[cpp.cond]p4:
Prior to evaluation, macro invocations in the list of preprocessing
tokens that will become the controlling constant expression are replaced
(except for those macro names modified by the 'defined' unary operator),
just as in normal text. If the token 'defined' is generated as a result
of this replacement process or use of the 'defined' unary operator does
not match one of the two specified forms prior to macro replacement, the
behavior is undefined.
This isn't an idle threat, consider this program:
#define FOO
#define BAR defined(FOO)
#if BAR
...
#else
...
#endif
clang and gcc will pick the #if branch while Visual Studio will take the
#else branch. Emit a warning about this undefined behavior.
One problem is that this also applies to function-like macros. While the
example above can be written like
#if defined(FOO) && defined(BAR)
#defined HAVE_FOO 1
#else
#define HAVE_FOO 0
#endif
there is no easy way to rewrite a function-like macro like `#define FOO(x)
(defined __foo_##x && __foo_##x)`. Function-like macros like this are used in
practice, and compilers seem to not have differing behavior in that case. So
this a default-on warning only for object-like macros. For function-like
macros, it is an extension warning that only shows up with `-pedantic`.
(But it's undefined behavior in both cases.)
llvm-svn: 258128
in-progress implementation of the Concepts TS. The recommended feature
test macro __cpp_experimental_concepts is set to 1 (as opposed to
201501) to indicate that the feature is enabled, but the
implementation is incomplete.
The link to the Concepts TS in cxx_status is updated to refer to the
PDTS (N4377). Additional changes related to __has_feature and
__has_extension are to follow in a later change.
Relevant tests include:
test/Lexer/cxx-features.cpp
The test file is updated with testing of the C++14 + Concepts TS mode.
The expected behaviour is the same as that of the C++14 modes except
for the case of __cpp_experimental_concepts."
- Hubert Tong.
Being committed for Hubert (as per his understanding with Richard Smith) as we start work on the concepts-ts following our preliminary strategy session earlier today.
The patch is tiny and seems quite standard.
Thanks Hubert!
llvm-svn: 237982
There are no widely deployed standard libraries providing sized
deallocation functions, so we have to punt and ask the user if they want
us to use sized deallocation. In the future, when such libraries are
deployed, we can teach the driver to detect them and enable this
feature.
N3536 claimed that a weak thunk from sized to unsized deallocation could
be emitted to avoid breaking backwards compatibility with standard
libraries not providing sized deallocation. However, this approach and
other variations don't work in practice.
With the weak function approach, the thunk has to have default
visibility in order to ensure that it is overridden by other DSOs
providing sized deallocation. Weak, default visibility symbols are
particularly expensive on MachO, so John McCall was considering
disabling this feature by default on Darwin. It also changes behavior
ELF linking behavior, causing certain otherwise unreferenced object
files from an archive to be pulled into the link.
Our second approach was to use an extern_weak function declaration and
do an inline conditional branch at the deletion call site. This doesn't
work because extern_weak only works on MachO if you have some archive
providing the default value of the extern_weak symbol. Arranging to
provide such an archive has the same challenges as providing the symbol
in the standard library. Not to mention that extern_weak doesn't really
work on COFF.
Reviewers: rsmith, rjmccall
Differential Revision: http://reviews.llvm.org/D8467
llvm-svn: 232788
JF Bastien