Fawzi Mohamed wrote:
On 12-apr-10, at 21:40, Steven Schveighoffer wrote:
On Mon, 12 Apr 2010 13:45:14 -0400, Jérôme M. Berger
<jeber...@free.fr> wrote:
Steven Schveighoffer wrote:
J�r�me M. Berger Wrote:
J�r�me M. Berger wrote:
OK, I found a solution that is optimal in over 99% of the cases
(99.195% to be precise), while still being fast (because it avoids
looping over the bits):
I've run my function and Andrei's on all possible min, max
pairs in
0..299 without checking, just for the timing. On my computer (Athlon
X2 64 @2GHz), my function took 50s and Andrei's took 266s. The
difference should be even bigger for larger numbers.
Can I ask, what is the point of having a non-exact solution (unless
you are just doing this for fun)?
We are talking about range propagation, a function of the compiler,
not a function of the compiled program. Therefore, slower but more
exact functions should be preferred, since they only affect the
compile time. Note that you will only need to do this range
propagation on lines that "or" two values together, and something
that reduces the runtime of the compiler by 216 seconds, but only
when compiling enough code to have over 8 billion 'or' operations in
it (300^4), I don't think is really that important. Give me the
exact solution that prevents me from having to cast when the
compiler can prove it, even if the runtime of the compiler is
slightly slower.
Funny you should say that given the current thread comparing the
speed of the D compiler to that of the Go compiler...
My point was simply that the amount of time saved is relative to the
size of the program being compiled. If we assume conservatively that
every other line in a program has bitwise or in it, then you are
talking about a 16 billion line program, meaning the 216 seconds you
save is insignificant compared to the total compile time. My point
was that your fast solution that is inaccurate is not preferable
because nobody notices the speed, they just notice the inaccuracy.
We are talking about range propagation, a function of the compiler,
not a function of the compiled program. Since we can't get a 100%
accurate representation of the possible values anyway (even yours
might leave holes in the middle after all), then it might be better
to choose a faster, slightly less precise algorithm if the
difference is not too great. That's the difference between a
compiler and a full-fledged static code analysis an program prover.
When we're talking about the difference between O(1) and O(lgn), I'll
take accuracy over speed in my compiler any day. The solution should
be 100% accurate for the problem statement. If the problem statement
is not what we need, then we need a new problem statement :) Solving
the problem statement for 99% of values is not good enough.
Anyway, the point is moot, I have a new version of my algorithm
with 100% precision and high speed.
Yes, I'm still trying to understand how it works :)
Sorry for the probably stupid question, but I don't understand much the
need of all this range propagation, in the compiler either you have a a
constant (and then you have the value at compile time, and you don't
need any range propagation, you can compare with the value), or you have
a runtime value.
It's been part of DMD2 for a while now. It allows you to do things like:
ubyte lowbits(int x)
{
return x & 7;
}
without an explicit cast. The compiler knows that x&7 can safely fit
inside a single byte. Whereas ((x&7) << 12) | (x&3);
does not fit, and requires an explicit cast.
