[Haskell-cafe] ghc-prof-options and libraries on Hackage

[John Lato]

On Mon, Sep 22, 2014 at 4:52 AM, Eric Seidel <eseidel at cs.ucsd.edu> wrote:

> > To provide an example, I'm currently working on a little game engine that
> > uses JuicyPixels to load images. I have a problem in my code that needs
> > optimizing, but the current state of things results in profiles that are
> > very difficult to work with. JuicyPixels specifies -auto-all in its cabal
> > file, which means I have no alternative but to profile JuicyPixels code.
> In
> > this scenario, the bottleneck is actually within my FRP game loop and
> > nothing to do with image loading! As a result, the profiles are fairly
> > useless to me.
>
> While in this case the extra profiling data may be useless, it seems to me
> that in general you won't know a priori. I would prefer to have as much
> data available as possible, and then filter it.
>
> HPC works sort of like this, it gathers coverage data for all of the
> modules compiled with -fhpc, but has command-line options to only report
> coverage for certain modules. Perhaps GHC's rts could add a similar flag to
> only report profiling data from certain modules or functions?


I am sympathetic to this concern, but the main problem here is that
-fprof-auto isn't free.  It can interfere quite heavily with optimization
passes, meaning that it ends up attributing much higher costs to certain
functions than they would have in a normal compilation path, actively
misleading users.  I for one would have no confidence that profiles were
pointing to the correct place if everything were built with -fprof-auto
from the start.  It's much better to start with just a few high-level
cost-centers and drill down from there.  This can be a tedious process, but
it's much more reliable.  As an alternative, compiling libraries with
-fprof-auto-exported is fairly reasonable IMHO.

John L.
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