RISCVMatInt.cpp

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//===- RISCVMatInt.cpp - Immediate materialisation -------------*- C++ -*--===//
//
//                     The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//

#include "RISCVMatInt.h"
#include "MCTargetDesc/RISCVMCTargetDesc.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/Support/MachineValueType.h"
#include "llvm/Support/MathExtras.h"
#include <cstdint>

namespace llvm {

namespace RISCVMatInt {
void generateInstSeq(int64_t Val, bool Is64Bit, InstSeq &Res) {
  if (isInt<32>(Val)) {
    // Depending on the active bits in the immediate Value v, the following
    // instruction sequences are emitted:
    //
    // v == 0                        : ADDI
    // v[0,12) != 0 && v[12,32) == 0 : ADDI
    // v[0,12) == 0 && v[12,32) != 0 : LUI
    // v[0,32) != 0                  : LUI+ADDI(W)
    int64_t Hi20 = ((Val + 0x800) >> 12) & 0xFFFFF;
    int64_t Lo12 = SignExtend64<12>(Val);

    if (Hi20)
      Res.push_back(Inst(RISCV::LUI, Hi20));

    if (Lo12 || Hi20 == 0) {
      unsigned AddiOpc = (Is64Bit && Hi20) ? RISCV::ADDIW : RISCV::ADDI;
      Res.push_back(Inst(AddiOpc, Lo12));
    }
    return;
  }

  assert(Is64Bit && "Can't emit >32-bit imm for non-RV64 target");

  // In the worst case, for a full 64-bit constant, a sequence of 8 instructions
  // (i.e., LUI+ADDIW+SLLI+ADDI+SLLI+ADDI+SLLI+ADDI) has to be emmitted. Note
  // that the first two instructions (LUI+ADDIW) can contribute up to 32 bits
  // while the following ADDI instructions contribute up to 12 bits each.
  //
  // On the first glance, implementing this seems to be possible by simply
  // emitting the most significant 32 bits (LUI+ADDIW) followed by as many left
  // shift (SLLI) and immediate additions (ADDI) as needed. However, due to the
  // fact that ADDI performs a sign extended addition, doing it like that would
  // only be possible when at most 11 bits of the ADDI instructions are used.
  // Using all 12 bits of the ADDI instructions, like done by GAS, actually
  // requires that the constant is processed starting with the least significant
  // bit.
  //
  // In the following, constants are processed from LSB to MSB but instruction
  // emission is performed from MSB to LSB by recursively calling
  // generateInstSeq. In each recursion, first the lowest 12 bits are removed
  // from the constant and the optimal shift amount, which can be greater than
  // 12 bits if the constant is sparse, is determined. Then, the shifted
  // remaining constant is processed recursively and gets emitted as soon as it
  // fits into 32 bits. The emission of the shifts and additions is subsequently
  // performed when the recursion returns.

  int64_t Lo12 = SignExtend64<12>(Val);
  int64_t Hi52 = (Val + 0x800) >> 12;
  int ShiftAmount = 12 + findFirstSet((uint64_t)Hi52);
  Hi52 = SignExtend64(Hi52 >> (ShiftAmount - 12), 64 - ShiftAmount);

  generateInstSeq(Hi52, Is64Bit, Res);

  Res.push_back(Inst(RISCV::SLLI, ShiftAmount));
  if (Lo12)
    Res.push_back(Inst(RISCV::ADDI, Lo12));
}
} // namespace RISCVMatInt
} // namespace llvm