[cfe-commits] [PATCH RFC] Stop defining __GNUC__ for MSVC builds

[Douglas Gregor]

On Mar 6, 2012, at 5:31 AM, Aaron Ballman wrote:

> On Mon, Mar 5, 2012 at 10:32 PM, Douglas Gregor <dgregor at apple.com> wrote:
>> 
>> On Mar 2, 2012, at 4:16 PM, Aaron Ballman wrote:
>> 
>>> There are a lot of references on the web which relate __GNUC__ to GCC
>>> for compiler discovery
>>> (<http://sourceforge.net/apps/mediawiki/predef/index.php?title=Compilers#GCC_C.2FC.2B.2B>,
>>> <http://www.ocf.berkeley.edu/~pad/tigcc/doc/html/cpp_SEC15_GNUC.html>,
>>> etc).  This translates into a fair number of cross-compiler projects
>>> with code like this:
>>> 
>>> #if defined(__GNUC__)
>>>  // Do GCC things
>>> #elif defined(_MSC_VER)
>>>  // Do MSVC things
>>> ...
>>> #endif
>>> 
>>> This currently leads to problems when compiling with Clang because
>>> __GNUC__ (and friends) are always defined, even when compiling for MS
>>> compatibility (PR 11790).  I've attached a patch which addresses this
>>> by only defining __GNUC__ et al when MSVC mode is not set.  This means
>>> code like the above will work as expected.
>>> 
>>> Thoughts?
>> 
>> -  // Currently claim to be compatible with GCC 4.2.1-5621.
>> -  Builder.defineMacro("__GNUC_MINOR__", "2");
>> -  Builder.defineMacro("__GNUC_PATCHLEVEL__", "1");
>> -  Builder.defineMacro("__GNUC__", "4");
>> -  Builder.defineMacro("__GXX_ABI_VERSION", "1002");
>> +  if (!LangOpts.MSCVersion) {
>> 
>> This should probably just check LangOpts.MicrosoftMode.
>> 
>> I'm not sure about the general approach, though. Even when compiling in Microsoft-compatible mode, Clang is still much more GCC-like than MSVC-like. I would venture a guess that
>> 
>>        // Do GCC things
>> 
>> is far more likely to be standards-conforming code that Clang will accept than
>> 
>>        // Do MSVC things
>> 
>> especially when dealing with template libraries in C++.
> 
> I think your guess may be off base.  A lot of those things are
> includes.  At work, we have several sources like:
> 
> #if defined( _MSC_VER )
> #include <Windows.h>
> #elif defined( __GNUC__ )
> #include <gtk/gtk.h>
> #endif

So the assumption is that _MSC_VER/__GNUC__ are used more as a platform indicator than a compiler indicator. Broken as that is, I agree that this could be the common case.

> I think the correct thing to do is define __clang__ and MSVC in
> MS-compat mode, like we already do for __clang__ and __GNUC__ for GNU
> mode.  It seems like a reasonable pattern.  If someone cares about
> clang, that's turned on appropriately, as is the compiler they're
> emulating.

Fair enough. Patch is fine with the change to check for LangOpts.MicrosoftMode.

> But I do wonder what should happen with -std=gnu++98
> -fms-compatibility as a combination.  Seems almost like an error?


I think it's fine to have GNU extensions and MS extensions available at the same time. -fms-compatibility essentially takes precedence when it is provided.

	- Doug