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/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*-
* vim: set ts=8 sts=2 et sw=2 tw=80:
* This Source Code Form is subject to the terms of the Mozilla Public
* License, v. 2.0. If a copy of the MPL was not distributed with this
* file, You can obtain one at http://mozilla.org/MPL/2.0/. */
//===- RISCVMatInt.cpp - Immediate materialisation -------------*- C++
//-*--===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM
// Exceptions. See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//
#include "mozilla/MathAlgorithms.h"
#include "mozilla/Maybe.h"
#include "gc/Marking.h"
#include "jit/AutoWritableJitCode.h"
#include "jit/ExecutableAllocator.h"
#include "jit/riscv64/Assembler-riscv64.h"
#include "jit/riscv64/disasm/Disasm-riscv64.h"
#include "vm/Realm.h"
namespace js {
namespace jit {
void Assembler::RecursiveLi(Register rd, int64_t val) {
if (val > 0 && RecursiveLiImplCount(val) > 2) {
unsigned LeadingZeros = mozilla::CountLeadingZeroes64((uint64_t)val);
uint64_t ShiftedVal = (uint64_t)val << LeadingZeros;
int countFillZero = RecursiveLiImplCount(ShiftedVal) + 1;
if (countFillZero < RecursiveLiImplCount(val)) {
RecursiveLiImpl(rd, ShiftedVal);
srli(rd, rd, LeadingZeros);
return;
}
}
RecursiveLiImpl(rd, val);
}
int Assembler::RecursiveLiCount(int64_t val) {
if (val > 0 && RecursiveLiImplCount(val) > 2) {
unsigned LeadingZeros = mozilla::CountLeadingZeroes64((uint64_t)val);
uint64_t ShiftedVal = (uint64_t)val << LeadingZeros;
// Fill in the bits that will be shifted out with 1s. An example where
// this helps is trailing one masks with 32 or more ones. This will
// generate ADDI -1 and an SRLI.
int countFillZero = RecursiveLiImplCount(ShiftedVal) + 1;
if (countFillZero < RecursiveLiImplCount(val)) {
return countFillZero;
}
}
return RecursiveLiImplCount(val);
}
inline int64_t signExtend(uint64_t V, int N) {
return int64_t(V << (64 - N)) >> (64 - N);
}
void Assembler::RecursiveLiImpl(Register rd, int64_t Val) {
if (is_int32(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 = Val << 52 >> 52;
if (Hi20) {
lui(rd, (int32_t)Hi20);
}
if (Lo12 || Hi20 == 0) {
if (Hi20) {
addiw(rd, rd, Lo12);
} else {
addi(rd, zero_reg, Lo12);
}
}
return;
}
// 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
// emitted. 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 = Val << 52 >> 52;
int64_t Hi52 = ((uint64_t)Val + 0x800ull) >> 12;
int ShiftAmount = 12 + mozilla::CountTrailingZeroes64((uint64_t)Hi52);
Hi52 = signExtend(Hi52 >> (ShiftAmount - 12), 64 - ShiftAmount);
// If the remaining bits don't fit in 12 bits, we might be able to reduce
// the shift amount in order to use LUI which will zero the lower 12 bits.
bool Unsigned = false;
if (ShiftAmount > 12 && !is_int12(Hi52)) {
if (is_int32((uint64_t)Hi52 << 12)) {
// Reduce the shift amount and add zeros to the LSBs so it will match
// LUI.
ShiftAmount -= 12;
Hi52 = (uint64_t)Hi52 << 12;
}
}
RecursiveLi(rd, Hi52);
if (Unsigned) {
} else {
slli(rd, rd, ShiftAmount);
}
if (Lo12) {
addi(rd, rd, Lo12);
}
}
int Assembler::RecursiveLiImplCount(int64_t Val) {
int count = 0;
if (is_int32(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 = Val << 52 >> 52;
if (Hi20) {
// lui(rd, (int32_t)Hi20);
count++;
}
if (Lo12 || Hi20 == 0) {
// unsigned AddiOpc = (IsRV64 && Hi20) ? RISCV::ADDIW : RISCV::ADDI;
// Res.push_back(RISCVMatInt::Inst(AddiOpc, Lo12));
count++;
}
return count;
}
// 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
// emitted. 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 = Val << 52 >> 52;
int64_t Hi52 = ((uint64_t)Val + 0x800ull) >> 12;
int ShiftAmount = 12 + mozilla::CountTrailingZeroes64((uint64_t)Hi52);
Hi52 = signExtend(Hi52 >> (ShiftAmount - 12), 64 - ShiftAmount);
// If the remaining bits don't fit in 12 bits, we might be able to reduce
// the shift amount in order to use LUI which will zero the lower 12 bits.
bool Unsigned = false;
if (ShiftAmount > 12 && !is_int12(Hi52)) {
if (is_int32((uint64_t)Hi52 << 12)) {
// Reduce the shift amount and add zeros to the LSBs so it will match
// LUI.
ShiftAmount -= 12;
Hi52 = (uint64_t)Hi52 << 12;
}
}
count += RecursiveLiImplCount(Hi52);
if (Unsigned) {
} else {
// slli(rd, rd, ShiftAmount);
count++;
}
if (Lo12) {
// addi(rd, rd, Lo12);
count++;
}
return count;
}
} // namespace jit
} // namespace js
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