Re: [PING][RFC] Assertions as optimization hints

[Allan Sandfeld Jensen <linux at carewolf dot com>]

On Wednesday 23 November 2016, Richard Biener wrote:
> On Tue, Nov 22, 2016 at 6:52 PM, Yuri Gribov <tetra2005@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.
> 
One option could be to provide such behaviour as new builtins, to be used for 
GSL implementations of Expects() and Ensures(). See 
https://github.com/isocpp/CppCoreGuidelines/blob/master/CppCoreGuidelines.md 
and https://github.com/Microsoft/GSL/blob/master/gsl/gsl_assert

But I think using the data from asserts should be safe and useful too, though 
there might be problems here and there with assert that only makes sense as 
#ifndef(NDEBUG) builds.

`Allan