[llvm] r200886 - Inliner uses a smaller inline threshold for callees with cold attribute.

[Chandler Carruth]

On Wed, Feb 5, 2014 at 3:58 PM, Manman Ren <manman.ren at gmail.com> wrote:

> I thought it is straight-forward to use a lower threshold for cold
> functions and didn't think this patch is going to have a wide & dramatic
> impact.
>
> I am not aware of any benchmark in lnt that uses cold attribute. If there
> is one, please let me know so I can collect some performance number.
>

Surely there are benchmarks with more representative code than SPEC? Maybe
not. There is also no cold attribute in the test suite.


>
> When Diego first introduced the Cold attribute and used it to adjust block
> weights, there was no request on benchmark numbers. So I assumed negligible
> performance impact for this patch.
>

Sure, but there wasn't any use of cold in LLVM before. Also, changing
inliner heuristics has an *incredibly* broader impact. It literally has the
largest impact in the entire compiler.

Notably, his change didn't introduce any particularly interesting
thresholds. It reused existing thresholds for __builtin_expect IIRC.

There at least needs to be *some* science behind the use of 75 for the
magical threshold here. When setting the other thresholds both Jakob and I
would run large benchmark suites across a wide range of values and try to
dig out the shape of the graph.


>
> Sorry if this caused regressions. The main purpose of this is performance
> of instrumentation based PGO.
>

I understand that this is the intent, but the whole point of PGO just
feeding the existing ways to represent things in the IR is because we
already use those when we *don't* have a profile, and any optimizations
will apply to both.


I'd like to see some numbers behind the use of "75" in the inliner. Until
then, I'd ask to keep the actual threshold unchanged and just introduce the
flag so that folks can conduct experiments to collect this data. Then you
can send a request to the mailing list that folks conduct such experiments
and collect feedback from the community. This pattern has worked extremely
well recently as Arnold has added new features and tuned thresholds in the
vectorizer. I think the inliner deserves the same or more scrutiny.
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-commits/attachments/20140205/290d2cee/attachment.html>