We cannot use Clang's __has_extension macro unless we really are
compiling on Clang, which means we cannot use this expression:
#if (defined(__clang__) && __has_extension(c_atomic)))
// ...
#endif
On GCC, this generates the following errors:
In file included from ./google/protobuf/stubs/atomicops.h:59:0,
from google/protobuf/stubs/atomicops_internals_x86_gcc.cc:36:
./google/protobuf/stubs/platform_macros.h:67:41: error: missing binary operator before token "("
(defined(__clang__) && __has_extension(c_atomic)))
^
In file included from google/protobuf/stubs/atomicops_internals_x86_gcc.cc:36:0:
./google/protobuf/stubs/atomicops.h:196:40: error: missing binary operator before token "("
(defined(__clang__) && __has_extension(c_atomic))
^
Instead, we have to protect the __has_extension expression by only
executing it when __clang__ is defined:
#if defined(__clang__)
# if __has_extension(c_atomic)
// ...
# endif
#endif
The generic atomicops implementation is only exposed if GCC >= 4.7 is
available, but Clang, where the underlying __atomic built-ins are also
available, typically only claims to be GCC 4.2. This causes build
failures when compiling protobuf or the output of protoc's C++ code
generator on an architecture that needs the generic atomicops
implementation with Clang.
Clang has a "c_atomic" extension which can be tested for which almost
does what we want:
C11 atomic operations
Use __has_feature(c_atomic) or __has_extension(c_atomic) to
determine if support for atomic types using _Atomic is enabled.
Clang also provides a set of builtins which can be used to implement
the <stdatomic.h> operations on _Atomic types.
I'm not sure if this guarantees that the GNU atomic builtins (the ones
with the __atomic prefix) are also available, but in practice this
should guarantee that Clang is new enough.
With this change in place, Clang generates several diagnostics when
compiling the generic atomicops implementation. These appear to be bugs
in the generic atomicops implementation and are not Clang-specific.
The macro GOOGLE_PROTOBUF_ARCH_PPC is not used anywhere in the protobuf
source; there is no Power-specific atomics implementation, etc.
Funnily enough, the macro __ppc__ is not actually defined on 32-bit
Power on GCC/Linux, according to the following webpage:
http://nadeausoftware.com/articles/2012/02/c_c_tip_how_detect_processor_type_using_compiler_predefined_macros#POWER
and verified on a 32-bit Debian sid 'powerpc' chroot:
(sid_powerpc-dchroot)edmonds@partch:~$ gcc -dM -E - < /dev/null | grep -c __ppc__
0
(sid_powerpc-dchroot)edmonds@partch:~$ gcc -dM -E - < /dev/null | grep -c __LP64__
0
With AM_PROG_AR may report "error: possibly underfined macro: AM_PROG_AR" (mingw, autoreconf-2.68).
Without AM_PROG_AR may report warning "archiver requires 'AM_PROG_AR' in 'configure.ac'" (cygwin, automake-1.12).
They do not affect the tests and install
