[Bug c/59562] New: __builtin_assume_aligned loses constness when used as initializer element

[stoeps at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=59562

            Bug ID: 59562
           Summary: __builtin_assume_aligned loses constness when used as
                    initializer element
           Product: gcc
           Version: 4.8.2
            Status: UNCONFIRMED
          Severity: minor
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: stoeps at gmail dot com

__builtin_assume_aligned appears to change the semantics of a constant
expression, but only in C.

The following snippet compiles in C++, but fails in C:

-- input
$ cat c.c
#define INT_PTR ((int*)0)
#define INT_PTR_BAA ((int*)__builtin_assume_aligned(0, 4))

int * i = INT_PTR;
int * i_baa = INT_PTR_BAA;

-- compiled as C
$ gcc -c c.c 
c.c:5:1: error: initializer element is not constant
$

-- compiled as C++
$ gcc -x c++ -c c.c 
$ 

I know about the different things that are considered "constant" between C++
and C, but wouldn't expect a GCC builtin to be among them. Am I wrong?

Reproduced with various GCC versions/targets:

$ gcc -v
Using built-in specs.
COLLECT_GCC=gcc
COLLECT_LTO_WRAPPER=/usr/lib/gcc/i486-linux-gnu/4.7/lto-wrapper
Target: i486-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Debian 4.7.2-5'
--with-bugurl=file:///usr/share/doc/gcc-4.7/README.Bugs
--enable-languages=c,c++,go,fortran,objc,obj-c++ --prefix=/usr
--program-suffix=-4.7 --enable-shared --enable-linker-build-id
--with-system-zlib --libexecdir=/usr/lib --without-included-gettext
--enable-threads=posix --with-gxx-include-dir=/usr/include/c++/4.7
--libdir=/usr/lib --enable-nls --with-sysroot=/ --enable-clocale=gnu
--enable-libstdcxx-debug --enable-libstdcxx-time=yes --enable-gnu-unique-object
--enable-plugin --enable-objc-gc --enable-targets=all --with-arch-32=i586
--with-tune=generic --enable-checking=release --build=i486-linux-gnu
--host=i486-linux-gnu --target=i486-linux-gnu
Thread model: posix
gcc version 4.7.2 (Debian 4.7.2-5) 

$ gcc -v
Using built-in specs.
COLLECT_GCC=c:\MinGW\bin\gcc.exe
COLLECT_LTO_WRAPPER=c:/mingw/bin/../libexec/gcc/mingw32/4.8.1/lto-wrapper.exe
Target: mingw32
Configured with: ../gcc-4.8.1/configure --prefix=/mingw --host=mingw32
--build=mingw32 --without-pic --enable-shared --enable-static --with-gnu-ld
--enable-lto --enable-libssp --disable-multilib
--enable-languages=c,c++,fortran,objc,obj-c++,ada --disable-sjlj-exceptions
--with-dwarf2 --disable-win32-registry --enable-libstdcxx-debug
--enable-version-specific-runtime-libs
--with-gmp=/usr/src/pkg/gmp-5.1.2-1-mingw32-src/bld
--with-mpc=/usr/src/pkg/mpc-1.0.1-1-mingw32-src/bld --with-mpfr=
--with-system-zlib --with-gnu-as --enable-decimal-float=yes --enable-libgomp
--enable-threads --with-libiconv-prefix=/mingw32 --with-libintl-prefix=/mingw
--disable-bootstrap LDFLAGS=-s CFLAGS=-D_USE_32BIT_TIME_T
Thread model: win32
gcc version 4.8.1 (GCC)

$ m68k-elf-gcc -v
Using built-in specs.
COLLECT_GCC=C:\MinGW\msys\1.0\local\gnu-m68k\bin\m68k-elf-gcc.exe
COLLECT_LTO_WRAPPER=c:/mingw/msys/1.0/local/gnu-m68k/bin/../libexec/gcc/m68k-elf/4.8.2/lto-wrapper.exe
Target: m68k-elf
Configured with: ../gcc-4.8.2/configure --build=i686-pc-linux-gnu
--host=i686-w64-mingw32 --target=m68k-elf --prefix=/usr/local/gnu-m68k
--enable-interwork --enable-languages=c,c++ --with-newlib --disable-shared
--disable-libssp --disable-nls --disable-libstdcxx-pch --with-gnu-as
--with-gnu-ld
--with-debug-prefix-map=/home/tom/m68k-build=/MinGW/msys/1.0/local/gnu-m68k/src
Thread model: single
gcc version 4.8.2 (GCC)

Thanks,
Tom