clang claim to be gcc 4.8.1 instead of 4.2.1

[Yaron Keren]

The doc say that clang still does not
support __builtin_va_arg_pack/__builtin_va_arg_pack_len:


http://clang.llvm.org/docs/UsersManual.html#gcc-extensions-not-implemented-yet

My use case for the newer gcc version is boost config, which conditions its
TR1 headers based on gcc version. clang claim to be 4.2 may introduce a
difference when switching to clang, since gcc 4.8 will use the libstdc++
boost headers while switching to clang will use the boost headers. In one
complex case actually fails to compile with #include <something> failing,
where it seems the boost code does not expect <something> to exist since it
did not in libstdc++ of gcc 4.2 at all while it does exist in libstdc++ of
gcc 4.9 which is the real version.

Other libraries than boost dont have their own TR1 replacement headers and
will probably have reduced functionality with clang = gcc 4.2.

It may be better have clang pretend gcc 4.8 and patch the glibc conditional
rather than all other libraries?

Examples from boost config:

//  C++0x headers in GCC 4.3.0 and later
//
#if __GNUC__ < 4 || (__GNUC__ == 4 && __GNUC_MINOR__ < 3) ||
!defined(__GXX_EXPERIMENTAL_CXX0X__)
#  define BOOST_NO_CXX11_HDR_ARRAY
#  define BOOST_NO_CXX11_HDR_TUPLE
#  define BOOST_NO_CXX11_HDR_UNORDERED_MAP
#  define BOOST_NO_CXX11_HDR_UNORDERED_SET
#  define BOOST_NO_CXX11_HDR_FUNCTIONAL
#endif

//  C++0x headers in GCC 4.4.0 and later
//
#if __GNUC__ < 4 || (__GNUC__ == 4 && __GNUC_MINOR__ < 4) ||
!defined(__GXX_EXPERIMENTAL_CXX0X__)
#  define BOOST_NO_CXX11_HDR_CONDITION_VARIABLE
#  define BOOST_NO_CXX11_HDR_FORWARD_LIST
#  define BOOST_NO_CXX11_HDR_INITIALIZER_LIST
#  define BOOST_NO_CXX11_HDR_MUTEX
#  define BOOST_NO_CXX11_HDR_RATIO
#  define BOOST_NO_CXX11_HDR_SYSTEM_ERROR
#  define BOOST_NO_CXX11_SMART_PTR
#else
#  define BOOST_HAS_TR1_COMPLEX_INVERSE_TRIG
#  define BOOST_HAS_TR1_COMPLEX_OVERLOADS
#endif

#if (!defined(_GLIBCXX_HAS_GTHREADS) ||
!defined(_GLIBCXX_USE_C99_STDINT_TR1)) &&
(!defined(BOOST_NO_CXX11_HDR_CONDITION_VARIABLE) ||
!defined(BOOST_NO_CXX11_HDR_MUTEX))
#  define BOOST_NO_CXX11_HDR_CONDITION_VARIABLE
#  define BOOST_NO_CXX11_HDR_MUTEX
#endif

...





2014-11-13 1:04 GMT+02:00 Nico Weber <thakis at chromium.org>:

> This was discussed in this thread
> http://marc.info/?t=133682946900003&r=1&w=2 People were generally
> supportive of the idea, but it was deemed very hard to get glibc to build
> with this ( http://marc.info/?l=cfe-dev&m=133717592322519&w=2 ) and the
> benefit of doing this wasn't seen as very big in the end.


On Wed, Nov 12, 2014 at 2:55 PM, Yaron Keren <yaron.keren at gmail.com> wrote:

> gcc 4.2.1 is really old from 2008.  Some configuration headers such as
> boost "libstdcpp3.hpp" behave differently for pre-gcc 4.3 or later.
>
> gcc 4.8.1 is more recent, reasonable claim for clang.
>
> I had tried going for 4.9.1 but encountered a problem with intrinsics in
> mingw 4.9.1 intrin.h which are treated differently for gcc 4.9 or later:
>
>  * On GCC 4.9 we may always include those headers. On older GCCs, we may
> do it only if CPU
>  * features used by them are enabled, so we need to check macros like
> __SSE__ or __MMX__ first.
>  */
> #if __MINGW_GNUC_PREREQ(4, 9)
> #define __MINGW_FORCE_SYS_INTRINS
> #endif
> ...
>
> this does not work correctly with clang since the headers try to use SSE3
> instructions without __SSE3__ being defined and fail.
>
>
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>
>
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