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    SubjectRe: [PATCH v2] err.h: allow IS_ERR_VALUE to handle properly more types
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    On 02/03/2016 01:33 AM, Andrew Morton wrote:
    > On Thu, 28 Jan 2016 09:27:28 +0100 Andrzej Hajda <a.hajda@samsung.com> wrote:
    >
    >> - use '<= 0' instead of '< 0' to silence gcc verbose warnings,
    >> - expand commit message.
    >> ---
    >> include/linux/err.h | 4 +++-
    >> 1 file changed, 3 insertions(+), 1 deletion(-)
    >>
    >> diff --git a/include/linux/err.h b/include/linux/err.h
    >> index 56762ab..43a6adb 100644
    >> --- a/include/linux/err.h
    >> +++ b/include/linux/err.h
    >> @@ -18,7 +18,9 @@
    >>
    >> #ifndef __ASSEMBLY__
    >>
    >> -#define IS_ERR_VALUE(x) unlikely((x) >= (unsigned long)-MAX_ERRNO)
    >> +#define IS_ERR_VALUE(x) ((typeof(x))(-1) <= 0 \
    >> + ? unlikely((x) < 0) \
    >> + : unlikely((x) >= (typeof(x))-MAX_ERRNO))
    > I'm still getting a bunch of
    >
    > include/linux/err.h: In function 'IS_ERR':
    > include/linux/err.h:37: warning: comparison of unsigned expression < 0 is always false
    > include/linux/err.h: In function 'IS_ERR_OR_NULL':
    > include/linux/err.h:42: warning: comparison of unsigned expression < 0 is always false
    >
    > with gcc-4.4.4.
    >
    >

    These warnings are false positives and gcc up to 4.7 emits them,
    gcc 4.8(which I use) behaves correctly (at least on x86 and arm64). I
    have tried
    to use __builtin_choose_expr instead of ?: operator but it did not help,
    although documentation says "the built-in function does not evaluate
    the expression that is not chosen"[1].

    The sanest gcc silencer I see for now is to replace:
    ? unlikely((x) < 0) \
    with
    ? unlikely((x) <= -1) \

    On the other side these warnings are caused by -Wtype-limits switch which
    is disabled by default in kernel build and treated as broken by Linus [2].
    Maybe it is good enough reason to disregard them? :)

    Anyway, I will post another iteration.

    [1]:
    https://gcc.gnu.org/onlinedocs/gcc/Other-Builtins.html#index-g_t_005f_005fbuiltin_005fchoose_005fexpr-4184
    [2]: http://permalink.gmane.org/gmane.linux.kernel/2053963

    Regards
    Andrzej

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