On Tue, Nov 22, 2016 at 6:52 PM, Yuri Gribov <tetra2...@gmail.com> wrote: > Hi all, > > I've recently revisited an ancient patch from Paolo > (https://gcc.gnu.org/ml/gcc-patches/2004-04/msg00551.html) which uses > asserts as optimization hints. I've rewritten the patch to be more > stable under expressions with side-effects and did some basic > investigation of it's efficacy. > > Optimization is hidden under !defined NDEBUG && defined > __ASSUME_ASSERTS__. !NDEBUG-part is necessary because assertions often > rely on special !NDEBUG-protected support code outside of assert > (dedicated fields in structures and similar stuff, collectively called > "ghost variables"). __ASSUME_ASSERTS__ gives user a choice whether to > enable optimization or not (should probably be hidden under a friendly > compiler switch e.g. -fassume-asserts). > > I do not have access to a good machine for speed benchmarks so I only > looked at size improvements in few popular projects. There are no > revolutionary changes (0.1%-1%) but some functions see good reductions > which may result in noticeable runtime improvements in practice. One > good example is MariaDB where you frequently find the following > pattern: > struct A { > virtual void foo() { assert(0); } > }; > ... > A *a; > a->foo(); > Here the patch will prevent GCC from inlining A::foo (as it'll figure > out that it's impossible to occur at runtime) thus saving code size. > > Does this approach make sense in general? If it does I can probably > come up with more measurements. > > As a side note, at least some users may consider this a useful feature: > http://www.nntp.perl.org/group/perl.perl5.porters/2013/11/msg209482.html
You should CC relevant maintainers or annotate the subject -- this is a C/C++ frontend patch introducing __builtin_has_side_effects_p plus a patch adding a GCC supplied assert.h header. Note that from a distribution point of view I wouldn't enable assume-asserts for a distro-build given the random behavior of __builtin_unreachable in case of assert failure. Richard. > -I
