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/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*-
* vim: set ts=8 sts=4 et sw=4 tw=99:
* 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/. */
#include "jit/AtomicOperations.h"
#if defined(__arm__)
# include "jit/arm/Architecture-arm.h"
#endif
#ifdef JS_HAVE_GENERATED_ATOMIC_OPS
# include <atomic>
# include "js/GCAPI.h"
using namespace js;
using namespace js::jit;
// A "block" is a sequence of bytes that is a reasonable quantum to copy to
// amortize call overhead when implementing memcpy and memmove. A block will
// not fit in registers on all platforms and copying it without using
// intermediate memory will therefore be sensitive to overlap.
//
// A "word" is an item that we can copy using only register intermediate storage
// on all platforms; words can be individually copied without worrying about
// overlap.
//
// Blocks and words can be aligned or unaligned; specific (generated) copying
// functions handle this in platform-specific ways.
static constexpr size_t WORDSIZE = sizeof(uintptr_t);
static constexpr size_t BLOCKSIZE = 8 * WORDSIZE; // Must be a power of 2
static_assert(BLOCKSIZE % WORDSIZE == 0,
"A block is an integral number of words");
// Constants must match the ones in GenerateAtomicOperations.py
static_assert(JS_GENERATED_ATOMICS_BLOCKSIZE == BLOCKSIZE);
static_assert(JS_GENERATED_ATOMICS_WORDSIZE == WORDSIZE);
static constexpr size_t WORDMASK = WORDSIZE - 1;
static constexpr size_t BLOCKMASK = BLOCKSIZE - 1;
namespace js {
namespace jit {
static bool UnalignedAccessesAreOK() {
# ifdef DEBUG
const char* flag = getenv("JS_NO_UNALIGNED_MEMCPY");
if (flag && *flag == '1') return false;
# endif
# if defined(__x86_64__) || defined(__i386__)
return true;
# elif defined(__arm__)
return !HasAlignmentFault();
# elif defined(__aarch64__)
// This is not necessarily true but it's the best guess right now.
return true;
# else
# error "Unsupported platform"
# endif
}
# ifndef JS_64BIT
void AtomicCompilerFence() {
std::atomic_signal_fence(std::memory_order_acq_rel);
}
# endif
void AtomicMemcpyDownUnsynchronized(uint8_t* dest, const uint8_t* src,
size_t nbytes) {
JS::AutoSuppressGCAnalysis nogc;
const uint8_t* lim = src + nbytes;
// Set up bulk copying. The cases are ordered the way they are on the
// assumption that if we can achieve aligned copies even with a little
// preprocessing then that is better than unaligned copying on a platform
// that supports it.
if (nbytes >= WORDSIZE) {
void (*copyBlock)(uint8_t* dest, const uint8_t* src);
void (*copyWord)(uint8_t* dest, const uint8_t* src);
if (((uintptr_t(dest) ^ uintptr_t(src)) & WORDMASK) == 0) {
const uint8_t* cutoff = (const uint8_t*)RoundUp(uintptr_t(src), WORDSIZE);
MOZ_ASSERT(cutoff <= lim); // because nbytes >= WORDSIZE
while (src < cutoff) {
AtomicCopyByteUnsynchronized(dest++, src++);
}
copyBlock = AtomicCopyBlockDownUnsynchronized;
copyWord = AtomicCopyWordUnsynchronized;
} else if (UnalignedAccessesAreOK()) {
copyBlock = AtomicCopyBlockDownUnsynchronized;
copyWord = AtomicCopyWordUnsynchronized;
} else {
copyBlock = AtomicCopyUnalignedBlockDownUnsynchronized;
copyWord = AtomicCopyUnalignedWordDownUnsynchronized;
}
// Bulk copy, first larger blocks and then individual words.
const uint8_t* blocklim = src + ((lim - src) & ~BLOCKMASK);
while (src < blocklim) {
copyBlock(dest, src);
dest += BLOCKSIZE;
src += BLOCKSIZE;
}
const uint8_t* wordlim = src + ((lim - src) & ~WORDMASK);
while (src < wordlim) {
copyWord(dest, src);
dest += WORDSIZE;
src += WORDSIZE;
}
}
// Byte copy any remaining tail.
while (src < lim) {
AtomicCopyByteUnsynchronized(dest++, src++);
}
}
void AtomicMemcpyUpUnsynchronized(uint8_t* dest, const uint8_t* src,
size_t nbytes) {
JS::AutoSuppressGCAnalysis nogc;
const uint8_t* lim = src;
src += nbytes;
dest += nbytes;
if (nbytes >= WORDSIZE) {
void (*copyBlock)(uint8_t* dest, const uint8_t* src);
void (*copyWord)(uint8_t* dest, const uint8_t* src);
if (((uintptr_t(dest) ^ uintptr_t(src)) & WORDMASK) == 0) {
const uint8_t* cutoff = (const uint8_t*)(uintptr_t(src) & ~WORDMASK);
MOZ_ASSERT(cutoff >= lim); // Because nbytes >= WORDSIZE
while (src > cutoff) {
AtomicCopyByteUnsynchronized(--dest, --src);
}
copyBlock = AtomicCopyBlockUpUnsynchronized;
copyWord = AtomicCopyWordUnsynchronized;
} else if (UnalignedAccessesAreOK()) {
copyBlock = AtomicCopyBlockUpUnsynchronized;
copyWord = AtomicCopyWordUnsynchronized;
} else {
copyBlock = AtomicCopyUnalignedBlockUpUnsynchronized;
copyWord = AtomicCopyUnalignedWordUpUnsynchronized;
}
const uint8_t* blocklim = src - ((src - lim) & ~BLOCKMASK);
while (src > blocklim) {
dest -= BLOCKSIZE;
src -= BLOCKSIZE;
copyBlock(dest, src);
}
const uint8_t* wordlim = src - ((src - lim) & ~WORDMASK);
while (src > wordlim) {
dest -= WORDSIZE;
src -= WORDSIZE;
copyWord(dest, src);
}
}
while (src > lim) {
AtomicCopyByteUnsynchronized(--dest, --src);
}
}
} // namespace jit
} // namespace js
#endif // JS_HAVE_GENERATED_ATOMIC_OPS
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