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    SubjectRe: [PATCH] x86, vsyscall: Fix build warning in vsyscall_64.c

    * Andrew Lutomirski <luto@mit.edu> wrote:

    > On Mon, Jun 13, 2011 at 5:29 AM, Ingo Molnar <mingo@elte.hu> wrote:
    > >
    > > * Andrew Lutomirski <luto@mit.edu> wrote:
    > >
    > >> On Sat, Jun 11, 2011 at 3:31 AM, Rakib Mullick <rakib.mullick@gmail.com> wrote:
    > >> > Due to commit 5cec93c216db77 (x86-64: Emulate legacy vsyscalls), we get the following warning:
    > >> >
    > >> >   arch/x86/kernel/vsyscall_64.c: In function ‘do_emulate_vsyscall’:
    > >> >   arch/x86/kernel/vsyscall_64.c:111:7: warning: ‘ret’ may be used uninitialized in this function
    > >>
    > >> What's the code path that uses ret without initializing it?
    > >
    > > If the code is correct but GCC got confused then please use the
    > > simplest possible patch to help GCC find its way around the code.
    >
    > The simplest patch is to mark ret as uninitialized_var.

    No - that primitive really sucks as it might hide *future* debug
    warnings and silently break code.

    The problem with uninitialized_var() is that such code:

    int test(void)
    {
    int uninitialized_var(ret);

    return ret;
    }

    Builds without a single warning but it is very broken code.

    So if we use uninitialized_var() and the code is changed in the
    future to have the above broken sequence, we'll have a silent runtime
    failure ...

    So we try to avoid using uninitialized_var() in arch/x86/ and use
    explicit initialization instead.

    That way GCC that can see through the flow will optimize away the
    superfluous initialization - GCC versions that are older will
    generate one more instruction but that's OK.

    Thanks,

    Ingo
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