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    SubjectRe: [PATCH] TRACING: Fix a copmile warning
    From
    Hi,

    [adding gcc-help@ to the Cc: list]

    On Mon, Jul 25, 2011 at 6:38 PM, Arnaud Lacombe <lacombar@gmail.com> wrote:
    > Hi,
    >
    > On Mon, Jul 25, 2011 at 4:19 PM, Steven Rostedt <rostedt@goodmis.org> wrote:
    >> On Mon, 2011-07-25 at 15:43 -0400, Arnaud Lacombe wrote:
    >>
    >>> Actually, we have a special uninitialized_var(x) macro to handle such
    >>> false positive. From include/linux/compiler-gcc.h:
    >>>
    >>> /*
    >>>  * A trick to suppress uninitialized variable warning without generating any
    >>>  * code
    >>>  */
    >>> #define uninitialized_var(x) x = x
    >>
    >> I'm aware of that too, but I think that is inappropriate as well. As I
    >> said, some versions of gcc report it, others don't. Seems that gcc 4.6.0
    >> says this is an error where 4.5.1 does not (I just tried both).
    >>
    gcc will only emits the warning at -Os. It seems to me that the
    resulting code clearly ends-up testing an uninitialized value, ie.
    assuming the following test-case:

    extern void *e(void);
    extern void *f(void);
    extern void g(void);

    void fn(void)
    {
    void *b, *a;

    a = e();
    if (a != 0)
    b = f();
    if (a != 0 && b != 0)
    g();
    }

    gcc 4.5.1 will generates the following x86-32 assembly:

    % gcc -m32 -Wall -Os -c -S -o - kernel/trace/trace_printk.c
    .file "trace_printk.c"
    kernel/trace/trace_printk.c: In function 'fn':
    kernel/trace/trace_printk.c:7:8: warning: 'b' may be used
    uninitialized in this function
    .text
    .globl fn
    .type fn, @function
    fn:
    pushl %ebp
    movl %esp, %ebp
    pushl %esi
    pushl %ebx
    call e
    testl %eax, %eax
    movl %eax, %ebx
    je .L2
    call f
    movl %eax, %esi
    .L2:
    testl %esi, %esi
    je .L1
    testl %ebx, %ebx
    je .L1
    popl %ebx
    popl %esi
    popl %ebp
    jmp g
    .L1:
    popl %ebx
    popl %esi
    popl %ebp
    ret
    .size fn, .-fn
    .ident "GCC: (GNU) 4.5.1 20100924 (Red Hat 4.5.1-4)"
    .section .note.GNU-stack,"",@progbits

    It seems gcc transforms the conditional from:

    if (a != NULL && b != NULL) ...

    to

    if (b != NULL && a != NULL) ...

    In which case the warning is fully valid. I'm not sure what's the C
    standard guarantee in term of conditional test order. gcc 4.7.0 has
    the same behavior.

    - Arnaud
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