On Mon, 28 Oct 2024 23:38:43 GMT, Dean Long <dl...@openjdk.org> wrote:
>>> Could the problem be solved with a resume adapter instead, like the >>> interpreter uses? >>> >> It will just move the task of adjusting the size of the frame somewhere else. > >> One way to get rid of this would be to have c2 just set last_Java_pc too >> along with last_Java_sp, so we don't need to push lr to be able to do >> last_Java_sp[-1] to make the frame walkable. > > If that would solve the problem, then that must mean we save/freeze > last_Java_pc as part of the virtual thread's state. So why can't we just > call make_walkable() before we freeze, to fix things up as if C2 had stored > last_Java_pc to the anchor? Then freeze could assert that the thread is > already walkable. I'm surprised it doesn't already. The issue is not when we make the frame walkable but how. The way it currently works is by pushing the last_Java_pc to the stack in the runtime stub before making the call to the VM (plus an alignment word). So to make the frame walkable we do last_Java_sp[-1] in the VM. But this approach creates a mismatch between the recorded cb->frame_size() (which starts from last_Java_sp) vs the physical size of the frame which starts with rsp right before the call. This is what the c2 runtime stub code for aarch64 looks like: 0xffffdfdba584: sub sp, sp, #0x10 0xffffdfdba588: stp x29, x30, [sp] 0xffffdfdba58c: ldrb w8, [x28, #1192] 0xffffdfdba590: cbz x8, 0xffffdfdba5a8 0xffffdfdba594: mov x8, #0x4ba0 0xffffdfdba598: movk x8, #0xf6a8, lsl #16 0xffffdfdba59c: movk x8, #0xffff, lsl #32 0xffffdfdba5a0: mov x0, x28 0xffffdfdba5a4: blr x8 0xffffdfdba5a8: mov x9, sp 0xffffdfdba5ac: str x9, [x28, #1000] <------- store last_Java_sp 0xffffdfdba5b0: mov x0, x1 0xffffdfdba5b4: mov x1, x2 0xffffdfdba5b8: mov x2, x28 0xffffdfdba5bc: adr x9, 0xffffdfdba5d4 0xffffdfdba5c0: mov x8, #0xe6a4 0xffffdfdba5c4: movk x8, #0xf717, lsl #16 0xffffdfdba5c8: movk x8, #0xffff, lsl #32 0xffffdfdba5cc: stp xzr, x9, [sp, #-16]! <------- Push two extra words 0xffffdfdba5d0: blr x8 0xffffdfdba5d4: nop 0xffffdfdba5d8: movk xzr, #0x0 0xffffdfdba5dc: movk xzr, #0x0 0xffffdfdba5e0: add sp, sp, #0x10 <------- Remove two extra words 0xffffdfdba5e4: str xzr, [x28, #1000] 0xffffdfdba5e8: str xzr, [x28, #1008] 0xffffdfdba5ec: ldr x10, [x28, #8] 0xffffdfdba5f0: cbnz x10, 0xffffdfdba600 0xffffdfdba5f4: ldp x29, x30, [sp] 0xffffdfdba5f8: add sp, sp, #0x10 0xffffdfdba5fc: ret 0xffffdfdba600: ldp x29, x30, [sp] 0xffffdfdba604: add sp, sp, #0x10 0xffffdfdba608: adrp x8, 0xffffdfc30000 0xffffdfdba60c: add x8, x8, #0x80 0xffffdfdba610: br x8 ------------- PR Review Comment: https://git.openjdk.org/jdk/pull/21565#discussion_r1821389434
