Re: Unaligned load/store opcodes

[Paul Cercueil]

Hi Paulo,

Le mercredi 05 avril 2023 à 13:08 -0300, Paulo César Pereira de Andrade
a écrit :
> Em qui., 23 de mar. de 2023 às 08:07, Paul Cercueil
> <paul@crapouillou.net> escreveu:
> > 
> > Hi Paulo,
> 
>   Hi Paul,
> 
> > I think Lightning would benefit from having support for 16/32/64-
> > bit
> > I/O to unaligned addresses. That's something I would actually use.
> > 
> > Something like:
> > ldur_s / ldur_us / ldur_i / ldur_ui / ldur_l
> > stur_s / stur_i / stur_l
> > 
> > I don't think we need ldx/stx variants.
> 
>   Please check the new instructions (not yet properly documented):
> 
> unldr r0 r1 i0 -- r0 = *(signed_i0_size*)r1
> unldi r0 i1 -- r0 = *(signed_i1_size*)i0
> unldr_u r0 r1 i0 -- r0 = *(unsigned_i0_size*)r1
> unldi_u r0 i1 -- r0 = *(unsigned_i1_size*)i0
> unstr r0 r1 i0 -- *(i0_size_integer*)r0 = r1
> unsti i0 r0 i1 -- *(i1_size_integer*)r0 = r0
> 
> valid size values are 1 to wordsize. This means it can help for
> integers
> with a size other than 1, 2, 4 or 8.
> 
> unldr_x r0 r1 i0 -- r0 = *(i0_size_float*)r1
> unldi_x r0 i0 i1 -- r0 = *(i1_size_float*)i0
> unstr_x r0 r1 i0 -- *(i0_size_float*)r0 = r1
> unstr_x i0 r0 i1 -- *(i1_size_float*)i0 = r0
> 
> valid size values are 4 and 8. There is no plan, but it is possible
> to
> add support for soft floats of a size other than 4 or 8. 1 and 2 byte
> floats could be very useful as a way to use less memory for low
> precision data, but if ever added, it would be only for load/store,
> otherwise, would need a conversion to a higher sized value.
> 
> > What do you think?
> 
>   It should be in a functional state, but some optimizations can
> still be
> done, or some redesign might be done as well.

Just with a quick glance at the MIPS implementation, it looks much more
complex than it needs to be... You can do a 32-bit (or 64-bit)
unaligned read with just LWL+LWR (LDL+LDR respectively) then clear or
sign-extend the bits you're not interested in, I don't see why you need
a loop and bit shifts for.

I really won't have time for anything before next week, unfortunately.
I'll give it a proper test then.

Cheers,
-Paul