Faster _EXIT_TRACE via jump patching

[Fidget-Spinner]

Feature or enhancement

Proposal:

When a trace ends in _EXIT_TRACE instead of JUMP_TO_TOP, it seems to cost a lot more to jump.

We should do like pypy and patch the trace exit to the actual machine code so it's a direct jump.

Has this already been discussed elsewhere?

No response given

Links to previous discussion of this feature:

No response

[Fidget-Spinner]

I tried, and performance results are too lacklustre to bother with this https://github.com/python/cpython/compare/main...Fidget-Spinner:cpython:faster_side_exits_selfmodifying?expand=1

[Fidget-Spinner]

This could be profitable for _EXIT_TRACE that leads to another executor. pypy seems to do it too.

[Fidget-Spinner]

Comparison:

void
emit__EXIT_TRACE(
    unsigned char *code, unsigned char *data, _PyExecutorObject *executor,
    const _PyUOpInstruction *instruction, jit_state *state)
{
    // 
    // _EXIT_TRACE.o: file format elf64-x86-64
    // 
    // Disassembly of section .text:
    // 
    // 0000000000000000 <_JIT_ENTRY>:
    // 0: 48 b8 00 00 00 00 00 00 00 00 movabsq $0x0, %rax
    // 0000000000000002:  R_X86_64_64  _JIT_OPERAND0
    // a: 49 89 86 18 01 00 00          movq    %rax, 0x118(%r14)
    // 11: 48 b8 00 00 00 00 00 00 00 00 movabsq $0x0, %rax
    // 0000000000000013:  R_X86_64_64  _JIT_OPERAND0+0x8
    // 1b: 48 8b 00                      movq    (%rax), %rax
    // 1e: 48 8b 40 70                   movq    0x70(%rax), %rax
    // 22: ff e0                         jmpq    *%rax
    const unsigned char code_body[36] = {
        0x48, 0xb8, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
        0x00, 0x00, 0x49, 0x89, 0x86, 0x18, 0x01, 0x00,
        0x00, 0x48, 0xb8, 0x00, 0x00, 0x00, 0x00, 0x00,
        0x00, 0x00, 0x00, 0x48, 0x8b, 0x00, 0x48, 0x8b,
        0x40, 0x70, 0xff, 0xe0,
    };
    memcpy(code, code_body, sizeof(code_body));
    patch_64(code + 0x2, instruction->operand0);
    patch_64(code + 0x13, instruction->operand0 + 0x8);
}

void
emit__EXIT_TRACE_REWRITTEN(
    unsigned char *code, unsigned char *data, _PyExecutorObject *executor,
    const _PyUOpInstruction *instruction, jit_state *state)
{
    // 
    // _EXIT_TRACE_REWRITTEN.o:       file format elf64-x86-64
    // 
    // Disassembly of section .text:
    // 
    // 0000000000000000 <_JIT_ENTRY>:
    // 0: 48 b8 00 00 00 00 00 00 00 00 movabsq $0x0, %rax
    // 0000000000000002:  R_X86_64_64  _JIT_OPERAND0
    // a: 49 89 86 18 01 00 00          movq    %rax, 0x118(%r14)
    // 11: e9 00 00 00 00                jmp     0x16 <_JIT_ENTRY+0x16>
    // 0000000000000012:  R_X86_64_PLT32       _JIT_JUMP_TARGET-0x4
    const unsigned char code_body[22] = {
        0x48, 0xb8, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
        0x00, 0x00, 0x49, 0x89, 0x86, 0x18, 0x01, 0x00,
        0x00, 0xe9,
    };
    memcpy(code, code_body, sizeof(code_body));
    patch_64(code + 0x2, instruction->operand0);
    patch_32r(code + 0x12, state->instruction_starts[instruction->jump_target] + -0x4);
}