On 7/1/19 3:08 PM, Jeff Law wrote: > External Email > > ---------------------------------------------------------------------- > 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 > Jeff,
I assume you mean Jan Hubicka? I'll certainly have a look at the code dump you mention. I do have a high level design in mind already but I'm always up for making my life easier. Thanks, Gary
