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    SubjectRe: [PATCH 1/2] UNREACHABLE() macro
    On Fri, Sep 11, 2009 at 10:52:28PM -0700, Andrew Morton wrote:
    > On Fri, 11 Sep 2009 21:43:42 -0700 "H. Peter Anvin" <hpa@kernel.org> wrote:
    >
    > > On 09/11/2009 08:49 PM, Arjan van de Ven wrote:
    > > > On Fri, 11 Sep 2009 14:55:25 -0700 (PDT)
    > > > Roland McGrath <roland@redhat.com> wrote:
    > > >
    > > >>> That's a bit of a mouthful. Did you consider a runtime probe with
    > > >>> scripts/Kbuild.include's try-run, cc-option, etc?
    > > >>
    > > >> I did not see any precedent in the sources for using those to test for
    > > >> features by compiling particular test sources (i.e. in autoconf
    > > >
    > > > look at the stackprotector flags.. they work this way already.
    > > > It gets done once per kernel build...
    > > >
    > >
    > > That works for flags, but not for the presence of builtin functions.
    > > You can't even just try compiling something, since it will turn into an
    > > ordinary function if not present... not obvious until link.
    > >
    >
    > Use -Wall -Werror and if the compiler doesn't know about
    > __builtin_unreachable() it will error out.

    Yeah, e.g. checking exit status of:
    echo '__attribute__((noreturn)) void foo (void) { asm volatile ("" : : : "memory"); __builtin_unreachable (); }' | gcc -S -O2 -W -Wall -Werror -xc - -o /dev/null > /dev/null 2>/dev/null
    works.

    Jakub


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