Hi,
Abstract :)
===========
A means to distinguish a patched GCC release from a vanilla GCC
release should be added. This would enable developers to work
around incompatibilities between GCC releases in public header
files. One macro, defined only by the respective distributor,
could uniquely identify the distribution and its version.
Problem
=======
As you're surely aware, most people don't actually use GCC as released by the
FSF (aka. "vanilla GCC") but use the versions as packaged by their
distribution (whether that be Linux, MacOS, or whatever else) (aka. "patched
GCC"). Often these patched GCCs include all patches from the SVN branch
available at the time of packaging + some distribution specific changes.
This leads to incompatiblities between GCC releases that all identify
themselves with the same version number. The most recent issue, which prompts
this mail, was the AVX maskstore interface change between 4.5.2 and 4.5.3 [1].
There are other conceivable examples; e.g. I experienced compilation errors
(failed inlining) with Fedora GCC, while the same versions of vanilla GCC
compiled my code just fine.
Ideally, a software project would be able to use configure checks and feature-
macros to work around those issues, and thus support all (vanilla and patched)
GCC releases. But it's not always that "easy". If the code in question resides
in a public header of a library, the feature-macros would have to be correctly
defined by every project that makes use of them, which is a major problem.
suggested solution
==================
GCC should provide (an) additional predefined macro(s) to distinguish a
patched GCC from vanilla GCC. This/These macro(s) should be sufficient to
uniquely identify every released GCC from each other. This must also include
updates to distribution packages, which could fix or introduce a problem.
Idea:
add the following macro:
__GNUC_DISTRIBUTOR_<name>__
This macro is defined in releases of GCC that are prepared by entities other
than the FSF. The actual name of the macro depends on the value set by the
packager. A list of known names can be found at <page to be provided>.
This macro expands to a number that uniquely identifies the package. The
actual format of the number is defined by the distributor, but it is
recommended that distributors define the value like this:
<distribution major version> * 0x10000 + <distribution minor version> * 0x100
+ <patchlevel>
(or call it __GNUC_<name>_VERSION__ ?)
<name> and the value of the macro would be set by a configure switch to GCC
and would have values like "REDHAT", "UBUNTU", "SUSE", ...
Rationale
=========
- We can't expect distributors to only ship vanilla GCC packages (even if I'd
prefer that).
- We can't expect that incompatibilities between GCC releases with the exact
same version number will never occur again.
- We can't expect software developers to correctly define compiler-specific
feature-macros for the header files of the libraries they use.
- A means to distinguish different releases of a given GCC version is
currently not available.
=> The suggested macro would make it possible for library headers to work with
all released GCCs, without additional work for the library user.
How to go forward
=================
I'd look into implementing this for GCC 4.7, if you like the idea. Unless, of
course, somebody else prefers to do it instead. :-)
[1] https://bugs.launchpad.net/bugs/780551
Cheers,
Matthias
--
Dipl.-Phys. Matthias Kretz
http://compeng.uni-frankfurt.de/?mkretz
SIMD easy and portable: http://compeng.uni-frankfurt.de/?vc
