[Lightning] Re: `pushr' on SPARC

[Ludovic Courtès]

Paolo Bonzini <address@hidden> writes:

>>   2. Include a fixed pushr/popr.
>>
>> While (2) incurs some overhead, I don't clearly understand the impact of
>> larger stack frames.  Are you concerned by cache misses?  Keep in mind
>> that lightning 1.2 allocated 120 octets per stack frame (it could have
>> allocated only 104) while this fix allocates 104 + 32 = 136 octets (that
>> is, "only" 16 additional octets compared to 1.2).
>>
> Then we could make the stack area only 16 bytes (4 words) and I would
> be forced to shut up. :-)

Indeed!  ;-)

BTW, you did not tell what your major concern is wrt. large stack
frames.

> Your solution may still not work if push
> and pop are in different basic blocks (e.g. push before a conditional
> jump, pop in both branches of the jump)

Hmmm, good point.

>> Solution (2b) could be a nice compromise, provided we clearly recommend
>> against using both `allocai' and `pushr' (which is needed anyway for the
>> other architectures, according to your previous post).
>>
> Yes.

So which one would you prefer?  Given the limitations of both, I have no
strong preference.  Maybe (2b) would be "less bad" since it would allow
legacy (i.e., pre-allocai) programs to work as expected.

I'll look at implementing it over the next few days.

Thanks,
Ludovic.