[Bug c/40693] New: atomic built-ins malfunction with 64-bit and optimization

[Bug c/40693] New: atomic built-ins malfunction with 64-bit and optimization

[m dot rosellini at f5 dot com]

#include <stdint.h>
#include <stdio.h>

int
main(int ac, char *av[])
{
    int64_t x;
    int64_t like_a_constant = -1;
    int64_t unlike_a_constant = -1;

    if (ac == 0) unlike_a_constant = 5;

    x = 0;
    __sync_add_and_fetch(&x, like_a_constant);
    printf("%016llx\n", x);

    x = 0;
    __sync_add_and_fetch(&x, unlike_a_constant);
    printf("%016llx\n", x);
}

Expected result:
ffffffffffffffff
ffffffffffffffff

Actual result:
00000000ffffffff
ffffffffffffffff


-- 
           Summary: atomic built-ins malfunction with 64-bit and
                    optimization
           Product: gcc
           Version: 4.1.2
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: m dot rosellini at f5 dot com


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40693

[Bug c/40693] atomic built-ins malfunction with 64-bit and optimization

[m dot rosellini at f5 dot com]

------- Comment #1 from m dot rosellini at f5 dot com  2009-07-09 00:10 -------
I forgot to add: You need to compile this with -O2 and -march=pentium.

The way that negative constants are handled in the code emitted for
__sync_blah_and_blah is incorrect when the pointer type is 64-bits and the
platform is 32-bit x86.  That's the real issue.  Calling
__sync_blah_and_blah(ptr, -1) is sufficient (where ptr is a pointer to a 64-bit
signed type).  Then you do not need to turn on optimization.

This doesn't really have anything to do with optimization... it's just that
there is a strange interplay with optimization whereby the function has the bad
behavior exhibited with constants even when it doesn't look like one is passing
a constant (such as when calling this through an inline function).


-- 

m dot rosellini at f5 dot com changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |m dot rosellini at f5 dot
                   |                            |com


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40693

[Bug target/40693] atomic built-ins malfunction with 64-bit signed ptrs and negative constants

[jakub at gcc dot gnu dot org]

------- Comment #2 from jakub at gcc dot gnu dot org  2009-07-09 05:54 -------
Can't reproduce.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40693

[Bug target/40693] atomic built-ins malfunction with 64-bit signed ptrs and negative constants

[m dot rosellini at f5 dot com]

------- Comment #3 from m dot rosellini at f5 dot com  2009-07-09 20:29 -------
OK, maybe some assembly will make the issue clearer.  Or perhaps you just want
to close this bug if you're not fixing bugs in the 4.1 compiler.  I verified
that the problem does occur with 4.1 and that it does not occur with 4.2.  The
relevant built-ins seem not to exist in 4.0.

Below is the output from a 4.1 compiler and a 4.2 compiler.  I have annotated
the relevant instructions with "<<<".  In the first example, from a 4.1
compiler, you can see that there is a constant of "0" passed to adcl (which is
incorrect), whereas in the second example, from a 4.2 compiler, there is a
constant of "-1" passed to adcl (which is correct).


seapddev(catalan) ~/ 503$ cat t.c
int
main(int ac, char **av)
{
    volatile long long x;
    __sync_fetch_and_add(&x, -1);
    return x;
}
seapddev(catalan) ~/ 504$ gcc --version
gcc (GCC) 4.1.2 20080704 (Red Hat 4.1.2-44)
Copyright (C) 2006 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

