RFC on a new optimization

[Jeff Law]

On 7/1/19 3:58 PM, Gary Oblock wrote:
> I've been looking at trying to optimize the performance of code for
> programs that use functions like qsort where a function is passed the
> name of a function and some constant parameter(s).
> 
> The function qsort itself is an excellent example of what I'm trying to show
> what I want to do, except for being in a library, so please ignore
> that while I proceed assuming that that qsort is not in a library.  In
> qsort the user passes in a size of the array elements and comparison
> function name in addition to the location of the array to be sorted. I
> noticed that for a given call site that the first two are always the
> same so why not create a specialized version of qsort that eliminates
> them and internally uses a constant value for the size parameter and
> does a direct call instead of an indirect call. The later lets the
> comparison function code be inlined.
> 
> This seems to me to be a very useful optimization where heavy use is
> made of this programming idiom. I saw a 30%+ overall improvement when
> I specialized a function like this by hand in an application.
> 
> My question is does anything inside gcc do something similar? I don't
> want to reinvent the wheel and I want to do something that plays
> nicely with the rest of gcc so it makes it into real world. Note, I
> should mention that I'm an experienced compiler developed and I'm
> planning on adding this optimization unless it's obvious from the
> ensuing discussion that either it's a bad idea or that it's a matter
> of simply tweaking gcc a bit to get this optimization to occur.
Jan is the expert in this space, but yes, GCC has devirtualization and
function specialization.  See ipa-devirt.c and ipa-cp.c  You can use the
-fdump-ipa-all-details option to produce debugging dumps for the IPA
passes.  THat might help guide you a bit.


jeff