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    SubjectRe: [patch] __volatile__ needed in get_cycles()?
    Hi Andrea,
    On Mon, 29 Mar 1999, Andrea Arcangeli wrote:

    > On Mon, 29 Mar 1999, Tigran Aivazian wrote:
    > >which would enforce "Von Neumann execution stream", e.g. by doing CPUID
    > What is a Von Neumann execution stream? ;)
    P6 architecture (PPro, PII etc.) introduce speculative execution, i.e. if
    you for example try to "profile" fdiv by putting a couple of rdtsc before
    and after you will be told that fdiv took 0 cycles which is obviously not
    true (I wish it was :). This happens because the processor decides that
    the second rdtsc is independent from the fdiv and executes it first. So,
    one needs to serialize it somehow and the easiest way I know of doing it
    is cpuid (but one needs to remember that it clobbers registers).

    > Why you need CPUID?
    > If you need to know some features from the CPU you probably want to use
    > the information we grabbed at boot time in the current_cpu_data struct.
    It is just for serializing, not to get any cpu features.

    > About the volatile thing the reason I thought it's not needed is that we
    > care that rdtsc is run always at the same offset of code. We care only
    > about the _delta_ between the two rdtsc. So basically I seen not using
    > __volatile__ as a feature. Comments?
    The first "care" you mean "don't care"? So I am not the only human being
    who switches "yes/no", "do/don't".... Good.

    Btw, __volatile__ in get_cycles() did go into 2.2.5. And the
    justification, I hope, is that by doing so it can be used later on for
    some other purpose. Putting __volatile__ does not make the current usage
    of get_cycles() any worse so why not, if it gives you extra choice?

    I personally use it to count the number of cycles it takes for a
    particular code path (i.e. without having to enable profiling globally). I
    know it is not very accurate (without the usual triple cache warmup
    thingy and deducting overheads etc) but it *can* be used to compare one
    instruction sequence with another.

    There is a Intel's paper on this on called something
    like "Using RDTSC for Performance Monitoring" (not exactly, can't remember
    the exact name).


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