Re: Optimizing byte-swap-and-store on PPC

[Paul Cercueil]

Le jeudi 09 février 2023 à 09:49 -0300, Paulo César Pereira de Andrade
a écrit :
> Em qui., 9 de fev. de 2023 às 08:18, Paul Cercueil
> <paul@crapouillou.net> escreveu:
> > 
> > Hi Paulo,
> 
>   Hi Paul,
> 
> > If you remember, I added an optimization (for my use case anyway)
> > for
> > the following code:
> > 
> > jit_ldr(rY, rX);
> > jit_bswapr(rY, rY);
> > 
> > Lightning will only generate a single LWBRX instruction for this.
> > 
> > Now I would like to do the same for stores. The problem is that the
> > pattern typically use a temporary register T:
> > 
> > jit_bswapr(T, rX);
> > jit_str(rY, T);
> > 
> > How can I know that the register T is dead after this pattern?
> 
>   Depends a bit on how T value is resolved.
> 
>   If from jit_get_reg() it is live until the jit_unget_reg() call.
> You
> should avoid branches that cannot be followed in such cases.
> 
>   Bellow is explanation when an explicit value, not from
> jit_get_reg().

In my case it is with an explicit value, yes.

>   It is considered live in the range it is set until the last use of
> the
> register.
>   It is considered dead in the range from last use (but not set)
> until the next value change. Branches might cause it to be in
> a state merged as live due to use in other paths.
>   The liveness range is broken if it is not a callee save register
> and when following the code it finds a function call or an indirect
> branch to a register or non jit address.
>   If it is a callee save register it is always considered live.

I myself do know when the registers are dead, if I look at the code -
that was not my question. What I'm wondering, is whether it is possible
to know if a given public register (as in JIT_V/JIT_R) is dead, at a
specific point in the code generation step within Lightning itself.

Something like a (private)
"bool reg_is_dead(jit_node_t node, jit_int32_t reg)" function.

The code generator for "jit_str" (and related) would then convert the
jit_bswapr(T, rX) + jit_str(rY, T) to a single "STBRX rY, rX"
instruction, if we know that T is dead after this point.

>   There isn't an explicit jit_dead() call because most times it would
> allow creating bugs that are difficult to debug. There is only a
> jive_live() call to allow telling a non callee save register is live
> at the point of an indirect branch or at a label entry, usually
> a point where an indirect branch lands, or some function returns
> a value in a non standard way.
> 
>   The easiest way to see what is happening is call jit_print() and
> see the state it prints at label entry point.
> 
>   If this reply is not clear, please provide some sample example
> code or jit_print() output.
> 
> > Cheers,
> > -Paul
> 
> Thanks,
> Paulo

Cheers,
-Paul