roo*_*eee 8 java x86 assembly jvm jvm-hotspot
正如标题所述,为什么OpenJDK JVM不会在Windows x86上发出预取指令？请参阅OpenJDK Mercurial @http://hg.openjdk.java.net/jdk8u/jdk8u/hotspot/file/c49dcaf78a65/src/os_cpu/windows_x86/vm/prefetch_windows_x86.inline.hpp
inline void Prefetch::read (void *loc, intx interval) {}
inline void Prefetch::write(void *loc, intx interval) {}
没有评论,我发现除了源代码之外没有其他资源.我问,因为它对Linux x86这样做,请参阅http://hg.openjdk.java.net/jdk8u/jdk8u/hotspot/file/c49dcaf78a65/src/os_cpu/linux_x86/vm/prefetch_linux_x86.inline.hpp
inline void Prefetch::read (void *loc, intx interval) {
#ifdef AMD64
__asm__ ("prefetcht0 (%0,%1,1)" : : "r" (loc), "r" (interval));
#endif // AMD64
}
inline void Prefetch::write(void *loc, intx interval) {
#ifdef AMD64
// Do not use the 3dnow prefetchw instruction. It isn't supported on em64t.
// __asm__ ("prefetchw (%0,%1,1)" : : "r" (loc), "r" (interval));
__asm__ ("prefetcht0 (%0,%1,1)" : : "r" (loc), "r" (interval));
#endif // AMD64
}
正如JDK-4453409所示,在JDK 1.4中的HotSpot JVM中实现了预取,以加速GC.那是超过15年前,没有人会记得为什么它没有在Windows上实现.我的猜测是Visual Studio(它一直用于在Windows上构建HotSpot)在这些时候基本上不理解预取指令.看起来像是一个改进的地方.
无论如何,您询问的代码是由JVM垃圾收集器在内部使用的.这不是JIT生成的.C2 JIT代码生成器规则位于体系结构定义文件x86_64.ad中,并且存在要转换的规则PrefetchRead,PrefetchWrite以及PrefetchAllocation指向相应x64指令的节点.
一个insteresting事实是,PrefetchRead和PrefetchWrite节点不会以代码的任何地方创建.它们仅用于支持Unsafe.prefetchX内在函数,但它们在JDK 9中被删除.
JIT生成预取指令的唯一情况是PrefetchAllocation节点.您可以验证-XX:+UnlockDiagnosticVMOptions -XX:+PrintAssembly该PREFETCHNTA对象分配之后确实产生,无论在Linux和Windows.
class Test {
public static void main(String[] args) {
byte[] b = new byte[0];
for (;;) {
b = Arrays.copyOf(b, b.length + 1);
}
}
}
java.exe -XX:+UnlockDiagnosticVMOptions -XX:+PrintAssembly Test
# {method} {0x00000000176124e0} 'main' '([Ljava/lang/String;)V' in 'Test'
...
