Source/JavaScriptCore/heap/GCMemoryOperations.h - WebKit - Git at Google

 /*
  * Copyright (C) 2016-2018 Apple Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
  *    documentation and/or other materials provided with the distribution.
  *
  * THIS SOFTWARE IS PROVIDED BY APPLE INC. ``AS IS'' AND ANY
  * EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
  * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
  * PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL APPLE INC. OR
  * CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,
  * EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
  * PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR
  * PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY
  * OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
  * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
  * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  */

 #pragma once

 #include "CPU.h"
 #include "JSCJSValue.h"

 namespace JSC {

 // We use these memory operations when modifying memory that might be scanned by the concurrent collector.
 // We don't call the default operations because they're not guaranteed to store to memory in eight byte aligned
 // chunks. If we happened to fall into the system's normal byte copy loop, we may see a torn JSValue in the
 // concurrent collector.

 constexpr size_t smallCutoff = 30 * 8;
 constexpr size_t mediumCutoff = 4 * 1024;

 // This is a forwards loop so gcSafeMemmove can rely on the direction.
 template <typename T>
 ALWAYS_INLINE void gcSafeMemcpy(T* dst, T* src, size_t bytes)
 {
     static_assert(sizeof(T) == sizeof(JSValue));
     RELEASE_ASSERT(bytes % 8 == 0);

 #if USE(JSVALUE64)

     auto slowPathForwardMemcpy = [&] {
         size_t count = bytes / 8;
         for (unsigned i = 0; i < count; ++i)
             bitwise_cast<volatile uint64_t*>(dst)[i] = bitwise_cast<volatile uint64_t*>(src)[i];
     };

 #if COMPILER(GCC_COMPATIBLE) && USE(JSVALUE64)
     if (bytes <= smallCutoff)
         slowPathForwardMemcpy();
     else if (isARM64() || bytes <= mediumCutoff) {
 #if CPU(X86_64)
         size_t alignedBytes = (bytes / 64) * 64;
         size_t tmp;
         size_t offset = 0;
         asm volatile(
             ".balign 32\t\n"
             "1:\t\n"
             "cmpq %q[offset], %q[alignedBytes]\t\n"
             "je 2f\t\n"
             "movups (%q[src], %q[offset], 1), %%xmm0\t\n"
             "movups 16(%q[src], %q[offset], 1), %%xmm1\t\n"
             "movups 32(%q[src], %q[offset], 1), %%xmm2\t\n"
             "movups 48(%q[src], %q[offset], 1), %%xmm3\t\n"
             "movups %%xmm0, (%q[dst], %q[offset], 1)\t\n"
             "movups %%xmm1, 16(%q[dst], %q[offset], 1)\t\n"
             "movups %%xmm2, 32(%q[dst], %q[offset], 1)\t\n"
             "movups %%xmm3, 48(%q[dst], %q[offset], 1)\t\n"
             "addq $64, %q[offset]\t\n"
             "jmp 1b\t\n"

             "2:\t\n"
             "cmpq %q[offset], %q[bytes]\t\n"
             "je 3f\t\n"
             "movq (%q[src], %q[offset], 1), %q[tmp]\t\n"
             "movq %q[tmp], (%q[dst], %q[offset], 1)\t\n"
             "addq $8, %q[offset]\t\n"
             "jmp 2b\t\n"

             "3:\t\n"

             : [alignedBytes] "+r" (alignedBytes), [bytes] "+r" (bytes), [tmp] "+r" (tmp), [offset] "+r" (offset), [dst] "+r" (dst), [src] "+r" (src)
             :
             : "xmm0", "xmm1", "xmm2", "xmm3", "memory", "cc"
         );
 #elif CPU(ARM64)
         size_t alignedBytes = (bytes / 16) * 16;
         size_t offset = 0;

         asm volatile(
             "1:\t\n"
             "cmp %x[offset], %x[alignedBytes]\t\n"
             "b.eq 2f\t\n"
             "ldr q0, [%x[src], %x[offset]]\t\n"
             "str q0, [%x[dst], %x[offset]]\t\n"
             "add %x[offset], %x[offset], #0x10\t\n"
             "b 1b\t\n"

