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Re: [Lightning] More on work on lightning

From: Paolo Bonzini
Subject: Re: [Lightning] More on work on lightning
Date: Sun, 26 Sep 2010 09:22:37 +0200

2010/9/25 Paulo César Pereira de Andrade
>  How to get forward/context information?
> 2. Add a standard field to jit_state_t or jit_local_state to be filled
>   by the programmer

That's possible.  However, I think this does not belong in lightning
at all.  lightning users could do inlining at a high level to ensure
big enough subroutines are generated and the prolog overhead is not
important.  Register allocation could be done at a higher level too,
and so could constant propagation.

>    printf("%d\n", JIT_R0);
> =>
>    subi_l %sp %sp 12
>    str_i %sp %r0
>    movi_p %r0 "%d\n"
>    pushr_l %r0
>    calli @printf
>    addi %sp %sp 16
> it could have allocated stack in prolog for all calls done in
> the function, and the above could have became:
>    stxi_i 4 %sp %r0
>    movi_p %r0 "%d\n"
>    str_p %r0 %sp
>    calli @printf

As a code-size efficiency concern I can share it.  But regarding code
performance, I think there's hardly a difference between the two.

Using the alignment padding more efficiently however is a good idea.

> it is even more appealing when there are several sequential
> function calls (like in my interpreter that currently uses
> lightning mostly to glue calls to C functions).

Why not use regparm(3) calling convention on i386 instead?

>  Another issue is things like:
> foo(0,0,0,0);
> =>
>    prepare 4
>    xorr_i %r0 %r0
>    pushr_i %r0
>    xorr_i %r0 %r0
>    pushr_i %r0
>    xorr_i %r0 %r0
>    pushr_i %r0
>    xorr_i %r0 %r0
>    pushr_i %r0
>    finish @foo
>    addi_l %sp %sp 16
> but this requires significant extra information to understand
> what is going on.

You can do constant propagation at a higher level for this, but
however again I don't think there's anything to worry about regarding

>  Another somewhat unrelated comments is about the
> initial trigonometric functions using x87. Probably they
> are not (easily?) available on other cpus

Not at all, actually.  And the inter-unit moves probably make them not
so much faster compared to libc's sin, which you have to use anyway on

I'm sorry that I disagree on (almost) everything you wrote in this
mail, it's usually not the case. :)


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