seapddev(catalan) ~/ 505$ gcc -march=pentium t.c
seapddev(catalan) ~/ 506$ cat t.s
        .file   "t.c"
        .text
.globl main
        .type   main, @function
main:
        leal    4(%esp), %ecx
        andl    $-16, %esp
        pushl   -4(%ecx)
        pushl   %ebp
        movl    %esp, %ebp
        pushl   %edi
        pushl   %esi
        pushl   %ebx
        pushl   %ecx
        subl    $40, %esp
        leal    -24(%ebp), %eax
        movl    %eax, -36(%ebp)
        movl    -36(%ebp), %edx
        movl    (%edx), %eax
        movl    4(%edx), %edx
        movl    %eax, -48(%ebp)
        movl    %edx, -44(%ebp)
.L2:
        movl    -48(%ebp), %eax
        movl    -44(%ebp), %edx
        movl    %eax, -56(%ebp)
        movl    %edx, -52(%ebp)
        movl    -56(%ebp), %ecx
        movl    -52(%ebp), %ebx
        addl    $-1, %ecx
        adcl    $0, %ebx                        <<<
        movl    %ecx, %esi
        movl    %ebx, %ecx
        movl    -36(%ebp), %edi
        movl    -56(%ebp), %eax
        movl    -52(%ebp), %edx
        movl    %esi, %ebx
        lock
        cmpxchg8b       (%edi)
        movl    %eax, -48(%ebp)
        movl    %edx, -44(%ebp)
        jne     .L2
        movl    -24(%ebp), %eax
        movl    -20(%ebp), %edx
        addl    $40, %esp
        popl    %ecx
        popl    %ebx
        popl    %esi
        popl    %edi
        popl    %ebp
        leal    -4(%ecx), %esp
        ret
        .size   main, .-main
        .ident  "GCC: (GNU) 4.1.2 20080704 (Red Hat 4.1.2-44)"
        .section        .note.GNU-stack,"",@progbits
seapddev(catalan) ~/ 507$ 


wayward ~/ 537$ cat t.c
int
main(int ac, char **av)
{
    volatile long long x;
    __sync_fetch_and_add(&x, -1);
    return x;
}
wayward ~/ 538$ gcc-4.2 --version
i686-apple-darwin9-gcc-4.2.1 (GCC) 4.2.1 (Apple Inc. build 5574)
Copyright (C) 2007 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

wayward ~/ 539$ rm t.s
rm: t.s: No such file or directory
wayward ~/ 540$ gcc-4.2 -march=pentium -S t.c
wayward ~/ 541$ cat t.s
        .text
.globl _main
_main:
        pushl   %ebp
        movl    %esp, %ebp
        pushl   %edi
        pushl   %esi
        subl    $64, %esp
        leal    -16(%ebp), %eax
        movl    %eax, -28(%ebp)
        movl    -28(%ebp), %edi
        movl    (%edi), %esi
        movl    4(%edi), %edi
        movl    %esi, -40(%ebp)
        movl    %edi, -36(%ebp)
L2:
        movl    -40(%ebp), %eax
        movl    -36(%ebp), %edx
        movl    %eax, -64(%ebp)
        movl    %edx, -60(%ebp)
        movl    -64(%ebp), %esi
        movl    -60(%ebp), %edi
        addl    $-1, %esi
        adcl    $-1, %edi                       <<<
        movl    %esi, -44(%ebp)
        movl    %edi, %ecx
        movl    -28(%ebp), %esi
        movl    -64(%ebp), %eax
        movl    -60(%ebp), %edx
        movl    -44(%ebp), %edi
        xchgl   %ebx, %edi
        lock
        cmpxchg8b       (%esi)
        xchgl   %ebx, %edi
        movl    %eax, -40(%ebp)
        movl    %edx, -36(%ebp)
        jne     L2
        movl    -16(%ebp), %eax
        movl    -12(%ebp), %edx
        addl    $64, %esp
        popl    %esi
        popl    %edi
        popl    %ebp
        ret
        .subsections_via_symbols
wayward ~/ 542$ 


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40693

[Bug target/40693] atomic built-ins malfunction with 64-bit signed ptrs and negative constants

[pinskia at gcc dot gnu dot org]

------- Comment #4 from pinskia at gcc dot gnu dot org  2009-10-14 04:00 -------
Fixed in 4.2.0 so closing.


-- 

pinskia at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|                            |FIXED
   Target Milestone|---                         |4.2.0


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40693