             "2:\t\n"
             "cmp %x[offset], %x[bytes]\t\n"
             "b.eq 3f\t\n"
             "ldr d0, [%x[src], %x[offset]]\t\n"
             "str d0, [%x[dst], %x[offset]]\t\n"
             "add %x[offset], %x[offset], #0x8\t\n"
             "b 2b\t\n"

             "3:\t\n"

             : [alignedBytes] "+r" (alignedBytes), [bytes] "+r" (bytes), [offset] "+r" (offset), [dst] "+r" (dst), [src] "+r" (src)
             :
             : "d0", "d1", "memory"
         );
 #else
 #error "Unknown architecture."
 #endif // CPU(X86_64)
     } else {
         RELEASE_ASSERT(isX86_64());
 #if CPU(X86_64)
         size_t count = bytes / 8;
         asm volatile(
             ".balign 16\t\n"
             "cld\t\n"
             "rep movsq\t\n"
             : "+D" (dst), "+S" (src), "+c" (count)
             :
             : "memory");
 #endif // CPU(X86_64)
     }
 #else
     slowPathForwardMemcpy();
 #endif // COMPILER(GCC_COMPATIBLE)
 #else
     memcpy(dst, src, bytes);
 #endif // USE(JSVALUE64)
 }

 template <typename T>
 ALWAYS_INLINE void gcSafeMemmove(T* dst, T* src, size_t bytes)
 {
     static_assert(sizeof(T) == sizeof(JSValue));
     RELEASE_ASSERT(bytes % 8 == 0);
 #if USE(JSVALUE64)
     if (bitwise_cast<uintptr_t>(src) >= bitwise_cast<uintptr_t>(dst)) {
         // This is written to do a forwards loop, so calling it is ok.
         gcSafeMemcpy(dst, src, bytes);
         return;
     }

     if ((bitwise_cast<uintptr_t>(src) + bytes) <= bitwise_cast<uintptr_t>(dst)) {
         gcSafeMemcpy(dst, src, bytes);
         return;
     }

     auto slowPathBackwardsMemmove = [&] {
         size_t count = bytes / 8;
         for (size_t i = count; i--; )
             bitwise_cast<volatile uint64_t*>(dst)[i] = bitwise_cast<volatile uint64_t*>(src)[i];
     };

 #if COMPILER(GCC_COMPATIBLE)
     if (bytes <= smallCutoff)
         slowPathBackwardsMemmove();
     else {
 #if CPU(X86_64)
         size_t alignedBytes = (bytes / 64) * 64;

         size_t tail = alignedBytes;
         size_t tmp;
         asm volatile(
             "2:\t\n"
             "cmpq %q[tail], %q[bytes]\t\n"
             "je 1f\t\n"
             "addq $-8, %q[bytes]\t\n"
             "movq (%q[src], %q[bytes], 1), %q[tmp]\t\n"
             "movq %q[tmp], (%q[dst], %q[bytes], 1)\t\n"
             "jmp 2b\t\n"

             "1:\t\n"
             "test %q[alignedBytes], %q[alignedBytes]\t\n"
             "jz 3f\t\n"

             ".balign 32\t\n"
             "100:\t\n"

             "movups -64(%q[src], %q[alignedBytes], 1), %%xmm0\t\n"
             "movups -48(%q[src], %q[alignedBytes], 1), %%xmm1\t\n"
             "movups -32(%q[src], %q[alignedBytes], 1), %%xmm2\t\n"
             "movups -16(%q[src], %q[alignedBytes], 1), %%xmm3\t\n"
             "movups %%xmm0, -64(%q[dst], %q[alignedBytes], 1)\t\n"
             "movups %%xmm1, -48(%q[dst], %q[alignedBytes], 1)\t\n"
             "movups %%xmm2, -32(%q[dst], %q[alignedBytes], 1)\t\n"
             "movups %%xmm3, -16(%q[dst], %q[alignedBytes], 1)\t\n"
             "addq $-64, %q[alignedBytes]\t\n"
             "jnz 100b\t\n"

