[Bug target/77308] surprisingly large stack usage for sha512 on arm

[wilco at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=77308

--- Comment #47 from wilco at gcc dot gnu.org ---
(In reply to Richard Earnshaw from comment #46)
> (In reply to wilco from comment #44)
> > (In reply to Bernd Edlinger from comment #38)
> > > Created attachment 39939 [details]
> > > proposed patch, v2
> > > 
> > 
> > > Unlike the previous patch, thumb1 stack usage stays at 1588 bytes,
> > > because thumb1 cannot split the adddi3 pattern, once it is emitted.
> > 
> > We can split into a new pattern that contains adds/adc together. Splitting
> > should help Thumb-1 the most as it has just 3 allocatable DI mode
> > registers...
> 
> Not on Thumb-1 we can't.  Because of register allocation limitations, we
> cannot expose the flags until after register allocation has completed. 
> (Since the register allocator needs to be able to insert loads, adds and
> copy instructions between any two insns.  The add and copy instructions
> clobber the flags, making early splitting impossible.

What I meant is splitting into a single new instruction using SI mode registers
rather than DI mode registers so that register allocation is more efficient.