[issue3166] Make conversions from long to float correctly rounded.

Mark Dickinson added the comment: (Slightly updated version of) patch applied in r71772 (trunk), r71773 (py3k). -- resolution: -> accepted stage: patch review -> committed/rejected status: open -> closed ___ Python tracker

[issue3166] Make conversions from long to float correctly rounded.

Mark Dickinson added the comment: Updated patch; applies cleanly to current trunk. No significant changes. Note that there's now a new reason to apply this patch: it ensures that the result of a long->float conversion is independent of whether we're using 30-bit digits or 15-bit digits for

[issue3166] Make conversions from long to float correctly rounded.

Changes by Mark Dickinson : -- assignee: -> marketdickinson priority: -> normal ___ Python tracker ___ ___ Python-bugs-list mailing l

[issue3166] Make conversions from long to float correctly rounded.

Changes by Mark Dickinson : Removed file: http://bugs.python.org/file12320/long_as_double2.patch ___ Python tracker ___ ___ Python-bugs-list ma

[issue3166] Make conversions from long to float correctly rounded.

Changes by Mark Dickinson : Removed file: http://bugs.python.org/file10694/long_as_double.patch ___ Python tracker ___ ___ Python-bugs-list mai

[issue3166] Make conversions from long to float correctly rounded.

Mark Dickinson added the comment: Updated patch; cleanup of comments and slight refactoring of code. Int->float conversions are even a speck faster than the current code, for small inputs. (On my machine, on a Friday night, during a full moon. Your results may differ. :)) Also, retarget

[issue3166] Make conversions from long to float correctly rounded.

Changes by Mark Dickinson <[EMAIL PROTECTED]>: Removed file: http://bugs.python.org/file12312/long_as_double2.patch ___ Python tracker <[EMAIL PROTECTED]> ___ _

[issue3166] Make conversions from long to float correctly rounded.

Mark Dickinson <[EMAIL PROTECTED]> added the comment: Minor cleanup of long_as_double2.patch. Added file: http://bugs.python.org/file12320/long_as_double2.patch ___ Python tracker <[EMAIL PROTECTED]>

[issue3166] Make conversions from long to float correctly rounded.

Mark Dickinson <[EMAIL PROTECTED]> added the comment: Thanks for your comments, Alexander. Here's a rewritten version of the patch that's better commented and somewhat less convoluted; I think it should be easier to verify the correctness of this one. Added file: http://bugs.python.org/file1

[issue3166] Make conversions from long to float correctly rounded.

Mark Dickinson <[EMAIL PROTECTED]> added the comment: > floating-point variable "x" has an exact nonnegative integer value > between 0 and 2**DBL_MANT_DIG. Hmm. On closer inspection that's not quite true. After the line x = x * PyLong_BASE + (dig & (PyLong_BASE - pmask)); x has a value of

[issue3166] Make conversions from long to float correctly rounded.

Mark Dickinson <[EMAIL PROTECTED]> added the comment: By the way, the algorithm here is essentially the same as the algorithm that I implemented for the float.fromhex method, except that the float.fromhex method is more complicated in that it may have to deal with signed zeros or subnormals.

[issue3166] Make conversions from long to float correctly rounded.

Alexander Belopolsky <[EMAIL PROTECTED]> added the comment: On Tue, Dec 9, 2008 at 12:39 PM, Mark Dickinson <[EMAIL PROTECTED]> wrote: .. > What would using Python's integer type solve, that isn't already solved by > the patch? > Speaking for myself, it would alleviate the irrational fear of any

[issue3166] Make conversions from long to float correctly rounded.

Mark Dickinson <[EMAIL PROTECTED]> added the comment: As you say, performance would suffer. What would using Python's integer type solve, that isn't already solved by the patch? I know the code isn't terribly readable; I'll add some comments explaining clearly what's going on.

[issue3166] Make conversions from long to float correctly rounded.

Changes by Gabriel Genellina <[EMAIL PROTECTED]>: -- nosy: +gagenellina ___ Python tracker <[EMAIL PROTECTED]> ___ ___ Python-bugs-list

[issue3166] Make conversions from long to float correctly rounded.

Alexander Belopolsky <[EMAIL PROTECTED]> added the comment: .. > The idea's attractive. The problem is finding an integer type that's > guaranteed to have enough bits to store the mantissa for the float > (probably plus one or two bits more for comfort); for IEEE 754 this > means a 64-bit integ

[issue3166] Make conversions from long to float correctly rounded.

Mark Dickinson <[EMAIL PROTECTED]> added the comment: [Alexander] > I also wonder whether round to nearest float can be implemented without > floating point arithmetics. I would think round towards zero should be > a simple matter of extracting an appropriate number of bits from the > long and

[issue3166] Make conversions from long to float correctly rounded.

