RFR: 8352251: Implement Cooperative JFR Sampling

[Aleksey Shipilev]

On Fri, 28 Mar 2025 15:38:59 GMT, Markus Grönlund <mgronlun at openjdk.org> wrote:

> Greetings,
> 
> This is the implementation of JEP [JDK-8350338 Cooperative JFR Sampling](https://bugs.openjdk.org/browse/JDK-8350338).
> 
> Implementations in this change set are provided and have been tested on the following platforms:
> 
> - windows-x64
> - windows-x64-debug
> - linux-x64
> - linux-x64-debug
> - macosx-x64
> - macosx-x64-debug
> - linux-aarch64
> - linux-aarch64-debug
> - macosx-aarch64
> - macosx-aarch64-debug
> 
> Testing: tier1-6, jdk_jfr, stress testing.
> 
> Platform porters note:
> Some platform-specific code needs to be provided, mainly in the interpreter. Take a look at the following files for changes:
> 
> - src/hotspot/cpu/x86/frame_x86.cpp
> - src/hotspot/cpu/x86/interp_masm_x86.cpp
> - src/hotspot/cpu/x86/interp_masm_x86.hpp
> - src/hotspot/cpu/x86/javaFrameAnchor_x86.hpp
> - src/hotspot/cpu/x86/macroAssembler_x86.cpp
> - src/hotspot/cpu/x86/macroAssembler_x86.hpp
> - src/hotspot/cpu/x86/templateInterpreterGenerator_x86.cpp
> - src/hotspot/cpu/x86/templateTable_x86.cpp
> - src/hotspot/os_cpu/linux_x86/javaThread_linux_x86.hpp
> 
> Thanks
> Markus

A drive-by comment, since I am cleaning up some of x86 code after x86_32 removals:

src/hotspot/cpu/x86/interp_masm_x86.cpp line 791:

> 789: 
> 790:   // Thread argument
> 791:   mov(c_rarg0, r15_thread);

`movptr`, I'd think?

src/hotspot/cpu/x86/interp_masm_x86.cpp line 796:

> 794:   Label L_ljf, L_valid_rbp;
> 795:   testptr(rbp, rbp);
> 796:   jcc(Assembler::notZero, L_valid_rbp);

Little trivia about x86: "normal" jumps are fairly long (6 bytes, IIRC), but you can have a "short jump" (about 3 bytes, IIRC) if the offset is small. This is normally handled by assembler itself, when it knows where the branch target is. That is known for _backward_ branches. For _forward_ branches like this, we unfortunately often need to claim the jump is short ahead of time. See `jccb` (note `b`). 

Basically, if you are sure the branch target is within 128 bytes (or, more likely, a few instructions) away, use `jccb`.

Same thing with `jmp` -> `jmpb`.

src/hotspot/cpu/x86/interp_masm_x86.cpp line 1045:

> 1043:   Label slow_path;
> 1044:   Label fast_path;
> 1045:   safepoint_poll(slow_path, rthread, current_fp, true /* at_return */, false /* in_nmethod */);

A little heads-up: I am going to propose a little cleanup soon to drop `rthread` from x86 safepoint_pool (we can trust it is `r15_thread` always). That would probably yield a minor merge conflict here.

-------------

PR Review: https://git.openjdk.org/jdk/pull/24296#pullrequestreview-2726620088
PR Review Comment: https://git.openjdk.org/jdk/pull/24296#discussion_r2019283641
PR Review Comment: https://git.openjdk.org/jdk/pull/24296#discussion_r2019283404
PR Review Comment: https://git.openjdk.org/jdk/pull/24296#discussion_r2019285766