Prefer open-coding vector integer division

[Matthew Fortune]

Richard Sandiford <richard.sandiford@linaro.org> writes:
> vect_recog_divmod_pattern currently bails out if the target has
> native support for integer division, but I think in practice
> it's always going to be better to open-code it anyway, just as
> we usually open-code scalar divisions by constants.
> 
> I think the only currently affected target is MIPS MSA, where for:
> 
>   void
>   foo (int *x)
>   {
>     for (int i = 0; i < 100; ++i)
>       x[i] /= 2;
>   }
> 
> we previously preferred to use division for powers of 2:
> 
>         .set    noreorder
>         bnz.w   $w1,1f
>         div_s.w $w0,$w0,$w1
>         break   7
>         .set    reorder
> 1:
> 
> (or just the div_s.w for -mno-check-zero-division), but after the patch
> we open-code them using shifts:
> 
>         clt_s.w $w1,$w0,$w2
>         subv.w  $w0,$w0,$w1
>         srai.w  $w0,$w0,1
> 
> I assume that's better.  Matthew, is that right?

Sorry for extreme tardiness. Yes, the alternate sequence has a max latency
of 6. Although I don't have the range of latencies to hand for the FDIV, as
far as I remember 6 cycles is better than the fastest FDIV case at least for
i6400/i6500.

Matthew