Feature or enhancement

Proposal:

This is really a follow up of #115802 and more focused on the AArch64 improvements of the code generated for the JIT.
This has been discussed with @brandtbucher during PyCon 2024.

There are a series of incremental improvements that we could implement when generating AArch64 code:

• Remove duplication of trampoline section (movk) at the end of every micro op assembly code.

    // 0000000000000140:  R_AARCH64_MOVW_UABS_G0_NC    PyObject_Free
    // 144: f2a00008      movk    x8, #0x0, lsl #16
    // 0000000000000144:  R_AARCH64_MOVW_UABS_G1_NC    PyObject_Free
    // 148: f2c00008      movk    x8, #0x0, lsl #32
    // 0000000000000148:  R_AARCH64_MOVW_UABS_G2_NC    PyObject_Free
    // 14c: f2e00008      movk    x8, #0x0, lsl #48
    // 000000000000014c:  R_AARCH64_MOVW_UABS_G3       PyObject_Free
    // 150: d61f0100      br      x8
    // 154: 00 00 00 00
    // 158: d2800008      mov     x8, #0x0
    // 0000000000000158:  R_AARCH64_MOVW_UABS_G0_NC    PyObject_Free
    // 15c: f2a00008      movk    x8, #0x0, lsl #16
    // 000000000000015c:  R_AARCH64_MOVW_UABS_G1_NC    PyObject_Free
    // 160: f2c00008      movk    x8, #0x0, lsl #32
    // 0000000000000160:  R_AARCH64_MOVW_UABS_G2_NC    PyObject_Free
    // 164: f2e00008      movk    x8, #0x0, lsl #48
    // 0000000000000164:  R_AARCH64_MOVW_UABS_G3       PyObject_Free
    // 168: d61f0100      br      x8

• Implement trampoline with LDR of a PC relative literal (instead of movk). It saves 8bytes in code size.
• Move the trampolines from the "code" section of a micro-op to the "data" section, so it's out-of-line.
• Emit all of the trampolines at the end of every trace, so that each opcode doesn't need its own copy of the trampolines it uses. Also write a function to generate the trampoline.
• Once we have a slab allocator from JIT: improve memory allocation #119730, a PR use one set of trampolines per-slab rather than per-trace.

Has this already been discussed elsewhere?

I have already discussed this feature proposal on Discourse

Links to previous discussion of this feature:

This has been discussed broadly at PyCon 2024 in person.

Linked PRs

• gh-119726: JIT: re-use trampolines on AArch64 #120250
• gh-119726: replace AArch64 trampolines with LDR #121001
• gh-119726: emit AArch64 trampolines in the data section #121280
• gh-119726: generate and patch AArch64 trampolines #123872