[Bug other/60783] New: unexpected address variation when taking address of reference, const reference, etc.

[Bug other/60783] New: unexpected address variation when taking address of reference, const reference, etc.

[parke.nexus at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=60783

            Bug ID: 60783
           Summary: unexpected address variation when taking address of
                    reference, const reference, etc.
           Product: gcc
           Version: 4.8.2
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: other
          Assignee: unassigned at gcc dot gnu.org
          Reporter: parke.nexus at gmail dot com

I am comparing passing pointers versus references into a function.  I
encountered unexpected variations when taking the address of passed in
references.

Perhaps most notably, if
f is declared as:       int* const & f
h is declared as: const int* const & h 
then
& f is not equal to & h

Below is:
1) test code that reproduces the issue
2) output when compiled with g++ 4.8.2 on Ubuntu 14.04 on x68_64
3) output when compiled with g++ 4.7.3 on Ubuntu 13.04 on i686

I in the output, I expect "e" through "l" to be identical.

In 2, e f g and l are identical.
h to k are identical to each other, but different from e f g and l.

In 3, e f g and l are identical.  h to k are each unique.

Here is code to reproduce the issue:

#include <stdio.h>

void foo_ab ( int* a, int & b ) {
  printf ( "a  %p\n",   a );
  printf ( "b  %p\n", & b ); }

void foo_cd ( void** c,  void* & d ) {
  printf ( "c  %p\n",   c );
  printf ( "d  %p\n", & d ); }

void foo_ef ( int** e,  int* const & f ) {
  printf ( "e  %p\n",   e );
  printf ( "f  %p\n", & f ); }

void foo_gh ( int** g,  const int* const & h ) {
  printf ( "g  %p\n",   g );
  printf ( "h  %p\n", & h ); }

void foo_i ( const void* const & i )  { printf ( "i  %p\n", & i ); }
void foo_j ( const int*  const & j )  { printf ( "j  %p\n", & j ); }
void foo_k (       void* const & k )  { printf ( "k  %p\n", & k ); }
void foo_l (       int*  const & l )  { printf ( "l  %p\n", & l ); }

int main () {

  int  n  =  5;
  int* p  =  & n;

  printf ( "\n" );
  foo_ab ( & n, n );

  // foo_cd ( & p, p );    // causes compile time error

  printf ( "\n" );
  foo_ef ( & p, p );

  printf ( "\n" );
  foo_gh ( & p, p );

  printf ( "\n" );
  foo_i ( p );
  foo_i ( p );

  printf ( "\n" );
  foo_j ( p );
  foo_j ( p );

  printf ( "\n" );
  foo_k ( p );
  foo_k ( p );

  printf ( "\n" );
  foo_l ( p );
  foo_l ( p );

