JVM安全点轮询汇编函数解析
OpenJDK 17 源码的实现逻辑，handle_polling_page_exception 函数在方法返回时的调用流程如下：
调用流程分析：
栈水印检查触发跳转：
当线程执行方法返回前的安全点轮询时（
MacroAssembler::safepoint_poll中at_return=true）如果栈指针超出安全区域（
cmpptr比较结果为above），会跳转到关联的slow_path标签这个
slow_path标签由C2SafepointPollStub管理
桩代码生成：
C2SafepointPollStubTable::emit()遍历所有安全点条目为每个条目调用
emit_stub()生成桩代码
cpp
void emit_stub(MacroAssembler& masm, C2SafepointPollStub* entry) const {select_emit_stub<VM_Version::supports_stack_watermark_barrier()>(masm, entry); }桩代码核心逻辑：
在
emit_stub_impl()中生成实际汇编代码：cpp
__ bind(entry->_stub_label); // 计算安全点返回地址 InternalAddress safepoint_pc(...);// 保存返回地址到线程控制块 __ lea(rscratch1, safepoint_pc); __ movptr(Address(r15_thread, JavaThread::saved_exception_pc_offset()), rscratch1);// 跳转到异常处理入口 __ jump(callback_addr); // callback_addr = polling_page_return_handler_blob
处理函数绑定：
polling_page_return_handler_blob在 JVM 初始化时创建：cpp
_polling_page_return_handler_blob = generate_handler_blob(CAST_FROM_FN_PTR(address, SafepointSynchronize::handle_polling_page_exception),POLL_AT_RETURN );
该 blob 将
handle_polling_page_exception函数地址嵌入到生成的机器代码中
关键调用路径：
text
执行线程 → 方法返回前的轮询检查│├─ 栈水印检查失败 → 跳转到 slow_path (C2SafepointPollStub)│ ││ └─ 桩代码执行:│ 1. 保存返回地址到 JavaThread::saved_exception_pc_offset()│ 2. 跳转到 polling_page_return_handler_blob│ ││ └─ 执行 handle_polling_page_exception()│ ││ └─ 安全点同步处理│└─ 正常继续执行
技术要点：
上下文保存：
桩代码通过
saved_exception_pc_offset()保存原始返回地址确保异常处理后能正确返回到原执行点
平台适配：
使用
select_emit_stub模板实现条件编译仅当
VM_Version::supports_stack_watermark_barrier()为 true 时生成桩代码
异常处理机制：
cpp
void SafepointSynchronize::handle_polling_page_exception(JavaThread* thread) {// 1. 检查安全点状态// 2. 阻塞线程直到安全点操作完成// 3. 恢复线程执行（通过 saved_exception_pc 返回） }水印检查优化：
在 nmethod 中使用
rsp而非rbp，因为：cpp
// 当 in_nmethod 时，栈指针已在轮询前递增 cmpptr(in_nmethod ? rsp : rbp, ...);
总结：
当线程在方法返回前检测到栈水印越界时，会通过预先生成的桩代码跳转到 handle_polling_page_exception 函数。该函数负责将线程挂起直到安全点操作完成，然后通过保存的返回地址恢复执行。这种机制实现了高效的安全点协作，同时保证了栈收缩时的内存安全。
##源码
#define __ masm.
