Vectorised copy

Richard Sandiford richard.sandiford at linaro.org
Tue Sep 6 14:14:18 UTC 2011


Michael Hope <michael.hope at linaro.org> writes:
> While out benchmarking today, I ran across code similar to this:
>
> int *a;
> int *b;
> int *c;
>
> const int ad[320];
> const int bd[320];
> const int cd[320];
>
> void fill()
> {
>   for (int i = 0; i < 320; i++)
>     {
>       a[i] = ad[i];
>       b[i] = bd[i];
>       c[i] = cd[i];
>     }
> }
>
> I was surprised and happy to see the vectoriser kick in for the copy.
> The inner loop looks like:
>
> 	add	r5, r3, ip
> 	adds	r4, r3, r7
> 	vldmia	r2!, {d16-d17}
> 	vldmia	r1!, {d18-d19}
> 	adds	r0, r3, r6
> 	vst1.32	{q9}, [r5]
> 	vst1.32	{q8}, [r4]
> 	vldmia	r3, {d16-d17}
> 	adds	r3, r3, #16
> 	cmp	r3, r8
> 	vst1.32	{q8}, [r0]
> 	bne	.L3
>
> so r3 is the loop variable and {ip,r7} are the offsets from r3 to the
> destination pointers.  Adding a __restrict doesn't change the code.

FWIW, this comes from ivopts.  I raised the "problem" on gcc@
a few months back, but it seems to be intentional behaviour:

    http://gcc.gnu.org/ml/gcc/2011-07/msg00050.html

That is, all things being equal, the current code tends to prefer
cases where it can hoist the difference between potential ivs
rather than creating separate ivs.

As far as the end of today's meeting goes: ivopts is one of those
things on my unwritten list of areas that it would be nice to look at.
I posted some benchmark comparing -fivopts with -fno-ivopts to the
benchmark list in July.  As expected, ivopts does help a lot cases,
but there were also a fair number of cases where turning it off
significantly improved performance.

> Richard, will your auto-inc/dec changes combine the final vldmia r3,
> add r3 into a vldmia r3! ?

Yeah, it should do.

Richard



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