  return 0; }


----

$ ./a.out 

a  0x7fff3a1e669c
b  0x7fff3a1e669c

e  0x7fff3a1e66a0
f  0x7fff3a1e66a0

g  0x7fff3a1e66a0
h  0x7fff3a1e66a8

i  0x7fff3a1e66a8
i  0x7fff3a1e66a8

j  0x7fff3a1e66a8
j  0x7fff3a1e66a8

k  0x7fff3a1e66a8
k  0x7fff3a1e66a8

l  0x7fff3a1e66a0
l  0x7fff3a1e66a0

$ g++ -v
Using built-in specs.
COLLECT_GCC=g++
COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-linux-gnu/4.8/lto-wrapper
Target: x86_64-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Ubuntu 4.8.2-19ubuntu1'
--with-bugurl=file:///usr/share/doc/gcc-4.8/README.Bugs
--enable-languages=c,c++,java,go,d,fortran,objc,obj-c++ --prefix=/usr
--program-suffix=-4.8 --enable-shared --enable-linker-build-id
--libexecdir=/usr/lib --without-included-gettext --enable-threads=posix
--with-gxx-include-dir=/usr/include/c++/4.8 --libdir=/usr/lib --enable-nls
--with-sysroot=/ --enable-clocale=gnu --enable-libstdcxx-debug
--enable-libstdcxx-time=yes --enable-gnu-unique-object --disable-libmudflap
--enable-plugin --with-system-zlib --disable-browser-plugin
--enable-java-awt=gtk --enable-gtk-cairo
--with-java-home=/usr/lib/jvm/java-1.5.0-gcj-4.8-amd64/jre --enable-java-home
--with-jvm-root-dir=/usr/lib/jvm/java-1.5.0-gcj-4.8-amd64
--with-jvm-jar-dir=/usr/lib/jvm-exports/java-1.5.0-gcj-4.8-amd64
--with-arch-directory=amd64 --with-ecj-jar=/usr/share/java/eclipse-ecj.jar
--enable-objc-gc --enable-multiarch --disable-werror --with-arch-32=i686
--with-abi=m64 --with-multilib-list=m32,m64,mx32 --with-tune=generic
--enable-checking=release --build=x86_64-linux-gnu --host=x86_64-linux-gnu
--target=x86_64-linux-gnu
Thread model: posix
gcc version 4.8.2 (Ubuntu 4.8.2-19ubuntu1) 


$ ./a.out 

a  0xbfc74a3c
b  0xbfc74a3c

e  0xbfc74a40
f  0xbfc74a40

g  0xbfc74a40
h  0xbfc74a44

i  0xbfc74a48
i  0xbfc74a4c

j  0xbfc74a50
j  0xbfc74a54

k  0xbfc74a58
k  0xbfc74a5c

l  0xbfc74a40
l  0xbfc74a40

$ g++ -v
Using built-in specs.
COLLECT_GCC=/usr/bin/g++-4.7.real
COLLECT_LTO_WRAPPER=/usr/lib/gcc/i686-linux-gnu/4.7/lto-wrapper
Target: i686-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Ubuntu/Linaro
4.7.3-1ubuntu1' --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
--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
--with-system-zlib --enable-objc-gc --enable-targets=all --with-cloog
--enable-cloog-backend=ppl --disable-cloog-version-check
--disable-ppl-version-check --enable-multiarch --disable-werror
--with-arch-32=i686 --with-multilib-list=m32,m64,mx32 --with-tune=generic
--enable-checking=release --build=i686-linux-gnu --host=i686-linux-gnu
--target=i686-linux-gnu
Thread model: posix
gcc version 4.7.3 (Ubuntu/Linaro 4.7.3-1ubuntu1)

[Bug other/60783] unexpected address variation when taking address of reference, const reference, etc.

[pinskia at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=60783

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|---                         |INVALID

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
I don't see why this is a bug, g and h should be different.  Here is the
reason, const int* const & cannot lvalue bind to int *, it can lvalue bind to
const int* or int* const.  So there is a lvalue to rvalue and then a conversion
from int * to const int* and then a rvalue to const lvalue reference binding to
const int* const &.  This is true for i, j, and k.

For l, int* const& can be lvalue bind to int*.

[Bug other/60783] unexpected address variation when taking address of reference, const reference, etc.

[redi at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=60783

--- Comment #2 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Parke from comment #0)
> I in the output, I expect "e" through "l" to be identical.

Your expectation is wrong.

The functions that produce a different values from what you expect are binding
a reference of some type X to an object that is not reference-compatible with
X, which requires the creation of a temporary of type X. The temporary has a
different address from the original object.

Any conforming C++ compiler will produce similar results.

[Bug other/60783] unexpected address variation when taking address of reference, const reference, etc.

[parke.nexus at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=60783

--- Comment #3 from Parke <parke.nexus at gmail dot com> ---
Thanks, I thought it might be something like the creation of temporaries, but
could not find it discussed anywhere, and was surprised that I got different
behavior from 4.7.3 (many temporaries) and 4.8.2 (apparently only one
temporary).

[Bug other/60783] unexpected address variation when taking address of reference, const reference, etc.

[pinskia at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=60783

--- Comment #4 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
(In reply to Parke from comment #3)
> Thanks, I thought it might be something like the creation of temporaries,
> but could not find it discussed anywhere, and was surprised that I got
> different behavior from 4.7.3 (many temporaries) and 4.8.2 (apparently only
> one temporary).

Well 4.8 is better at coalescing the temporaries as there was a fix to the
front-end to produce a CLOBBER after the statement has ended.