             "3:\t\n"

             : [alignedBytes] "+r" (alignedBytes), [tail] "+r" (tail), [bytes] "+r" (bytes), [tmp] "+r" (tmp), [dst] "+r" (dst), [src] "+r" (src)
             :
             : "xmm0", "xmm1", "xmm2", "xmm3", "memory", "cc"
         );
 #elif CPU(ARM64)
         size_t alignedBytes = (bytes / 16) * 16;

         asm volatile(
             "1:\t\n"
             "cmp %x[alignedBytes], %x[bytes]\t\n"
             "b.eq 2f\t\n"
             "sub %x[bytes], %x[bytes], #0x8\t\n"
             "ldr d0, [%x[src], %x[bytes]]\t\n"
             "str d0, [%x[dst], %x[bytes]]\t\n"
             "b 1b\t\n"

             "2:\t\n"
             "cbz %x[alignedBytes], 3f\t\n"
             "sub %x[alignedBytes], %x[alignedBytes], #0x10\t\n"
             "ldr q0, [%x[src], %x[alignedBytes]]\t\n"
             "str q0, [%x[dst], %x[alignedBytes]]\t\n"
             "b 2b\t\n"

             "3:\t\n"

             : [alignedBytes] "+r" (alignedBytes), [bytes] "+r" (bytes), [dst] "+r" (dst), [src] "+r" (src)
             :
             : "d0", "d1", "memory"
         );
 #else
 #error "Unknown architecture."
 #endif // CPU(X86_64)
     }
 #else
     slowPathBackwardsMemmove();
 #endif // COMPILER(GCC_COMPATIBLE)
 #else
     memmove(dst, src, bytes);
 #endif // USE(JSVALUE64)
 }

 template <typename T>
 ALWAYS_INLINE void gcSafeZeroMemory(T* dst, size_t bytes)
 {
     static_assert(sizeof(T) == sizeof(JSValue));
     RELEASE_ASSERT(bytes % 8 == 0);
 #if USE(JSVALUE64)
 #if COMPILER(GCC_COMPATIBLE)
 #if CPU(X86_64)
     uint64_t zero = 0;
     size_t count = bytes / 8;
     asm volatile (
         "rep stosq\n\t"
         : "+D"(dst), "+c"(count)
         : "a"(zero)
         : "memory"
     );
 #elif CPU(ARM64)
     size_t alignedBytes = (bytes / 64) * 64;
     char* end = bitwise_cast<char*>(dst) + bytes;
     char* alignedEnd = bitwise_cast<char*>(dst) + alignedBytes;
     asm volatile(
         "movi d0, #0\t\n"
         "movi d1, #0\t\n"

         ".p2align 4\t\n"
         "2:\t\n"
         "cmp %x[dst], %x[alignedEnd]\t\n"
         "b.eq 4f\t\n"
         "stnp q0, q0, [%x[dst]]\t\n"
         "stnp q0, q0, [%x[dst], #0x20]\t\n"
         "add %x[dst], %x[dst], #0x40\t\n"
         "b 2b\t\n"

         "4:\t\n"
         "cmp %x[dst], %x[end]\t\n"
         "b.eq 5f\t\n"
         "str d0, [%x[dst]], #0x8\t\n"
         "b 4b\t\n"

         "5:\t\n"

         : [alignedBytes] "+r" (alignedBytes), [bytes] "+r" (bytes), [dst] "+r" (dst), [end] "+r" (end), [alignedEnd] "+r" (alignedEnd)
         :
         : "d0", "d1", "memory"
     );
 #else
 #error "Unknown architecture."
 #endif // CPU(X86_64)
 #else
     size_t count = bytes / 8;
     for (size_t i = 0; i < count; ++i)
         bitwise_cast<volatile uint64_t*>(dst)[i] = 0;
 #endif // COMPILER(GCC_COMPATIBLE)
 #else
     memset(dst, 0, bytes);
 #endif // USE(JSVALUE64)
 }

