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

[Igor Munkin]

Sasha,

On 04.12.20, Alexander V. Tikhonov wrote:
> Hi Igor, thanks for the patch, as I see no new degradations found [1],
> patch LGTM.

Great, thanks!

Kirill, please proceed with the patch.

> 
> [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

-- 
Best regards,
IM