[Libre-soc-dev] change sv ld/st to allow non-zero immediates for unit-strided load/store

Jacob Lifshay programmerjake at gmail.com
Wed Oct 12 00:35:18 BST 2022

On Tue, Oct 11, 2022, 16:08 lkcl <luke.leighton at gmail.com> wrote:

> On Tuesday, October 11, 2022, Jacob Lifshay <programmerjake at gmail.com>
> wrote:
> > even if we just enable it only for RA=r1 rather than having a separate
> mode bit, it is that important.
> it's simply not possible.  take a look at the page pseudocode.
> non-uniform behaviour is where the problems start.
> RA=r1 becoming "special"

r1 is already special because it's the stack pointer, rendering strided
load/store with a base of r1 completely useless and wasted. see risc-v's c
extension (and arm's thumb) for examples of stack-pointer-specific

will be hated universally by
> the compiler teams.

they will hate even more not having efficient spill/fill ops that don't
require a temporary address register (which causes even more problems for
register allocation because spills/fills are generated precisely when you
just ran out of registers, so needing *even more* registers for all the
address calculation isn't helpful).


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