 } // namespace JSC
	/*
	* Copyright (C) 2016-2018 Apple Inc. All rights reserved.
	*
	* Redistribution and use in source and binary forms, with or without
	* modification, are permitted provided that the following conditions
	* are met:
	* 1. Redistributions of source code must retain the above copyright
	* notice, this list of conditions and the following disclaimer.
	* 2. Redistributions in binary form must reproduce the above copyright
	* notice, this list of conditions and the following disclaimer in the
	* documentation and/or other materials provided with the distribution.
	*
	* THIS SOFTWARE IS PROVIDED BY APPLE INC. ``AS IS'' AND ANY
	* EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
	* IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
	* PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL APPLE INC. OR
	* CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,
	* EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
	* PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR
	* PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY
	* OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
	* (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
	* OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
	*/

	#pragma once

	#include "CPU.h"
	#include "JSCJSValue.h"

	namespace JSC {

	// We use these memory operations when modifying memory that might be scanned by the concurrent collector.
	// We don't call the default operations because they're not guaranteed to store to memory in eight byte aligned
	// chunks. If we happened to fall into the system's normal byte copy loop, we may see a torn JSValue in the
	// concurrent collector.

	constexpr size_t smallCutoff = 30 * 8;
	constexpr size_t mediumCutoff = 4 * 1024;

	// This is a forwards loop so gcSafeMemmove can rely on the direction.
	template <typename T>
	ALWAYS_INLINE void gcSafeMemcpy(T* dst, T* src, size_t bytes)
	{
	static_assert(sizeof(T) == sizeof(JSValue));
	RELEASE_ASSERT(bytes % 8 == 0);

	#if USE(JSVALUE64)

	auto slowPathForwardMemcpy = [&] {
	size_t count = bytes / 8;
	for (unsigned i = 0; i < count; ++i)
	bitwise_cast<volatile uint64_t>(dst)[i] = bitwise_cast<volatile uint64_t>(src)[i];
	};

	#if COMPILER(GCC_COMPATIBLE) && USE(JSVALUE64)
	if (bytes <= smallCutoff)
	slowPathForwardMemcpy();
	else if (isARM64() \|\| bytes <= mediumCutoff) {
	#if CPU(X86_64)
	size_t alignedBytes = (bytes / 64) * 64;
	size_t tmp;
	size_t offset = 0;
	asm volatile(
	".balign 32\t\n"
	"1:\t\n"
	"cmpq %q[offset], %q[alignedBytes]\t\n"
	"je 2f\t\n"
	"movups (%q[src], %q[offset], 1), %%xmm0\t\n"
	"movups 16(%q[src], %q[offset], 1), %%xmm1\t\n"
	"movups 32(%q[src], %q[offset], 1), %%xmm2\t\n"
	"movups 48(%q[src], %q[offset], 1), %%xmm3\t\n"
	"movups %%xmm0, (%q[dst], %q[offset], 1)\t\n"
	"movups %%xmm1, 16(%q[dst], %q[offset], 1)\t\n"
	"movups %%xmm2, 32(%q[dst], %q[offset], 1)\t\n"
	"movups %%xmm3, 48(%q[dst], %q[offset], 1)\t\n"
	"addq $64, %q[offset]\t\n"
	"jmp 1b\t\n"

	"2:\t\n"
	"cmpq %q[offset], %q[bytes]\t\n"
	"je 3f\t\n"
	"movq (%q[src], %q[offset], 1), %q[tmp]\t\n"
	"movq %q[tmp], (%q[dst], %q[offset], 1)\t\n"
	"addq $8, %q[offset]\t\n"
	"jmp 2b\t\n"

	"3:\t\n"

	: [alignedBytes] "+r" (alignedBytes), [bytes] "+r" (bytes), [tmp] "+r" (tmp), [offset] "+r" (offset), [dst] "+r" (dst), [src] "+r" (src)
	:
	: "xmm0", "xmm1", "xmm2", "xmm3", "memory", "cc"
	);
	#elif CPU(ARM64)
	size_t alignedBytes = (bytes / 16) * 16;
	size_t offset = 0;

	asm volatile(
	"1:\t\n"
	"cmp %x[offset], %x[alignedBytes]\t\n"
	"b.eq 2f\t\n"
	"ldr q0, [%x[src], %x[offset]]\t\n"
	"str q0, [%x[dst], %x[offset]]\t\n"
	"add %x[offset], %x[offset], #0x10\t\n"
	"b 1b\t\n"

