On 7/1/19 5:01 PM, Gary Oblock wrote: > 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? Yes.
Jeff
