On 13-apr-10, at 12:02, Don wrote:
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.
ah ok I understand, I thought that was treated like x & cast(ubyte)7 ,
and so as comparison of the compile time value with the ranges of
ubyte (no range propagation needed).
But I can understand why it is treated as cast(ubyte)((cast(int)x) &
7), as it is probably easier for the compiler, as it upcasts by default.
Thanks for the explanation.