Re: SP-GiST micro-optimizations

On 28.08.2012 22:50, Ants Aasma wrote:
> On Tue, Aug 28, 2012 at 9:42 PM, Tom Lane<tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>> Seems like that's down to the CPU not doing "rep stosq" particularly
>> quickly, which might well be chip-specific.
>
> AMD optimization manual[1] states the following:
>
> For repeat counts of less than 4k, expand REP string instructions
> into equivalent sequences of simple
> AMD64 instructions.
>
> Intel optimization manual[2] doesn't provide equivalent guidelines,
> but the graph associated with string instructions states about 30
> cycles of startup latency. The mov based code on the other hand
> executes in 6 cycles and can easily overlap with other non-store
> instructions.
>
> [1] http://support.amd.com/us/Processor_TechDocs/25112.PDF
> [2] http://www.intel.com/content/dam/doc/manual/64-ia-32-architectures-optimization-manual.pdf

Hmm, sounds like gcc just isn't doing a very good job then. I also tried
replacing the memset with variable initialization: "spgChooseOut out = {
0 }" (and I moved that to where the memset was). In that case, gcc
produced the same (fast) sequence of movq's I got with
-mstringop=unrolled_loop.

Out of curiosity, I also tried this on clang. It produced this,
regardless of whether I used MemSet or memset or variable initializer:

pxor %xmm0, %xmm0
.loc 1 2040 4 # spgdoinsert.c:2040:4
movaps %xmm0, -1280(%rbp)
movaps %xmm0, -1296(%rbp)
movaps %xmm0, -1312(%rbp)

So, it's using movaps to clear it in 16-byte chunks. perf annotate shows
that that's comparable in speed to the gcc's code produced for MemSet.

--
Heikki Linnakangas
EnterpriseDB http://www.enterprisedb.com