Re: PPC64 libmvec sincos/sincosf ABI

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

On 8/8/19 10:25 AM, Bill Schmidt wrote:
> On 8/6/19 12:42 PM, Wilco Dijkstra wrote:
>> Hi,
>>
>>> 1. What is the best vector ABI (best performance) for sincos on PPC64?  
>>> That may be a function of the particular vector instructions available on 
>>> PPC64; the best choice of ABI on PPC64 need not correspond to the best 
>>> choice on x86_64.
>> I don't think it is related to the target - the fastest ABI is one that avoids
>> unnecessary work. For example scalar sincos is slow due to the inefficient
>> ABI which forces the results through memory (fixing that gives a 50% speedup). 
>>
>> Similarly for the vector ABI I think returning 2 vectors in registers will be the
>> fastest option in all cases. The actual vector instructions shouldn't affect the
>> ABI beyond the vector widths that can be supported.
>>
>> Wilco
>>
> Let me jump in here to answer a general question that I think Bert has
> had for a while.
>
> For the PPC64LE ABI, we should be returning everything through registers
> wherever possible.  The ABI supports multiple return values of the same
> type (up to 8 vector return values, for example), using the same
> registers used for passing parameters.  For simplicity in this example,
> I'll use the AltiVec-style types (vector double), but this works
> identically if you use more generically defined vector types.
>
> #include <altivec.h>
>
> struct sincosret
> {
>     vector double sinvals;
>     vector double cosvals;
> };
>
> struct sincosret
> mysincos (vector double a)
> {
>     struct sincosret scr;
>     scr.sinvals = a+a;  // May be slightly incorrect
>     scr.cosvals = a*a;  // Ditto
>     return scr;
> }
>
> This will result in the values being returned in VR2 and VR3:
>
>     xvmuldp 35,34,34
>     xvadddp 34,34,34
>     blr
>
> This is preferable to returning values indirectly through memory, which
> on older POWER processors can result in stalls from the store and load
> being too close together and possibly executed out of order.  The cost
> is pretty much negligible compared to the cost of computing sin/cos, but
> we might as well do it the best way that the ABI provides.

Important caveat to the above.  This is the ELFv2 ABI, used for
little-endian.  For the older ELFv1 ABI, the returned values will still
go through memory.

This doesn't restrict us from supporting ELFv1, but we just won't get
the benefit.

Thanks,
Bill
>
> Now, as I've said elsewhere, dealing with sincos in the -mveclibabi
> framework in GCC may be less than straightforward, due to the different
> description of the output types, but perhaps AArch64 has already laid
> some groundwork here.  I'm not up to date on the pending patches.
>
> Hope this helps,
> Bill
>