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[RFC] Assertions as optimization hints


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

-I

Attachment: 0001-Optionally-treat-assertions-as-optimization-hints-dr.patch
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