Re: [Lightning] jit_qdivr_u trashes JIT_R0 on x86_64

[Paul Cercueil]

Le mer. 28 août 2019 à 18:15, Paulo =?iso-8859-1?b?Q+lzYXI=?= Pereira 
de Andrade <address@hidden> a écrit :
> Em qui, 22 de ago de 2019 às 16:24, Paul Cercueil 
> <address@hidden> escreveu:
> [...]
> >  This is with the latest master. Good job on the RISC-V port btw :)
> Thanks :)
>
> [...]
> >  I call jit_qdiv(reg1, reg2, reg2, reg1)
> >  (and also jit_qdiv_u with the same arguments). reg1 and reg2 cannot
> >  represent the same register.
>
>   Ok. I see it can fail on some ports, actually, probably only on x86.
> I will add extra tests for these conditions, and fix any register 
> clobbers
> that should be being generated.
>   Note that by doing the way you described, it might cause lightning 
> to
> generate code to get at least one extra temporary depending on 
> registers
> input. It might look clean in the lightning input, but may generate
> significantly larger code due to moves to/from %rax and %rdx, as well 
> as
> spill reloads.

Since I have my own register allocator on top of Lightning, sometimes 
in a function all the registers offered by Lightning have been written 
to, and I guess there's no way for Lightning to know which registers 
are safe to use without spilling. A good improvement would be to have 
an API function to mark a register as freely usable.

Thanks,
-Paul