Subject: Re: bug#18711: Numerous unknown attribute '__alloc_size__' warnings when using clang

From: Mark H Weaver <[email-masked]>
Date: Mon, 13 Oct 2014 23:53:57 -0400

[Forwarding this bug report to [email-masked]]

William S Fulton <[email-masked]> writes:

> A simple
>
> #include <libguile.h>
>
> generates the numerous warnings given below:
>
>
> In file included from example_wrap.c:702:
> In file included from /usr/local/Cellar/guile/2.0.11_1/include/guile/2.0/libguile.h:37:
> In file included from /usr/local/Cellar/guile/2.0.11_1/include/guile/2.0/libguile/async.h:28:
> In file included from /usr/local/Cellar/guile/2.0.11_1/include/guile/2.0/libguile/threads.h:36:
> In file included from /usr/local/Cellar/guile/2.0.11_1/include/guile/2.0/libguile/pthread-threads.h:33:
> In file included from /usr/local/Cellar/guile/2.0.11_1/include/guile/2.0/libguile/bdw-gc.h:47:
> /usr/local/Cellar/bdw-gc/7.4.2/include/gc/gc.h:411:23: warning: unknown attribute '__alloc_size__' ignored [-Wattributes]
> GC_API GC_ATTR_MALLOC GC_ATTR_ALLOC_SIZE(1) void * GC_CALL
> ^
> /usr/local/Cellar/bdw-gc/7.4.2/include/gc/gc_config_macros.h:249:54: note: expanded from macro 'GC_ATTR_ALLOC_SIZE'
> # define GC_ATTR_ALLOC_SIZE(argnum) __attribute__((__alloc_size__(argnum)))
> ^

This is clearly an issue with bdwgc on clang, and possibly only on
Apple's version of clang.  Here's the relevant section of
gc/gc_config_macros.h from bdwgc 7.4.2:

--8<---------------cut here---------------start------------->8---
#ifndef GC_ATTR_ALLOC_SIZE
  /* 'alloc_size' attribute improves __builtin_object_size correctness. */
  /* Only single-argument form of 'alloc_size' attribute is used.       */
# if defined(__GNUC__) && (__GNUC__ > 4 \
        || (__GNUC__ == 4 && __GNUC_MINOR__ >= 3 && !defined(__ICC)) \
        || __clang_major__ > 3 \
        || (__clang_major__ == 3 && __clang_minor__ >= 2))
#   define GC_ATTR_ALLOC_SIZE(argnum) __attribute__((__alloc_size__(argnum)))
# else
#   define GC_ATTR_ALLOC_SIZE(argnum)
# endif
#endif
--8<---------------cut here---------------end--------------->8---

You can see that the bdwgc developers have made an effort to check both
GCC and clang version numbers before using the __alloc_size__ attribute.
The code above seems to suggest that they believed clang 3.2 or later
supported this attribute, whereas your version of clang seems to be
based on upstream clang 3.5.  Perhaps Apple removed support for this
attribute from their clang?

In any case, this needs to be taken up with the bdwgc developers.

    Regards,
      Mark

> System information:
>
> $ guile --version | head -n 1
> guile (GNU Guile) 2.0.11
>
> $ clang --version
> Apple LLVM version 6.0 (clang-600.0.51) (based on LLVM 3.5svn)
> Target: x86_64-apple-darwin13.4.0
> Thread model: posix
>
> $ xcodebuild -version -sdk | head -n 2
> MacOSX10.9.sdk - OS X 10.9 (macosx10.9)
> SDKVersion: 10.9
>
> $ sw_vers
> ProductName: Mac OS X
> ProductVersion: 10.9.5
> BuildVersion: 13F34
>
> $ uname -a
> Darwin itako16071.miso 13.4.0 Darwin Kernel Version 13.4.0: Sun Aug 17
> 19:50:11 PDT 2014; root:xnu-2422.115.4~1/RELEASE_X86_64 x86_64