Re: [Tarantool-patches] [PATCH luajit 1/2] Fix register mask for stack check in head of side trace.

[Sergey Kaplun via Tarantool-patches <tarantool-patches@dev.tarantool.org>]

Hi, Maxim!
Thanks for the review!
Please, consider my answers below.

On 13.10.23, Maxim Kokryashkin wrote:
> Hi, Sergey!
> Thanks for the patch!
> LGTM, except for a few nits below.
> On Wed, Oct 11, 2023 at 06:04:09PM +0300, Sergey Kaplun wrote:
> > From: Mike Pall <mike>
> > 
> > Analyzed by Sergey Kaplun.
> > 
> > (cherry-picked from commit b7a8c7c184257858699454408420dd5f0b6c8a75)
> > 
> > Assume we have parent and child traces with the following IRs from the
> > IR dump:
> > 
> > Parent:
> > | 0009 rax   >  tab TNEW   0     0
> > | 0010          p32 FLOAD  0008  tab.node
> > | 0011          p32 HREFK  0010  "Name" @1
> > | 0012  {0008}  tab HSTORE 0011  0009
> > | ....              SNAP   2    [ ---- 0001 0002 0008 ---- ]
> > | 0013  {sink}  tab TNEW   0     0
> > | 0014  {0008}  fal HSTORE 0011  false
> > | ....              SNAP   3    [ ---- 0001 0002 0008 ---- ]
> > 
> > Child:
> > | 0001 r15      tab SLOAD  1     PI
> > | 0002 rbp      tab SLOAD  2     PI
> > | 0003          tab PVAL   9
> > 
> > As we can see from the trace dump above, the `rax` register is missing
> > in the `0003 PVAL` IR for the side trace -- so it is assumed to be
> > available in the allow RegSet inside `asm_stack_check()` and its value
> > is spoiled during this check, so if we are restoring from the 3rd
> Typo: s/spoiled/spilled/

I mean spoiled (damaged) (with incorrect value) here -- there is no spill, but
incorrect value as `ir->r`.

> > snapshot by stack overflow -- we are in trouble.
> > 
> > The moment when IR is spoiled is when we set a hint on the register
> Typo: s/spoiled/spilled/

Ditto.

> > inherited from the parent trace (see `asm_setup_regsp()` for details).

<snipped>

> > 

-- 
Best regards,
Sergey Kaplun