0x000000000340e512: cmp $0x100000,%r11d
0x000000000340e519: ja 0x000000000340e60f
0x000000000340e51f: movslq 0x24(%rsp),%r10
0x000000000340e524: add $0x1,%r10
0x000000000340e528: add $0x17,%r10
0x000000000340e52c: mov %r10,%r8
0x000000000340e52f: and $0xfffffffffffffff8,%r8
0x000000000340e533: cmp $0x100000,%r11d
0x000000000340e53a: ja 0x000000000340e496
0x000000000340e540: mov 0x60(%r15),%rbp
0x000000000340e544: mov %rbp,%r9
0x000000000340e547: add %r8,%r9
0x000000000340e54a: cmp 0x70(%r15),%r9
0x000000000340e54e: jae 0x000000000340e496
0x000000000340e554: mov %r9,0x60(%r15)
0x000000000340e558: prefetchnta 0xc0(%r9)
0x000000000340e560: movq $0x1,0x0(%rbp)
0x000000000340e568: prefetchnta 0x100(%r9)
0x000000000340e570: movl $0x200000f5,0x8(%rbp) ; {metadata({type array byte})}
0x000000000340e577: mov %r11d,0xc(%rbp)
0x000000000340e57b: prefetchnta 0x140(%r9)
0x000000000340e583: prefetchnta 0x180(%r9) ;*newarray
; - java.util.Arrays::copyOf@1 (line 3236)
; - Test::main@9 (line 9)
你引用的文件都有asm代码片段(内联汇编程序),某些C/C++软件在自己的代码中使用它(如apangin,JVM专家 指出,主要是在GC代码中).实际上存在差异:x86_64热点的Linux,Solaris和BSD变体在热点中都有预取,而Windows则禁用/未实现,这是部分奇怪的,部分无法解释的原因,它也可能使JVM位(某些百分比;更多)没有硬件预取的平台)在Windows上速度较慢,但仍无法为Sun/Oracle销售更多solaris/solaris付费支持合同. Ross还猜测 MS C++编译器可能不支持内联asm语法,但_mm_prefetch应该(谁将打开JDK bug将其添加到文件中？).
JVM热点是JIT,JIT由JIT作为字节发出(生成)(虽然JIT可以将代码从其自己的函数复制到生成的代码中或者发出对支持函数的调用,但预取是以字节形式发出的.热点).我们怎样才能发现它是如何排放的？简单的在线方式是找到一些在线可搜索的jdk8u副本(或更好的交叉引用,如metager),例如在github:https://github.com/JetBrains/jdk8u_hotspot上,并搜索prefetch或prefetch emit或prefetchr或者lir_prefetchr.有一些相关的结果:
JVM的c1编译器/LIR中发出的实际字节jdk8u_hotspot/src/cpu/x86/vm/assembler_x86.cpp:
void Assembler::prefetch_prefix(Address src) {
prefix(src);
emit_int8(0x0F);
}
void Assembler::prefetchnta(Address src) {
NOT_LP64(assert(VM_Version::supports_sse(), "must support"));
InstructionMark im(this);
prefetch_prefix(src);
emit_int8(0x18);
emit_operand(rax, src); // 0, src
}
void Assembler::prefetchr(Address src) {
assert(VM_Version::supports_3dnow_prefetch(), "must support");
InstructionMark im(this);
prefetch_prefix(src);
emit_int8(0x0D);
emit_operand(rax, src); // 0, src
}
void Assembler::prefetcht0(Address src) {
NOT_LP64(assert(VM_Version::supports_sse(), "must support"));
InstructionMark im(this);
prefetch_prefix(src);
emit_int8(0x18);
emit_operand(rcx, src); // 1, src
}
void Assembler::prefetcht1(Address src) {
NOT_LP64(assert(VM_Version::supports_sse(), "must support"));
InstructionMark im(this);
prefetch_prefix(src);
emit_int8(0x18);
emit_operand(rdx, src); // 2, src
}
void Assembler::prefetcht2(Address src) {
NOT_LP64(assert(VM_Version::supports_sse(), "must support"));
InstructionMark im(this);
prefetch_prefix(src);
emit_int8(0x18);
emit_operand(rbx, src); // 3, src
}
void Assembler::prefetchw(Address src) {
assert(VM_Version::supports_3dnow_prefetch(), "must support");
InstructionMark im(this);
prefetch_prefix(src);
emit_int8(0x0D);
emit_operand(rcx, src); // 1, src
}
用于c1 LIR: src/share/vm/c1/c1_LIRAssembler.cpp
void LIR_Assembler::emit_op1(LIR_Op1* op) {
switch (op->code()) {
...
