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Re: [5/9] Main target-independent support for direct interleaving
Richard Guenther <richard.guenther@gmail.com> writes:
> On Mon, Apr 18, 2011 at 1:24 PM, Richard Sandiford
> <richard.sandiford@linaro.org> wrote:
>> Richard Guenther <richard.guenther@gmail.com> writes:
>>> On Tue, Apr 12, 2011 at 3:59 PM, Richard Sandiford
>>> <richard.sandiford@linaro.org> wrote:
>>>> Index: gcc/doc/md.texi
>>>> ===================================================================
>>>> --- gcc/doc/md.texi   2011-04-12 12:16:46.000000000 +0100
>>>> +++ gcc/doc/md.texi   2011-04-12 14:48:28.000000000 +0100
>>>> @@ -3846,6 +3846,48 @@ into consecutive memory locations. ÂOper
>>>> Âconsecutive memory locations, operand 1 is the first register, and
>>>> Âoperand 2 is a constant: the number of consecutive registers.
>>>>
>>>> +@cindex @code{vec_load_lanes@var{m}@var{n}} instruction pattern
>>>> +@item @samp{vec_load_lanes@var{m}@var{n}}
>>>> +Perform an interleaved load of several vectors from memory operand 1
>>>> +into register operand 0. ÂBoth operands have mode @var{m}. ÂThe register
>>>> +operand is viewed as holding consecutive vectors of mode @var{n},
>>>> +while the memory operand is a flat array that contains the same number
>>>> +of elements. ÂThe operation is equivalent to:
>>>> +
>>>> +@smallexample
>>>> +int c = GET_MODE_SIZE (@var{m}) / GET_MODE_SIZE (@var{n});
>>>> +for (j = 0; j < GET_MODE_NUNITS (@var{n}); j++)
>>>> + Âfor (i = 0; i < c; i++)
>>>> + Â Âoperand0[i][j] = operand1[j * c + i];
>>>> +@end smallexample
>>>> +
>>>> +For example, @samp{vec_load_lanestiv4hi} loads 8 16-bit values
>>>> +from memory into a register of mode @samp{TI}@. ÂThe register
>>>> +contains two consecutive vectors of mode @samp{V4HI}@.
>>>
>>> So vec_load_lanestiv2qi would load ... ? Âc == 8 here. ÂIntuitively
>>> such operation would have adjacent blocks of siv2qi memory. ÂBut
>>> maybe you want to constrain the mode size to GET_MODE_SIZE (@var{n})
>>> * GET_MODE_NUNITS (@var{n})? ÂIn which case the mode m is
>>> redundant? ÂYou could specify that we load NUNITS adjacent vectors into
>>> an integer mode of appropriate size.
>>
>> Like you say, vec_load_lanestiv2qi would load 16 QImode elements into
>> 8 consecutive V2QI registers. ÂThe first element from register vector I
>> would come from operand1[I] and the second element would come from
>> operand1[I + 8]. ÂThat's meant to be a valid combination.
>
> Ok, but the C loop from the example doesn't seem to match. Or I couldn't
> wrap my head around it despite looking for 5 minutes and already having
> coffee ;) I would have expected the vectors being in memory as
>
> v0[0], v1[0], v0[1], v1[1], v2[0], v3[1]. v2[1], v3[1], ...
>
> not
>
> v0[0], v1[0], v2[0], ...
>
> as I would have thought the former is more useful (simple unrolling for
> stride 2).
The second one's right. All lane 0 elements, followed by all lane 1
elements, etc. I think that's what the C loop says.
> We'd need a separate set of optabs for such an interleaving
> scheme? In which case we might want to come up with a more
> specific name than load_lane?
Yeah, if someone has a single instruction that does your first example,
then it would need a new optab. The individual vector pairs could be
represented using the current optab though, if each pair needs a
separate instruction. E.g. with your v2qi example, vec_load_lanessiv2qi
would load:
v0[0], v1[0], v0[1], v1[1]
and you could repeat for the others. So load_lanes (as defined here)
could be treated as a primitive, and your first example could be something
like "repeat_load_lanes".
If you don't like the name "load_lanes" though, I'm happy to use
something else.
Richard