      case X86::BX: DestReg = X86::RBX; break;
      case X86::SI: DestReg = X86::RSI; break;
      case X86::DI: DestReg = X86::RDI; break;
      case X86::BP: DestReg = X86::RBP; break;
      case X86::SP: DestReg = X86::RSP; break;
      }
      if (DestReg) {
        Res.first = DestReg;
        Res.second = X86::GR64RegisterClass;
      }
    }
  } else if (Res.second == X86::FR32RegisterClass ||
             Res.second == X86::FR64RegisterClass ||
             Res.second == X86::VR128RegisterClass) {
    // Handle references to XMM physical registers that got mapped into the
    // wrong class.  This can happen with constraints like {xmm0} where the
    // target independent register mapper will just pick the first match it can
    // find, ignoring the required type.
    if (VT == MVT::f32)
      Res.second = X86::FR32RegisterClass;
    else if (VT == MVT::f64)
      Res.second = X86::FR64RegisterClass;
    else if (X86::VR128RegisterClass->hasType(VT))
      Res.second = X86::VR128RegisterClass;
  }

  return Res;
}