[RL78] Questions about code-generation

[Richard Hulme]

On 22/03/14 01:47, Jeff Law wrote:
> On 03/21/14 18:35, DJ Delorie wrote:
>>
>> I've found that "removing uneeded moves through registers" is
>> something gcc does poorly in the post-reload optimizers.  I've written
>> my own on some occasions (for rl78 too).  Perhaps this is a good
>> starting point to look at?
>>
>>> much needless copying, which strengthens my suspicion that it's
>>> something in the RL78 backend that needs 'tweaking'.
>>
>> Of course it is, I've said that before I think.  The RL78 uses a
>> virtual model until reload, then converts each virtual instructions
>> into multiple real instructions, then optimizes the result.  This is
>> going to be worse than if the real model had been used throughout
>> (like arm or x86), but in this case, the real model *can't* be used
>> throughout, because gcc can't understand it well enough to get through
>> regalloc and reload.  The RL78 is just to "weird" to be modelled
>> as-is.
>>
>> I keep hoping that gcc's own post-reload optimizers would do a better
>> job, though.  Combine should be able to combine, for example, the "mov
>> r8,ax; cmp r8,#4" types of insns together.
> The virtual register file was the only way I could see to make RL78
> work.  I can't recall the details, but when you described the situation
> to me the virtual register file was the only way I could see to make the
> RL78 work in the IRA+reload world.
>
> What would be quite interesting to try would be to continue to use the
> virtualized register set, but instead use the IRA+LRA path.  Presumably
> that wouldn't be terribly hard to try and there's a reasonable chance
> that'll improve the code in a noticeable way.

Looking at how that's done by other backends, as far as I can tell, I 
just need to add something like:

#undef  TARGET_LRA_P
#define TARGET_LRA_P rl78_enable_lra

static bool
rl78_enable_lra (void)
{
   return true;
}

to rl78.c?  At least in theory, even if other work is needed elsewhere 
to make things run smoothly.

Unfortunately, that function never seems to be called.

How does TARGET_LRA_P get used, anyway?  I can't find anything that 
tries to use it, only places where it gets set.  Is there some funky 
preprocessor stuff going on that's stopping me grepping for it?

> The next obvious thing to try, and it's probably a lot more work, would
> be to see if IRA+LRA is smart enough (or can be made so with a
> reasonable amount of work) to eliminate the virtual register file
> completely.
>
> Just to be clear, I'm not planning to work on this; my participation and
> interest in the RL78 was limited to providing a few tips to DJ.

And from my side, I'm not trying to get anyone to work on it (though 
obviously I'm not averse to it).  I'm just looking for hints and tips so 
that I can try to understand the causes (and hopefully find some solutions).

Regards,

Richard.