[llvm-dev] [RFC][SLP] Let's turn -slp-vectorize-hor on by default

[Nadav Rotem via llvm-dev]

> On Nov 10, 2015, at 6:49 AM, Charlie Turner <charlesturner7c5 at gmail.com> wrote:
> 
>> Out of curiosity, how much of the compile time are we spending in the SLP vectorizer nowadays ?
> 
> My measurements were originally based off the "real time" reports from
> /usr/bin/time (not the bash built-in), so I didn't have per-pass
> statistics to hand. I did a quick experiment in which I compiled each
> of the SPEC files with opt's -time-passes feature.
> 
> The "raw" numbers show that SLP can take anywhere from 0 to 30% of the
> total optimization time. At the high end of that scale, things are a
> bit fast and loose. Some of the biggest offenders are in rather small
> bitcode files (where the total compile time is getting very small as
> well)
> 
> The largest bitcode file[*] I had in SPEC2006 was about 1MiB. For that
> particular example, SLP took less than 1% of the opt time.
> 
> For all bitcode files in SPEC2006 between 100KiB and 1MiB, SLP takes
> less than 5% of compile time.
> 
> In tensor.bc (~ 80KiB) from SPEC2006, SLP took around 9.5% (+- 1%).
> This was a borderline case of a compile-time impact with horizontal
> reductions (about a 0.8% regression, so within stddev). There were
> actually swings the other way as well (i.e., SLP slower without
> horizontal reduction detection, so it's hard to make any judgment
> here)
> 
> Another pretty interesting one is fnpovfpu.bc (~ 40KiB), where SLP
> took 17% of compile time.

Thanks for the detailed analysis Charlie. We should probably look into fnpovfpu.bc and figure out what’s going on there. Overall I think that the compile time numbers are reasonable. 

> 
> Anyway, I hope that gives a rough impression of what's going on. I was
> taking the wall clock time measurement from -time-passes.
> 
> [*] I screwed up initially not reporting the overall compile time in
> my haste, so as a proxy metric, I went back and collected bitcode file
> sizes, which saved me from having to rerun everything :/