RFR: 8338383: Implement JEP 491: Synchronize Virtual Threads without Pinning [v12]

[Patricio Chilano Mateo]

On Mon, 28 Oct 2024 23:38:43 GMT, Dean Long <dlong at 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

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PR Review Comment: https://git.openjdk.org/jdk/pull/21565#discussion_r1821389434