case lir_prefetchr:
prefetchr(op->in_opr());
break;
case lir_prefetchw:
prefetchw(op->in_opr());
break;
现在我们知道操作码lir_prefetchr和可以进行搜索或OpenGrok外部参照和lir_prefetchw,找到唯一的例子,在src/share/vm/c1/c1_LIR.cpp
void LIR_List::prefetch(LIR_Address* addr, bool is_store) {
append(new LIR_Op1(
is_store ? lir_prefetchw : lir_prefetchr,
LIR_OprFact::address(addr)));
}
存在其中预取指令的定义(对于C2,如其他地方由apangin指出),在src/cpu/x86/vm/x86_64.ad:
// Prefetch instructions. ...
instruct prefetchr( memory mem ) %{
predicate(ReadPrefetchInstr==3);
match(PrefetchRead mem);
ins_cost(125);
format %{ "PREFETCHR $mem\t# Prefetch into level 1 cache" %}
ins_encode %{
__ prefetchr($mem$$Address);
%}
ins_pipe(ialu_mem);
%}
instruct prefetchrNTA( memory mem ) %{
predicate(ReadPrefetchInstr==0);
match(PrefetchRead mem);
ins_cost(125);
format %{ "PREFETCHNTA $mem\t# Prefetch into non-temporal cache for read" %}
ins_encode %{
__ prefetchnta($mem$$Address);
%}
ins_pipe(ialu_mem);
%}
instruct prefetchrT0( memory mem ) %{
predicate(ReadPrefetchInstr==1);
match(PrefetchRead mem);
ins_cost(125);
format %{ "PREFETCHT0 $mem\t# prefetch into L1 and L2 caches for read" %}
ins_encode %{
__ prefetcht0($mem$$Address);
%}
ins_pipe(ialu_mem);
%}
instruct prefetchrT2( memory mem ) %{
predicate(ReadPrefetchInstr==2);
match(PrefetchRead mem);
ins_cost(125);
format %{ "PREFETCHT2 $mem\t# prefetch into L2 caches for read" %}
ins_encode %{
__ prefetcht2($mem$$Address);
%}
ins_pipe(ialu_mem);
%}
instruct prefetchwNTA( memory mem ) %{
match(PrefetchWrite mem);
ins_cost(125);
format %{ "PREFETCHNTA $mem\t# Prefetch to non-temporal cache for write" %}
ins_encode %{
__ prefetchnta($mem$$Address);
%}
ins_pipe(ialu_mem);
%}
// Prefetch instructions for allocation.
instruct prefetchAlloc( memory mem ) %{
predicate(AllocatePrefetchInstr==3);
match(PrefetchAllocation mem);
ins_cost(125);
format %{ "PREFETCHW $mem\t# Prefetch allocation into level 1 cache and mark modified" %}
ins_encode %{
__ prefetchw($mem$$Address);
%}
ins_pipe(ialu_mem);
%}
instruct prefetchAllocNTA( memory mem ) %{
predicate(AllocatePrefetchInstr==0);
match(PrefetchAllocation mem);
ins_cost(125);
format %{ "PREFETCHNTA $mem\t# Prefetch allocation to non-temporal cache for write" %}
ins_encode %{
__ prefetchnta($mem$$Address);
%}
ins_pipe(ialu_mem);
%}
instruct prefetchAllocT0( memory mem ) %{
predicate(AllocatePrefetchInstr==1);
match(PrefetchAllocation mem);
ins_cost(125);
format %{ "PREFETCHT0 $mem\t# Prefetch allocation to level 1 and 2 caches for write" %}
ins_encode %{
__ prefetcht0($mem$$Address);
%}
ins_pipe(ialu_mem);
%}
instruct prefetchAllocT2( memory mem ) %{
predicate(AllocatePrefetchInstr==2);
match(PrefetchAllocation mem);
ins_cost(125);
format %{ "PREFETCHT2 $mem\t# Prefetch allocation to level 2 cache for write" %}
ins_encode %{
__ prefetcht2($mem$$Address);
%}
ins_pipe(ialu_mem);
%}
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