On Jul 3, 2008, at 9:01 AM, Jim Wilson wrote:
x z wrote:
If we want to fix this, gcc must change. And this may
also require GNU libc changes and linux kernel changes, etc.
Maybe you can enlighten us a bit on why GNU libc and linux kernel
need changes so that we can realize better how complicated the
issue is.
Because there are header files in /usr/include that test __GNUC__.
In order for these header files to do the right thing, the Intel
compiler (and other compilers) need to define __GNUC__.
Now suppose we add a new macro __GCC_COMPILER__ that is intended to
be unambiguous, and mean only that this is the GCC compiler. What
happens next? Within a few months, someone will post a patch to
glibc and/or the linux kernel that uses __GCC_COMPILER__, based on
the misconception that because it is new, that they are supposed to
use it. A few months later, there is a glibc release and/or linux
kernel release that contains this code. A few months later it gets
into a linux release. Then Intel discovers that their compiler no
longer works as intended on linux, and in order to fix it, they have
to define __GCC_COMPILER__. And now we are back where we started,
except now we have two useless ambiguous macros instead of one, and
hence we are worse off than before.
IMO, the whole notion of a compiler-specific macro has pretty limited
usefulness. Why not add macros for specific *features* offered by the
compiler. For example:
#ifdef __SUPPORTS_NESTED_FUNCTIONS__
is much better than some mismash of version checking, which isn't
guaranteed to be right in the future. One disadvantage of this is
that it will put even more burden on the already overloaded
preprocessor. It would be much nicer to have a feature query system
that doesn't rely on one macro per system. Perhaps something like:
#if __feature_supported(nested_functions) &&
__feature_supported(transparent_union) &&
__feature_supported(attribute_aligned)
...
Taking an approach reduces startup time of the preprocessor, because
it doesn't have to populate the identifier table with tons of
predefines.
-Chris
