Re: [PATCH, rs6000] Prefer vspltisw/h over xxspltib+instruction when available

[Bill Schmidt <wschmidt@linux.vnet.ibm.com>]

> On Jun 21, 2016, at 5:34 PM, Segher Boessenkool <segher@kernel.crashing.org> wrote:
> 
> On Tue, Jun 21, 2016 at 03:14:51PM -0500, Bill Schmidt wrote:
>> I discovered recently that, with -mcpu=power9, an attempt to generate a vspltish instruction resulted instead in an xxspltib followed by a vupkhsb.  This is semantically correct but the extra instruction is not optimal.  I found that there was some logic in xxspltib_constant_p to do special casing for const_vector with small constants, but not for vec_duplicate with small constants.  This patch duplicates that logic so we can generate the single instruction when possible.
> 
> This part is okay.
> 
>> When I did this, I ran into a problem with an existing test case.  We end up matching the *vsx_splat_v4si_internal pattern instead of falling back to the altivec_vspltisw pattern.  The constraints don't match for constant input.  To avoid this, I added a pattern ahead of this one that will match for VMX output registers and produce the vspltisw as desired.  This corrected the failing test and produces the expected code.
> 
> Why does the predicate allow constant input, while the constraints do not?

I have no idea why it was built that way.  The predicate seems to provide for all sorts of things, but this and the subsequent pattern both handle only a subset of the constraints implied by it.  To be honest, I didn't feel competent to try to fix the existing patterns.  Do you have any suggestions for what to do instead?

Thanks!
Bill

> 
>> I've added a test case to demonstrate the code works properly now in the usual case.
> 
> Thanks :-)
> 
> 
> Segher
>