On Mon, Jul 1, 2019 at 11:58 PM Gary Oblock <gobl...@marvell.com> 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.
GCC performs intraprocedural constant propagation (IPA-CP) and this should catch your case already. The IPA-CP function cloning might have too constrained limits (on code bloat) to apply on a specific testcase but all functionality for the qsort case should be available. Richard. > Thanks, > > Gary Oblock
