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    SubjectRe: [PATCH] perfcounters: fix type/event_id layout on big-endian systems

    * Paul Mackerras <paulus@samba.org> wrote:

    > Ingo Molnar writes:
    >
    > > hm, this ifdef really looks ugly. How about just changing event_id
    > > to 64 bits and having a separate u32 type field? The size impact is
    > > minimal, the cleanliness win is significant :-)
    >
    > We wanted to get a complete and unique identifier for the event
    > into 64 bits so that we could put it into the ring buffer for
    > PERF_RECORD_GROUP and have it take up only one 8-byte slot and yet
    > identify uniquely which counter's value follows it. I don't know
    > that that is absolutely necessary but it sounds like a nice
    > property.
    >
    > We could easily go back to a 1-bit raw field and have the type be
    > either a 64-bit raw value or an 8-bit type plus 32 or 56-bit
    > event_id.
    >
    > Or we could keep the current layout but use explicit shifts and
    > masks rather than bitfields.
    >
    > I don't know the current C rules concerning unions very well, but
    > I have the impression that writing to one member of a union and
    > reading another is undefined behaviour, which is another strike
    > against the current code if true...
    >
    > Anyway, the point is that the current code doesn't compile on
    > powerpc and wouldn't work properly even if it did, so we need to
    > do something.

    yeah - i pulled your fix, thanks Paul.

    Maybe the best option is to get rid of the bitfields and use masks
    ...

    Ingo


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