This is the mail archive of the mailing list for the GCC project.

Index Nav: [Date Index] [Subject Index] [Author Index] [Thread Index]
Message Nav: [Date Prev] [Date Next] [Thread Prev] [Thread Next]
Other format: [Raw text]

[PING][RFC] Assertions as optimization hints

Hi all,

I've recently revisited an ancient patch from Paolo
( 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
  struct A {
    virtual void foo() { assert(0); }
  A *a;
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:


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

Index Nav: [Date Index] [Subject Index] [Author Index] [Thread Index]
Message Nav: [Date Prev] [Date Next] [Thread Prev] [Thread Next]