[Tarantool-patches] [PATCH luajit] x64: Fix __call metamethod return dispatch.

[Alexander V. Tikhonov]

Hi Igor, thanks for the patch, as I see no new degradations found [1],
patch LGTM.

[1] - https://gitlab.com/tarantool/tarantool/-/pipelines/225349795

On Fri, Dec 04, 2020 at 07:24:41PM +0300, Igor Munkin wrote:
> Sasha,
> 
> Could you please confirm whether CI is OK?
> 
> On 04.12.20, Igor Munkin wrote:
> > From: Mike Pall <mike>
> > 
> > After linking new cframe to the chain KBASEa still stores the address of
> > the previous one. If the execution proceeds to <lj_vmeta_call> KBASE
> > value (i.e. low 32 bits of the stored address) might be equal to the
> > current BASE address value so the execution takes the invalid path. Such
> > address clashing occurs only on x86_64 platform with disabled LJ_GC64,
> > so 64-bit registers have to be compared in x64 build.
> > 
> > NB: Though there is only 32-bit load to restore BASE value prior to the
> > comparison, the high 32 bits of RDX are reset to zeros, according to x86
> > long mode semantics.
> > 
> > Igor Munkin:
> > * backported the original patch to tarantool/luajit repo
> > * extended the original commit message with the rationale
> > 
> > For more info and explanation see LuaJIT/LuaJIT#636.
> > 
> > Relates to tarantool/tarantool#4518
> > Relates to tarantool/tarantool#4649
> > 
> > Signed-off-by: Igor Munkin <imun at tarantool.org>
> > ---
> > 
> > Issues:
> > * https://github.com/tarantool/tarantool/issues/4518
> > * https://github.com/tarantool/tarantool/issues/4649
> > Branch:
> > * https://github.com/tarantool/luajit/tree/imun/gh-4518-cmp-64-bit-regs-in-vmeta-call
> > 
> > CI is kinda green, considering C6 EOL and the corresponding failures:
> > * https://gitlab.com/tarantool/tarantool/-/pipelines/225349795
> > 
> > @ChangeLog:
> > * Fixed address clashing occurring while __call metamethod dispatching
> >   (gh-4518, gh-4649).
> > 
> > Unfortunately, there is neither test nor reproducer for this failure, so
> > we'll know that the patch works only on production installations.
> > 
> 
> <snipped>
> 
> > 
> 
> -- 
> Best regards,
> IM