	"2:\t\n"
	"cmp %x[offset], %x[bytes]\t\n"
	"b.eq 3f\t\n"
	"ldr d0, [%x[src], %x[offset]]\t\n"
	"str d0, [%x[dst], %x[offset]]\t\n"
	"add %x[offset], %x[offset], #0x8\t\n"
	"b 2b\t\n"

	"3:\t\n"

	: [alignedBytes] "+r" (alignedBytes), [bytes] "+r" (bytes), [offset] "+r" (offset), [dst] "+r" (dst), [src] "+r" (src)
	:
	: "d0", "d1", "memory"
	);
	#else
	#error "Unknown architecture."
	#endif // CPU(X86_64)
	} else {
	RELEASE_ASSERT(isX86_64());
	#if CPU(X86_64)
	size_t count = bytes / 8;
	asm volatile(
	".balign 16\t\n"
	"cld\t\n"
	"rep movsq\t\n"
	: "+D" (dst), "+S" (src), "+c" (count)
	:
	: "memory");
	#endif // CPU(X86_64)
	}
	#else
	slowPathForwardMemcpy();
	#endif // COMPILER(GCC_COMPATIBLE)
	#else
	memcpy(dst, src, bytes);
	#endif // USE(JSVALUE64)
	}

	template <typename T>
	ALWAYS_INLINE void gcSafeMemmove(T* dst, T* src, size_t bytes)
	{
	static_assert(sizeof(T) == sizeof(JSValue));
	RELEASE_ASSERT(bytes % 8 == 0);
	#if USE(JSVALUE64)
	if (bitwise_cast<uintptr_t>(src) >= bitwise_cast<uintptr_t>(dst)) {
	// This is written to do a forwards loop, so calling it is ok.
	gcSafeMemcpy(dst, src, bytes);
	return;
	}

	if ((bitwise_cast<uintptr_t>(src) + bytes) <= bitwise_cast<uintptr_t>(dst)) {
	gcSafeMemcpy(dst, src, bytes);
	return;
	}

	auto slowPathBackwardsMemmove = [&] {
	size_t count = bytes / 8;
	for (size_t i = count; i--; )
	bitwise_cast<volatile uint64_t>(dst)[i] = bitwise_cast<volatile uint64_t>(src)[i];
	};

	#if COMPILER(GCC_COMPATIBLE)
	if (bytes <= smallCutoff)
	slowPathBackwardsMemmove();
	else {
	#if CPU(X86_64)
	size_t alignedBytes = (bytes / 64) * 64;

	size_t tail = alignedBytes;
	size_t tmp;
	asm volatile(
	"2:\t\n"
	"cmpq %q[tail], %q[bytes]\t\n"
	"je 1f\t\n"
	"addq $-8, %q[bytes]\t\n"
	"movq (%q[src], %q[bytes], 1), %q[tmp]\t\n"
	"movq %q[tmp], (%q[dst], %q[bytes], 1)\t\n"
	"jmp 2b\t\n"

	"1:\t\n"
	"test %q[alignedBytes], %q[alignedBytes]\t\n"
	"jz 3f\t\n"

	".balign 32\t\n"
	"100:\t\n"

	"movups -64(%q[src], %q[alignedBytes], 1), %%xmm0\t\n"
	"movups -48(%q[src], %q[alignedBytes], 1), %%xmm1\t\n"
	"movups -32(%q[src], %q[alignedBytes], 1), %%xmm2\t\n"
	"movups -16(%q[src], %q[alignedBytes], 1), %%xmm3\t\n"
	"movups %%xmm0, -64(%q[dst], %q[alignedBytes], 1)\t\n"
	"movups %%xmm1, -48(%q[dst], %q[alignedBytes], 1)\t\n"
	"movups %%xmm2, -32(%q[dst], %q[alignedBytes], 1)\t\n"
	"movups %%xmm3, -16(%q[dst], %q[alignedBytes], 1)\t\n"
	"addq $-64, %q[alignedBytes]\t\n"
	"jnz 100b\t\n"