void C2SafepointPollStubTable::emit_stub_impl(MacroAssembler& masm, C2SafepointPollStub* entry) const {assert(SharedRuntime::polling_page_return_handler_blob() != NULL,"polling page return stub not created yet");address stub = SharedRuntime::polling_page_return_handler_blob()->entry_point();RuntimeAddress callback_addr(stub);__ bind(entry->_stub_label);InternalAddress safepoint_pc(masm.pc() - masm.offset() + entry->_safepoint_offset);
#ifdef _LP64__ lea(rscratch1, safepoint_pc);__ movptr(Address(r15_thread, JavaThread::saved_exception_pc_offset()), rscratch1);
#elseconst Register tmp1 = rcx;const Register tmp2 = rdx;__ push(tmp1);__ push(tmp2);__ lea(tmp1, safepoint_pc);__ get_thread(tmp2);__ movptr(Address(tmp2, JavaThread::saved_exception_pc_offset()), tmp1);__ pop(tmp2);__ pop(tmp1);
#endif__ jump(callback_addr);
}
#undef __void C2SafepointPollStubTable::emit(CodeBuffer& cb) {MacroAssembler masm(&cb);for (int i = _safepoints.length() - 1; i >= 0; i--) {// Make sure there is enough space in the code bufferif (cb.insts()->maybe_expand_to_ensure_remaining(PhaseOutput::MAX_inst_size) && cb.blob() == NULL) {ciEnv::current()->record_failure("CodeCache is full");return;}C2SafepointPollStub* entry = _safepoints.at(i);emit_stub(masm, entry);}
}// The selection logic below relieves the need to add dummy files to unsupported platforms.template <bool enabled>typename EnableIf<enabled>::typeselect_emit_stub(MacroAssembler& masm, C2SafepointPollStub* entry) const {emit_stub_impl(masm, entry);}template <bool enabled>typename EnableIf<!enabled>::typeselect_emit_stub(MacroAssembler& masm, C2SafepointPollStub* entry) const {}void emit_stub(MacroAssembler& masm, C2SafepointPollStub* entry) const {select_emit_stub<VM_Version::supports_stack_watermark_barrier()>(masm, entry);}_polling_page_return_handler_blob = generate_handler_blob(CAST_FROM_FN_PTR(address, SafepointSynchronize::handle_polling_page_exception), POLL_AT_RETURN);static SafepointBlob* polling_page_return_handler_blob() { return _polling_page_return_handler_blob; }##gdb堆栈
(gdb) bt
#0 C2SafepointPollStubTable::emit (this=0x7fffd0bfbb98, cb=...) at /home/yym/openjdk17/jdk17-master/src/hotspot/share/opto/output.cpp:237
#1 0x00007ffff6896363 in PhaseOutput::fill_buffer (this=0x7fffd0bfb950, cb=0x7fffd0bfb970, blk_starts=0x7fffb002a750)at /home/yym/openjdk17/jdk17-master/src/hotspot/share/opto/output.cpp:1824
#2 0x00007ffff6890444 in PhaseOutput::Output (this=0x7fffd0bfb950) at /home/yym/openjdk17/jdk17-master/src/hotspot/share/opto/output.cpp:432
#3 0x00007ffff602fe6b in Compile::Code_Gen (this=0x7fffd0bfde90) at /home/yym/openjdk17/jdk17-master/src/hotspot/share/opto/compile.cpp:2775
#4 0x00007ffff6027214 in Compile::Compile (this=0x7fffd0bfde90, ci_env=0x7fffd0bfeac0, generator=0x7ffff6959ff0 <OptoRuntime::new_instance_Type()>,stub_function=0x7ffff6958c68 <OptoRuntime::new_instance_C(Klass*, JavaThread*)> "\363\017\036\372UH\211\345H\203\354`H\211}\250H\211u\240H\213U\240H\215E\320H\211\326H\211\307\350\067\204N\377H\213M\240H\215E\340\272\001", stub_name=0x7ffff72815ea "_new_instance_Java", is_fancy_jump=0, pass_tls=true, return_pc=false, directive=0x7ffff01b1f90)at /home/yym/openjdk17/jdk17-master/src/hotspot/share/opto/compile.cpp:925
