[Bug rtl-optimization/63259] Detecting byteswap sequence

["olegendo at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63259

--- Comment #17 from Oleg Endo <olegendo at gcc dot gnu.org> ---
(In reply to thopre01 from comment #16)
> 
> Did we? All I can find is you and Andreas mentionning that it should work
> because it will be sign extended to int when doing the bitwise AND with
> 0xFF00.
> 
> What did I miss?

Ah sorry ... me bad.  I haven't tried increasing the 'level' with the bswaphi
expander pattern in place.  Will do that later.

> > > If you have a bswap instruction it seems better to define a pattern for that
> > > which the expander will use. That's the job of the bswap pass to detect a
> > > bswap, it shouldn't be done by combine.
> > 
> > The combine parts I was talking about are to eliminate the anding and oring
> > of higher 16 bits when a 16 bit byte swap is done on a 32 bit value.
> 
> I'm surprised that it's not the semantic of a bswaphi.

As far as I know...
bswaphi's inputs/output modes are HImode and it operates on the whole reg, not
on a subreg.  If applied to a SImode subreg the other bits need to be
saved/restored, which is what happens wiit the anding/oring.