On Fri, May 12, 2017 at 1:27 AM, Ehsan Akhgari <ehsan.akhg...@gmail.com> wrote: > I realized we haven't had a performance mini-story for a while -- I sort of > dropped the ball on that. Running over this bug made me want to talk about > a pretty well known sort of slowness in C++ code, virtual functions. The > cost of virtual functions comes from several different aspects, firstly they > effectively prevent the compiler from doing inlining the function which > enables a host of compiler optimizations, essentially by enabling the > compiler to see more of the code and optimize more effectively based on > that. But then there is the runtime cost of the function, which mostly > comes from the indirect call. The majority of the performance penalty here > on modern hardware is due to branch midpredictions when different > implementations of a virtual function get called at a call site. You should > remember that on modern desktop processors, the cost of a branch > misprediction can be around 15-20 cycles (depending on the processor) so if > what your function does is very trivial and it has many overrides that can > be called in hot code chances are that you are spending a considerable > amount of time waiting for the instruction cache misses on the calls to the > virtual function in question. Of course, finding which virtual functions in > your program are these expensive ones requires profiling the workloads you > care about improving, but always keep an eye for this problem as > unfortunately the object-oriented programming model in C++ really encourages > writing code like this. This is the kind of issue that a native profiler is > probably more suitable for discovering, for example if you are using a > simple native sampling profiler these issues typically show up as a long > amount of time being spent on the first instruction of the virtual function > being called (which is typically an inexpensive instruction otherwise.)
This reminded me of https://bugzilla.mozilla.org/show_bug.cgi?id=1332680 (and https://bugzilla.mozilla.org/show_bug.cgi?id=1332682 ) Adding -Wsuggest-final-types and -Wsuggest-final-methods and looking at the output seems pretty low-effort to find a lot of more opportunities for this. (Unless I'm misunderstanding things). Plus, it benefits security! -tom _______________________________________________ dev-platform mailing list dev-platform@lists.mozilla.org https://lists.mozilla.org/listinfo/dev-platform