#5 0x00007ffff6958b70 in OptoRuntime::generate_stub (env=0x7fffd0bfeac0, gen=0x7ffff6959ff0 <OptoRuntime::new_instance_Type()>,C_function=0x7ffff6958c68 <OptoRuntime::new_instance_C(Klass*, JavaThread*)> "\363\017\036\372UH\211\345H\203\354`H\211}\250H\211u\240H\213U\240H\215E\320H\211\326H\211\307\350\067\204N\377H\213M\240H\215E\340\272\001", name=0x7ffff72815ea "_new_instance_Java", is_fancy_jump=0, pass_tls=true, return_pc=false)at /home/yym/openjdk17/jdk17-master/src/hotspot/share/opto/runtime.cpp:171
#6 0x00007ffff6958625 in OptoRuntime::generate (env=0x7fffd0bfeac0) at /home/yym/openjdk17/jdk17-master/src/hotspot/share/opto/runtime.cpp:139
#7 0x00007ffff5f07753 in C2Compiler::init_c2_runtime () at /home/yym/openjdk17/jdk17-master/src/hotspot/share/opto/c2compiler.cpp:78
#8 0x00007ffff5f077d7 in C2Compiler::initialize (this=0x7ffff02c01f0) at /home/yym/openjdk17/jdk17-master/src/hotspot/share/opto/c2compiler.cpp:91
#9 0x00007ffff6043287 in CompileBroker::init_compiler_runtime () at /home/yym/openjdk17/jdk17-master/src/hotspot/share/compiler/compileBroker.cpp:1801
#10 0x00007ffff6043806 in CompileBroker::compiler_thread_loop () at /home/yym/openjdk17/jdk17-master/src/hotspot/share/compiler/compileBroker.cpp:1938
#11 0x00007ffff6065c02 in CompilerThread::thread_entry (thread=0x7ffff02c08c0, __the_thread__=0x7ffff02c08c0)at /home/yym/openjdk17/jdk17-master/src/hotspot/share/compiler/compilerThread.cpp:59
#12 0x00007ffff6b5e498 in JavaThread::thread_main_inner (this=0x7ffff02c08c0) at /home/yym/openjdk17/jdk17-master/src/hotspot/share/runtime/thread.cpp:1305
#13 0x00007ffff6b5e32e in JavaThread::run (this=0x7ffff02c08c0) at /home/yym/openjdk17/jdk17-master/src/hotspot/share/runtime/thread.cpp:1288
#14 0x00007ffff6b5ba31 in Thread::call_run (this=0x7ffff02c08c0) at /home/yym/openjdk17/jdk17-master/src/hotspot/share/runtime/thread.cpp:394
#15 0x00007ffff68752bb in thread_native_entry (thread=0x7ffff02c08c0) at /home/yym/openjdk17/jdk17-master/src/hotspot/os/linux/os_linux.cpp:720
#16 0x00007ffff7c94ac3 in start_thread (arg=<optimized out>) at ./nptl/pthread_create.c:442
#17 0x00007ffff7d26850 in clone3 () at ../sysdeps/unix/sysv/linux/x86_64/clone3.S:81##gdb堆栈
Thread 15 "C1 CompilerThre" hit Breakpoint 1, SharedRuntime::polling_page_return_handler_blob () at /home/yym/openjdk17/jdk17-master/src/hotspot/share/runtime/sharedRuntime.hpp:250
250 static SafepointBlob* polling_page_return_handler_blob() { return _polling_page_return_handler_blob; }
(gdb) bt
#0 SharedRuntime::polling_page_return_handler_blob () at /home/yym/openjdk17/jdk17-master/src/hotspot/share/runtime/sharedRuntime.hpp:250
