RE: Mode change for bswap pattern expansion

[Paulo Matos <pmatos@broadcom.com>]

> -----Original Message-----
> From: Richard Sandiford [mailto:rsandifo@linux.vnet.ibm.com]
> Sent: 27 January 2014 16:50
> To: Paulo Matos
> Cc: gcc@gcc.gnu.org
> Subject: Re: Mode change for bswap pattern expansion
>  
> Sorry, I meant we use an unspec for the first ("V2HI") rotate.
> I.e. rather than:
> 
>   (set (subreg:V2HI (match_dup 2) 0)
>        (rotate:V2HI (subreg:V2HI (match_dup 1) 0)
>                     (const_int 8)))
>   (set (match_dup 0)
>        (rotate:SI (match_dup 2)
>                   (const_int 16)))
> 
> we have:
> 
>   (set (match_dup 2) (unspec:SI [(match_dup 1)] UNSPEC_FOO))
>   (set (match_dup 0)
>        (rotate:SI (match_dup 2)
>                   (const_int 16)))
> 
> In your case the define_insn for the UNSPEC_FOO pattern would have the
> same attributes as a V2HI rotate, so it should get scheduled in the same way.
> 

In that case it would work. My only concern would then be if it prevents further optimizations. On the other hand I am not sure if GCC would try to optimize a rotate with vector V2HI mode...
Might give both solutions a try and see what results I get.

Thanks,

Paulo Matos

> Thanks,
> Richard