Re: constexpr is really very smart!

Michael S <already5chosen@yahoo.com> writes:

On Thu, 19 Dec 2024 00:57:20 -0800
Tim Rentsch <tr.17687@z991.linuxsc.com> wrote:
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Michael S <already5chosen@yahoo.com> writes:
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On Wed, 18 Dec 2024 09:45:38 -0800
Tim Rentsch <tr.17687@z991.linuxsc.com> wrote:
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Michael S <already5chosen@yahoo.com> writes:

[...]

According to my measurement gear, in range 0 to 92 there are few
points where [the code I posted] is faster than simple loop, but
in majority of cases it is slower.

>
I'm at a loss to understand how this could happen.  In my own
measurements, the code shown above runs faster than a simple loop
in all cases above n > 12, more than twice as fast when n > 17,
more than three times as fast when n > 42, and going up from
there.  What might account for these radically different results?

>
May be, number of repetitions?
I run the test only once.  That gives relative advantage to smaller
code which is less sensitive to cold ICache and to cold branch
predictors.

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That's an interesting idea.  Can you also run a measurement where
the code is run inside loops?  I think it would be instructive
to compare the results under the two approaches.

[...]

>
I feel that running fib(n) with the same n in loop too unrealistic.
So I decided to run, at least, with different values of n.

I agree, that is a better choice for a measurement test load.

Ended up spending about a hour just to build a test bench.
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The answer is - in a loop of more than dozen iterations your code is
indeed faster.  Esp. so for hundred or more iterations.

I'm happy to learn that my instincts were validated here.  And I
also learned something, about the effects of code warmup.  Very
interesting.

Here is my test bench.  [...]

Thank you for that.  Sadly I am not able to run it because my
test environment is lacking _rdtsc(), and I haven't been able
to find out how to install it.

By the way, I send you an email earlier today.  Did that get to
you?