[Lightning] Re: `pushr' on SPARC

[Paolo Bonzini]

>   1. Keep the non-working pushr/popr.
>
> Solution (1) seems quite harmful since the resulting programs produce
> erroneous results.
>   
(No more than previous versions, but I agree that it is not nice.)
>   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. :-)  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), but it would work in 99% of the cases, 
though, and this limitation may be documented in the NEWS file together 
with the deprecation of pushr/popr.
> 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.

Paolo