Mark Dickinson <[EMAIL PROTECTED]> added the comment: [Alexander] > The flags you may be looking for are -msse2 -mfpmath=sse Thanks, Alexander! [Alexander again, from an earlier post...] > I noticed that you replaced a call to _PyLong_AsScaledDouble with your > round to nearest algorithm. I w

[issue3166] Make conversions from long to float correctly rounded.

Alexander Belopolsky <[EMAIL PROTECTED]> added the comment: On Tue, Dec 9, 2008 at 11:02 AM, Mark Dickinson <[EMAIL PROTECTED]> wrote: ... > If your Intel machine is Pentium 4 or newer, you can get > around this by using the SSE2 extensions, which work with 64-bit doubles > throughout. I don't r

[issue3166] Make conversions from long to float correctly rounded.

Mark Dickinson <[EMAIL PROTECTED]> added the comment: > Intel uses 80 bits float in internals, but load/store uses 64 bits > float. Load/store looses least significant bits. Exactly. If your Intel machine is Pentium 4 or newer, you can get around this by using the SSE2 extensions, which work w

[issue3166] Make conversions from long to float correctly rounded.

STINNER Victor <[EMAIL PROTECTED]> added the comment: An interresting document: "Request for Comments: Rounding in PHP" http://wiki.php.net/rfc/rounding ___ Python tracker <[EMAIL PROTECTED]> _

[issue3166] Make conversions from long to float correctly rounded.

STINNER Victor <[EMAIL PROTECTED]> added the comment: About -O0 vs -O1, I think that I understood (by reading the assembler). pseudocode of the -O0 version: while () { load x from the stack x = x * ... + ... write x to the stack } pseudocode of the -O1 version: while

[issue3166] Make conversions from long to float correctly rounded.

Mark Dickinson <[EMAIL PROTECTED]> added the comment: Victor, what does >>> 1e16 + 2. give on your Ubuntu 2.5 machine? (Humor me. :) ) ___ Python tracker <[EMAIL PROTECTED]> ___ _

[issue3166] Make conversions from long to float correctly rounded.

STINNER Victor <[EMAIL PROTECTED]> added the comment: Ok, I understand why different versions of the same code gives different results: compiler flags! Python 2.5.1 is my Ubuntu version (should be compiled with -O3) whereas Python 2.7 and 3.1a0 are compiled by me with -00. Results with Python

[issue3166] Make conversions from long to float correctly rounded.

STINNER Victor <[EMAIL PROTECTED]> added the comment: Python 2.5.1 (r251:54863, Jul 31 2008, 23:17:40) >>> reduce(lambda x,y: x*32768.0 + y, [256, 0, 0, 1, 32767]) 2.9514790517935283e+20 >>> float(295147905179352891391L) 2.9514790517935289e+20 Python 2.7a0 (trunk:67679M, Dec 9 2008, 14:29:12) >

[issue3166] Make conversions from long to float correctly rounded.

STINNER Victor <[EMAIL PROTECTED]> added the comment: float(295147905179352891391L) gives different result on Python 2.5 and Python 2.6: - 2.9514790517935289e+20 # Python 2.5.1 - 2.9514790517935283e+20 # 2.7a0 whereas the code is the same!? ___ Python tr

[issue3166] Make conversions from long to float correctly rounded.

Alexander Belopolsky <[EMAIL PROTECTED]> added the comment: Mark, I noticed that you replaced a call to _PyLong_AsScaledDouble with your round to nearest algorithm. I wonder if _PyLong_AsScaledDouble itself would benefit from your change. Currently it is used in PyLong_AsDouble and long_tru

[issue3166] Make conversions from long to float correctly rounded.

STINNER Victor <[EMAIL PROTECTED]> added the comment: You may use "if (nbits == (size_t)-1 && PyErr_Occurred())" to check _PyLong_NumBits() error (overflow). Well, "if (numbits > DBL_MAX_EXP)" should already catch overflow, but I prefer explicit test to check the error case. Anyway, interrest

[issue3166] Make conversions from long to float correctly rounded.

Changes by STINNER Victor <[EMAIL PROTECTED]>: -- nosy: +haypo ___ Python tracker <[EMAIL PROTECTED]> ___ ___ Python-bugs-list mailing l

[issue3166] Make conversions from long to float correctly rounded.

David Jones <[EMAIL PROTECTED]> added the comment: I agree, longs should be correctly rounded when coerced to floats. There is an ugly (but amusing) workaround while people wait for this patch: Go via a string: int(float(repr(295147905179352891391)[:-1])) Though I assume this relies on the p

[issue3166] Make conversions from long to float correctly rounded.

New submission from Mark Dickinson <[EMAIL PROTECTED]>: If n is a Python long, then one might expect float(n) to return the closest float to n. Currently it doesn't do this. For example (with Python 2.6, on OS X 10.5.2/Intel): >>> n = 295147905179352891391L The closest float to n is equal t