[Bug c/49012] New: weak const optimisations

[Bug c/49012] New: weak const optimisations

[etienne_lorrain at yahoo dot fr]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49012

           Summary: weak const optimisations
           Product: gcc
           Version: 4.6.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: etienne_lorrain@yahoo.fr


With newer GCC compilers,

const struct { int a,b; } mystruct = {15, 0};
int adder (int x) { return x + mystruct.b; };

there isn't any addition performed in adder() because GCC knows that mystruct.b
is null,
and assumes that nobody is initialising mystruct.b is assembler or declaring
mystruct as
non-const in another module.

But if we add the weak attribute to mystruct, GCC-4.4.5 does the addition in
case mystruct
has been pre-loaded by for instance LD_PRELOAD, GCC-4.6 do not do the addition.

const struct { int a,b; } mystruct __attribute__((weak))= {15, 0};
int adder (int x) { return x + mystruct.b; };

LD_PRELOAD=/home/etienne/projet/toolchain/lib/libmpc.so.2:/home/etienne/projet/toolchain/lib/libgmp.so.10
/home/etienne/projet/toolchain/bin/gcc -Os -fomit-frame-pointer -S t.c -o t.s

gives:

    .file    "t.c"
    .text
    .globl    adder
    .type    adder, @function
adder:
.LFB0:
    .cfi_startproc
    movl    4(%esp), %eax
    ret
    .cfi_endproc
.LFE0:
    .size    adder, .-adder
    .weak    mystruct
    .section    .rodata
    .align 4
    .type    mystruct, @object
    .size    mystruct, 8
mystruct:
    .long    15
    .long    0
    .ident    "GCC: (GNU) 4.6.0"
    .section    .note.GNU-stack,"",@progbits

[Bug c/49012] weak const optimisations

[jakub at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49012

Jakub Jelinek <jakub at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |jakub at gcc dot gnu.org

--- Comment #1 from Jakub Jelinek <jakub at gcc dot gnu.org> 2011-05-16 13:46:10 UTC ---
You aren't compiling with -fpic, therefore the weak attribute is there
irrelevant - the symbol is known to be defined in the current TU, and for
anything that goes into the executable it can't be overridden.  Note LD_PRELOAD
inserts preloaded lib symbol search scope after executables, not before.

[Bug c/49012] weak const optimisations

[etienne_lorrain at yahoo dot fr]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49012

--- Comment #2 from etienne_lorrain at yahoo dot fr 2011-05-16 14:36:41 UTC ---
Well, with gcc-4.4.5-8 the weak attribute did the trick:

$ gcc -v
Using built-in specs.
Target: i486-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Debian 4.4.5-8'
--with-bugurl=file:///usr/share/doc/gcc-4.4/README.Bugs
--enable-languages=c,c++,fortran,objc,obj-c++ --prefix=/usr
--program-suffix=-4.4 --enable-shared --enable-multiarch
--enable-linker-build-id --with-system-zlib --libexecdir=/usr/lib
--without-included-gettext --enable-threads=posix
--with-gxx-include-dir=/usr/include/c++/4.4 --libdir=/usr/lib --enable-nls
--enable-clocale=gnu --enable-libstdcxx-debug --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 4.4.5 (Debian 4.4.5-8) 

$ cat t.c
//const struct { int a,b; } mystruct __attribute__((weak))= {15, 0};
const struct { int a,b; } mystruct = {15, 0};
int adder (int x) { return x + mystruct.b; };

$ gcc -Os -fomit-frame-pointer t.c -S -o t.s
$ cat t.s
    .file    "t.c"
    .text
.globl adder
    .type    adder, @function
adder:
    movl    4(%esp), %eax
    ret
    .size    adder, .-adder
.globl mystruct
    .section    .rodata
    .align 4
    .type    mystruct, @object
    .size    mystruct, 8
mystruct:
    .long    15
    .long    0
    .ident    "GCC: (Debian 4.4.5-8) 4.4.5"
    .section    .note.GNU-stack,"",@progbits

$ cat t.c
const struct { int a,b; } mystruct __attribute__((weak))= {15, 0};
//const struct { int a,b; } mystruct = {15, 0};
int adder (int x) { return x + mystruct.b; };

$ cat t.s
    .file    "t.c"
    .text
.globl adder
    .type    adder, @function
adder:
    movl    4(%esp), %eax
    addl    mystruct+4, %eax
    ret
    .size    adder, .-adder
    .weak    mystruct
    .section    .rodata
    .align 4
    .type    mystruct, @object
    .size    mystruct, 8
mystruct:
    .long    15
    .long    0
    .ident    "GCC: (Debian 4.4.5-8) 4.4.5"
    .section    .note.GNU-stack,"",@progbits

But in fact I just discovered "volatile const" structures which enables me to
have a C constant
initialised at run-time by the assembly preceding main(), so personally I do
not need this
"weak" trick.

Thanks for the quick answer,
Etienne.

[Bug c/49012] weak const optimisations

[rguenth at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49012

--- Comment #3 from Richard Guenther <rguenth at gcc dot gnu.org> 2011-05-17 08:39:50 UTC ---
I suppose PR47278 might be related.

[Bug middle-end/49012] weak const optimisations

[pinskia at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49012

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
          Component|c                           |middle-end
         Resolution|                            |INVALID

--- Comment #4 from Andrew Pinski <pinskia at gcc dot gnu.org> 2011-07-23 23:37:49 UTC ---
Invalid as you need to compile with -fPIC to be able to change weak symbols.