Re: [Tarantool-patches] [PATCH luajit] x86/x64: Check for jcc when using xor r, r in emit_loadi().

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

Hi, Igor!

Thanks for the review!

I've updated the commit message and the comment in the test considering
your proposal.

On 11.11.22, Igor Munkin wrote:
> Sergos,
> 
> On 06.07.22, sergos wrote:
> > Hi!
> > 
> > Thanks for the patch!
> > 

<snipped>

> > 
> > > 3) the comparison operation to perform a fused load, which in
> > >   turn needs to allocate a register, and for there to be no
> > >   free registers at that moment, and for the register chosen
> > >   for sacrifice to be holding the constant zero.
> > > 
> > Unfortunately, it’s not clear what this register (I suppose it’s r14d) is used for.
> > Is it an argument, needed after the fall-through in the same trace?
> > Why not it sank down below the branch? 
> > IIRC it is a dedicated register used for dispatch, so why is it used for sacrificing?
> 
> Like many things in LuaJIT register allocation is "state-of-the-art" (or
> as I called this "ad-hoc") entity. Since it's implemented as a linear
> scan RA working in a reverse direction of the trace being recorded,
> register is "allocated" for IR slot being used as a source and "freed"
> at the moment it becomes a destination.
> 
> To describe the process Sergey fit in a three-bullet list above, we have
> the following.
> 0) IR for either "internal" (e.g. type check, hmask check) or "external"
>    (e.g. branch or loop condition) guard is begin emitted to mcode.
> 1) JCC to side exit is emitted to the trace mcode at the beginning.
> 2) Condition (i.e. comparison) is going to be emitted.
> 3) Fuse optimization takes its place, that ought to allocate a register
>    for the load base.
> 4) There is no free registers at this point.
> 5) The one storing the constant zero is chosen to be sacrificed and
>    reallocated (consider allocation cost in ra_alloc for constant
>    materialization).
> 6) Before (or in the sense of trace execution, after) register is
>    being used in the aforementioned comparison, register (r14 in our
>    case) is reset by XOR emitted right after (before) jump instruction.
> 7) The comparison with fused load within is emitted.
> 
> As a result flags set by comparison are reset by XOR emitted in between
> of condition and jump instructions.
> 

Updated branch with force-push.

<snipped>

> 
> > > -- 
> > > 2.34.1
> > > 
> > 
> 
> -- 
> Best regards,
> IM

-- 
Best regards,
Sergey Kaplun