#1 0x00007ffff5e14d46 in C1SafepointPollStub::emit_code (this=0x7ffff0385490, ce=0x7fffd0afe0f0) at /home/yym/openjdk17/jdk17-master/src/hotspot/cpu/x86/c1_CodeStubs_x86.cpp:104
#2 0x00007ffff5e64987 in LIR_Assembler::emit_stubs (this=0x7fffd0afe0f0, stub_list=0x7fffa409c5f0) at /home/yym/openjdk17/jdk17-master/src/hotspot/share/c1/c1_LIRAssembler.cpp:150
#3 0x00007ffff5e649ff in LIR_Assembler::emit_slow_case_stubs (this=0x7fffd0afe0f0) at /home/yym/openjdk17/jdk17-master/src/hotspot/share/c1/c1_LIRAssembler.cpp:159
#4 0x00007ffff5e19a9d in Compilation::emit_code_epilog (this=0x7fffd0afe4d0, assembler=0x7fffd0afe0f0) at /home/yym/openjdk17/jdk17-master/src/hotspot/share/c1/c1_Compilation.cpp:291
#5 0x00007ffff5e19e18 in Compilation::emit_code_body (this=0x7fffd0afe4d0) at /home/yym/openjdk17/jdk17-master/src/hotspot/share/c1/c1_Compilation.cpp:355
#6 0x00007ffff5e1a12a in Compilation::compile_java_method (this=0x7fffd0afe4d0) at /home/yym/openjdk17/jdk17-master/src/hotspot/share/c1/c1_Compilation.cpp:404
#7 0x00007ffff5e1a470 in Compilation::compile_method (this=0x7fffd0afe4d0) at /home/yym/openjdk17/jdk17-master/src/hotspot/share/c1/c1_Compilation.cpp:460
#8 0x00007ffff5e1ac88 in Compilation::Compilation (this=0x7fffd0afe4d0, compiler=0x7ffff0231910, env=0x7fffd0afe9f0, method=0x7fffa40922b0, osr_bci=-1, buffer_blob=0x7fffe111cf90,install_code=true, directive=0x7ffff01b56b0) at /home/yym/openjdk17/jdk17-master/src/hotspot/share/c1/c1_Compilation.cpp:584
#9 0x00007ffff5e1f184 in Compiler::compile_method (this=0x7ffff0231910, env=0x7fffd0afe9f0, method=0x7fffa40922b0, entry_bci=-1, install_code=true, directive=0x7ffff01b56b0)at /home/yym/openjdk17/jdk17-master/src/hotspot/share/c1/c1_Compiler.cpp:247
#10 0x00007ffff6044f06 in CompileBroker::invoke_compiler_on_method (task=0x7ffff030e790) at /home/yym/openjdk17/jdk17-master/src/hotspot/share/compiler/compileBroker.cpp:2312
#11 0x00007ffff6043a81 in CompileBroker::compiler_thread_loop () at /home/yym/openjdk17/jdk17-master/src/hotspot/share/compiler/compileBroker.cpp:1985
#12 0x00007ffff6065c02 in CompilerThread::thread_entry (thread=0x7ffff02c2340, __the_thread__=0x7ffff02c2340)at /home/yym/openjdk17/jdk17-master/src/hotspot/share/compiler/compilerThread.cpp:59
#13 0x00007ffff6b5e498 in JavaThread::thread_main_inner (this=0x7ffff02c2340) at /home/yym/openjdk17/jdk17-master/src/hotspot/share/runtime/thread.cpp:1305
#14 0x00007ffff6b5e32e in JavaThread::run (this=0x7ffff02c2340) at /home/yym/openjdk17/jdk17-master/src/hotspot/share/runtime/thread.cpp:1288
#15 0x00007ffff6b5ba31 in Thread::call_run (this=0x7ffff02c2340) at /home/yym/openjdk17/jdk17-master/src/hotspot/share/runtime/thread.cpp:394
#16 0x00007ffff68752bb in thread_native_entry (thread=0x7ffff02c2340) at /home/yym/openjdk17/jdk17-master/src/hotspot/os/linux/os_linux.cpp:720
#17 0x00007ffff7c94ac3 in start_thread (arg=<optimized out>) at ./nptl/pthread_create.c:442
#18 0x00007ffff7d26850 in clone3 () at ../sysdeps/unix/sysv/linux/x86_64/clone3.S:81