[Bug rtl-optimization/63259] Detecting byteswap sequence

["thopre01 at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

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

--- Comment #16 from thopre01 at gcc dot gnu.org ---
(In reply to Oleg Endo from comment #15)
> (In reply to thopre01 from comment #14)
> 
> > You mean with the added bswaphi2 pattern the pattern is still unchanged?
> > 
> 
> After adding bswaphi2, the bswap pass does the transformation.  Except for
> the non-working 'signed short' mentioned above.  But we already figured that
> out earlier.

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?

> 
> The expected sequence for the function above is:
> 
>         rts
>         swap.b  r4,r0
> 
> i.e. no anding and oring of lower/higher 16 bit word, since the swap.b insn
> operates on a SImode value and does not alter the high 16 bits.

Oh yeah right.

> 
> > 
> > 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.

Best regards,

Thomas