	"3:\t\n"

	: [alignedBytes] "+r" (alignedBytes), [tail] "+r" (tail), [bytes] "+r" (bytes), [tmp] "+r" (tmp), [dst] "+r" (dst), [src] "+r" (src)
	:
	: "xmm0", "xmm1", "xmm2", "xmm3", "memory", "cc"
	);
	#elif CPU(ARM64)
	size_t alignedBytes = (bytes / 16) * 16;

	asm volatile(
	"1:\t\n"
	"cmp %x[alignedBytes], %x[bytes]\t\n"
	"b.eq 2f\t\n"
	"sub %x[bytes], %x[bytes], #0x8\t\n"
	"ldr d0, [%x[src], %x[bytes]]\t\n"
	"str d0, [%x[dst], %x[bytes]]\t\n"
	"b 1b\t\n"

	"2:\t\n"
	"cbz %x[alignedBytes], 3f\t\n"
	"sub %x[alignedBytes], %x[alignedBytes], #0x10\t\n"
	"ldr q0, [%x[src], %x[alignedBytes]]\t\n"
	"str q0, [%x[dst], %x[alignedBytes]]\t\n"
	"b 2b\t\n"

	"3:\t\n"

	: [alignedBytes] "+r" (alignedBytes), [bytes] "+r" (bytes), [dst] "+r" (dst), [src] "+r" (src)
	:
	: "d0", "d1", "memory"
	);
	#else
	#error "Unknown architecture."
	#endif // CPU(X86_64)
	}
	#else
	slowPathBackwardsMemmove();
	#endif // COMPILER(GCC_COMPATIBLE)
	#else
	memmove(dst, src, bytes);
	#endif // USE(JSVALUE64)
	}

	template <typename T>
	ALWAYS_INLINE void gcSafeZeroMemory(T* dst, size_t bytes)
	{
	static_assert(sizeof(T) == sizeof(JSValue));
	RELEASE_ASSERT(bytes % 8 == 0);
	#if USE(JSVALUE64)
	#if COMPILER(GCC_COMPATIBLE)
	#if CPU(X86_64)
	uint64_t zero = 0;
	size_t count = bytes / 8;
	asm volatile (
	"rep stosq\n\t"
	: "+D"(dst), "+c"(count)
	: "a"(zero)
	: "memory"
	);
	#elif CPU(ARM64)
	size_t alignedBytes = (bytes / 64) * 64;
	char* end = bitwise_cast<char*>(dst) + bytes;
	char* alignedEnd = bitwise_cast<char*>(dst) + alignedBytes;
	asm volatile(
	"movi d0, #0\t\n"
	"movi d1, #0\t\n"

	".p2align 4\t\n"
	"2:\t\n"
	"cmp %x[dst], %x[alignedEnd]\t\n"
	"b.eq 4f\t\n"
	"stnp q0, q0, [%x[dst]]\t\n"
	"stnp q0, q0, [%x[dst], #0x20]\t\n"
	"add %x[dst], %x[dst], #0x40\t\n"
	"b 2b\t\n"

	"4:\t\n"
	"cmp %x[dst], %x[end]\t\n"
	"b.eq 5f\t\n"
	"str d0, [%x[dst]], #0x8\t\n"
	"b 4b\t\n"

	"5:\t\n"

	: [alignedBytes] "+r" (alignedBytes), [bytes] "+r" (bytes), [dst] "+r" (dst), [end] "+r" (end), [alignedEnd] "+r" (alignedEnd)
	:
	: "d0", "d1", "memory"
	);
	#else
	#error "Unknown architecture."
	#endif // CPU(X86_64)
	#else
	size_t count = bytes / 8;
	for (size_t i = 0; i < count; ++i)
	bitwise_cast<volatile uint64_t*>(dst)[i] = 0;
	#endif // COMPILER(GCC_COMPATIBLE)
	#else
	memset(dst, 0, bytes);
	#endif // USE(JSVALUE64)
	}

	} // namespace JSC