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    SubjectRe: [PATCH RFC 2/4] include/linux/compiler.h: allow memory operands
    On Wed, Jan 02, 2019 at 03:57:54PM -0500, Michael S. Tsirkin wrote:
    > We don't really care whether the variable is in-register
    > or in-memory. Relax the constraint accordingly.
    >
    > Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
    > ---
    > include/linux/compiler.h | 2 +-
    > 1 file changed, 1 insertion(+), 1 deletion(-)
    >
    > diff --git a/include/linux/compiler.h b/include/linux/compiler.h
    > index 1ad367b4cd8d..6601d39e8c48 100644
    > --- a/include/linux/compiler.h
    > +++ b/include/linux/compiler.h
    > @@ -154,7 +154,7 @@ void ftrace_likely_update(struct ftrace_likely_data *f, int val,
    > #ifndef OPTIMIZER_HIDE_VAR
    > /* Make the optimizer believe the variable can be manipulated arbitrarily. */
    > #define OPTIMIZER_HIDE_VAR(var) \
    > - __asm__ ("" : "=r" (var) : "0" (var))
    > + __asm__ ("" : "=rm" (var) : "0" (var))
    > #endif

    I think this can break for architectures with write-back addressing modes
    such as arm, where the "m" constraint is assumed to be evaluated precisely
    once in the asm block.

    Will

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