Mark Dickinson dicki...@gmail.com added the comment:
Because pfval in this example doesn't exist but is merely a temporary,
there is no aliasing.
Maybe aliasing wasn't the right word to use, then. The exact rule being
violated is C99 6.5, para. 7 (or C11 6.5 para. 7 if you prefer):
An
Kristján Valur Jónsson krist...@ccpgames.com added the comment:
Interesting.
I declare that this rule does not apply here since the code is a deliberate
hack: We are pretending that a certain address points to integers and checking
those integers.
If you insist on following the standard,
Changes by Kristján Valur Jónsson krist...@ccpgames.com:
--
Removed message: http://bugs.python.org/msg158808
___
Python tracker rep...@bugs.python.org
http://bugs.python.org/issue14381
___
Kristján Valur Jónsson krist...@ccpgames.com added the comment:
Interesting.
I declare that this rule does not apply here since the code is a deliberate
hack: We are pretending that a certain address points to integers and checking
those integers.
If you insist on following the standard,
Mark Dickinson dicki...@gmail.com added the comment:
I declare that this rule does not apply here ...
Clearly the gcc developers disagree. :-)
iwasawa:~ mdickinson$ cat test2.c
int is_positive_zero(double f) {
return *(long long*)f == 0;
}
iwasawa:~ mdickinson$ gcc -fstrict-aliasing -O3
Kristján Valur Jónsson krist...@ccpgames.com added the comment:
Dang. I yield!
--
___
Python tracker rep...@bugs.python.org
http://bugs.python.org/issue14381
___
___
Mark Dickinson dicki...@gmail.com added the comment:
Why do you think it isn't safe, Antoine?
It violates C's strict aliasing rules; Google for 'C strict aliasing' or 'C
type punning' for more information. This isn't just a theoretical concern:
gcc is known to make optimizations based on
Mark Dickinson dicki...@gmail.com added the comment:
We only support IEEE platforms.
I don't think that's true, BTW.
--
___
Python tracker rep...@bugs.python.org
http://bugs.python.org/issue14381
___
Kristján Valur Jónsson krist...@ccpgames.com added the comment:
return *(PY_LONG_LONG*)fval == 0;
There is no aliasing, because there are no pointer variables in existence.
If we did this:
double *pfval = fval;
PY_LONG_LONG *pl = (PY_LONG_LONG*)pfval
return *pfval == 0
Then we would have
Changes by Jesús Cea Avión j...@jcea.es:
--
nosy: +jcea
___
Python tracker rep...@bugs.python.org
http://bugs.python.org/issue14381
___
___
Python-bugs-list mailing list
Kristján Valur Jónsson krist...@ccpgames.com added the comment:
Why do you think it isn't safe, Antoine?
We only support IEEE platforms, and IEEE defines positive 0.0 to be all bits
cleared.
I personally don't think that even a small measurable performance degradation
in creating new floats
Changes by Eric Snow ericsnowcurren...@gmail.com:
--
nosy: +eric.snow
___
Python tracker rep...@bugs.python.org
http://bugs.python.org/issue14381
___
___
Python-bugs-list
Serhiy Storchaka storch...@gmail.com added the comment:
PyBench contains a lot of different tests and executes them not too long.
Random fluctuations of a few percent are possible. We need a more stable
realistic test working with floats.
$ ./python -m timeit 'x,y,d=1,0,0.01
for i in
Raymond Hettinger raymond.hettin...@gmail.com added the comment:
FWIW, I'm -1 on making the change. Any memory savings would be microscopic for
most Python programs. And anything that slows down float creation (even a
teeny bit) is detrimental to all Python programs. We're better off not
Kristján Valur Jónsson krist...@ccpgames.com added the comment:
Yes, there is a measurable performance decrease in pybench arithmetic tests.
Integers don't fall out of arithmetic that often, true. But integral floats
are incredibly common in tabular data. In a game such as Eve Online,
Antoine Pitrou pit...@free.fr added the comment:
Integers don't fall out of arithmetic that often, true. But integral
floats are incredibly common in tabular data. In a game such as Eve
Online, configuration data contains a lot of 0.0, 1.0, -1.0 and so
on. This patch saved us many
Kristján Valur Jónsson krist...@ccpgames.com added the comment:
The -10..11 range was determined empirically. As you see from the values,
only -10 shows up as significant... In fact, just storing 0 to 5 would capture
the bulk of the savings.
Right, I'll do some test with the hardcoded
Antoine Pitrou pit...@free.fr added the comment:
Right, I'll do some test with the hardcoded values you mentioned.
Btw, I don't think -0.0 is worth it, that value is a typical
arithmetic result and not something you typically get from input data.
I was suggesting -0.0 in the hope that it
Serhiy Storchaka storch...@gmail.com added the comment:
Try moving your changes from PyFloat_FromDouble to PyFloat_FromString. Look at
memory and perfomance.
--
___
Python tracker rep...@bugs.python.org
http://bugs.python.org/issue14381
Kristján Valur Jónsson krist...@ccpgames.com added the comment:
PyFloat_FromString() is very restrictive. In our case, for example, the floats
in question don't come from text files.
I'm adding a new file with changes suggested by Antoine. only 0.0 and 1.0 are
interned. It turns out that
Antoine Pitrou pit...@free.fr added the comment:
Ok. In the current state it doesn't look detrimental, although I'm not
convinced it's that useful either.
That said, the implementation needs a bit of work, I don't think it's ok for
float_is_positive_zero() to use the Py_LONG_LONG casting
New submission from Kristján Valur Jónsson krist...@ccpgames.com:
Michael Foord reminded me of this issue recently. It was discussed on pydev a
few years back and met with limited enthusiasm. I speak from experience in
live production with EVE that this simple change saved us a lot of
Serhiy Storchaka storch...@gmail.com added the comment:
Does it not decrease the performance?
Falling out integral floating point numbers in the mathematical floating point
calculations seems unlikely. I suggest interning integral floats only in
conversions str - float and int - float.
Mark Dickinson dicki...@gmail.com added the comment:
This looks like a duplicate of issue 4024.
--
nosy: +mark.dickinson
___
Python tracker rep...@bugs.python.org
http://bugs.python.org/issue14381
___
24 matches
Mail list logo
