[Mesa-dev] [PATCH 2/2] mesa: Speedup the xrgb -> argb special case in fast_read_rgba_pixels_memcpy

[Ian Romanick]

On 03/11/2013 11:30 AM, Jose Fonseca wrote:
> ----- Original Message -----
>> On 03/11/2013 07:56 AM, Jose Fonseca wrote:
>>> I'm surprised this is is faster.
>>>
>>> In particular, for big things we'll be touching memory twice.
>>>
>>> Did you measure the speed up?
>>
>> The second hit is cache-hot, so it may not be too expensive.
>
> Yes, but the size in question is 1900x1200, ie, 9MB, which will trash L1-L2 caches, and won't even fit on the L3 cache of several processors.

But it's doing it line-by-line, right?  So 1900 * 4bpp is only ~8kb.

> I'm afraid we'd be optimizing some cases at expense of others.

That is probably true either way.  To optimize this for everything, we'd 
need a lot more tests.

> I think that at very least we should do this in 16KB/32KB or so chunks to avoid trashing the lower level caches.
>
>> I suspect
>> memcpy is optimized to fill the cache in a more efficient manner than
>> the old loop.  Since the old loop did a read and a bit-wise or, it's
>> also possible the compiler generated some really dumb code.  We'd have
>> to look at the assembly output to know.
>>
>> As Patrick suggests, there's probably an SSE2 method to do this even
>> faster.  That may be worth investigating.
>
> An SSE2 is quite easy with intrinsics:
>
>    _m128i pixels = _mm_loadu_si128((const __m128i *)src); // could use _mm_load_si128 with some checks
>    pixels = _mm_or_si128(pixels, _mm_set1_epi32(0xff000000));
>    _mm_storeu_si128((__m128i *)dst, pixels);
>    src += sizeof(__m128i) / sizeof *src;
>    dst += sizeof(__m128i) / sizeof *dst;
>
> the hard part is the runtime check for sse2 support...

We could start by doing something like this for 64-bit builds.  SSE2 is 
always available there. :)  If we're using the intrinsics anyway, it's 
probably even better to use PREFETCHNTA on the read.

Mesa has some code for detecting CPU capabilities, but I don't think it 
has been updated in ages...  It looks like src/mesa/x86/common_x86.c 
detects MMX and SSE, but there's no code for anything after that.

> Jose