[PATCH] Don't check for optab for 16bit bswap

[Richard Henderson]

On 01/21/2015 11:52 PM, Richard Biener wrote:
> On January 21, 2015 10:23:56 PM CET, Richard Henderson <rth@redhat.com> wrote:
>> On 12/29/2014 06:04 AM, Thomas Preud'homme wrote:
>>> Since 16bit byteswap can be done via an 8 bit rotation (and it is the
>> canonical form),
>>> the check for an optab only serves to prevent the bswap optimization
>> for
>>> targets that don't have a 16bit byteswap (but do have a rotation
>> instruction). See
>>> PR63259 (comments 6 and onwards) for more details.
>>
>> I question the choice to have rotate be the canonical form.
>>
>> Doesn't this make things more complicated for targets that
>> don't have rotate?  Or, equivalently, no 16-bit rotate?
>> That would seem to cover 99% of all 32-bit risc machines.
> 
> I was asking for the generic expander to consider bswapHI.  Rotates are
> certainly more likely to get combined with sth else.

Maybe.  Alternately, don't we canonicalize byte-swapping memory ops as

 (set (reg:HI) (bswap:HI (mem:HI)))

 (set (mem:HI) (bswap:HI (reg:HI)))

Certainly this is true for powerpc.

(It appears that both sparc and s390 are missing similar patterns,
despite having instructions that can perform this operation...)


r~


r~