round ties behaviour

[simonbyrne]

This has come up before (#5983), but probably deserves its own issue. How should round handle ties, that is, values with fractional part of 1/2.

The two most common options are:
a) round ties away from zero: round(0.5) = 1.0, round(1.5) = 2.0.
b) round ties to even (aka banker's rounding): round(0.5) = 0.0, round(1.5) = 2.0.

LLVM provides an intrinsic for (a), but this typically requires multiple CPU instructions, and doesn't vectorize (see #8364). On the other hand (b) often corresponds to a native CPU instruction (e.g. table 4-15 of the Intel developer manual), but I don't know if LLVM offers a way to access it. (UPDATE: rint can be used, but depends on rounding mode).

The IEEE spec requires that we offer both, but doesn't have anything to say about names or interface.

Here is what other languages do:

Language	a) Ties away from zero	b) Ties to even	Other
C (1999+) / C++ (2007+)	round and variants	via rint/nearbyint (depends on global rounding mode) , roundeven in future
Python	2.x: round	3+: round
Matlab/Octave	round
R		round
Mathematica		Round
.NET (C#, F#, VB)	via optional argument	Math.Round
Java			round: ties to +Inf
JavaScript			round: ties to +Inf
Fortran (77+)	ANINT
ALGOL		ROUND (though not specified in standard)
Common Lisp		fround
Scheme		round
Emacs lisp		fround
Haskell		round
OCaml			does not have a round function
Clojure			round: ties to +Inf
Erlang		round
Racket		round
Ruby	round
Rust	round
Go			does not have a Round function
Lua			does not have a round function
Pascal	ISO: round	Free Pascal: round	GNU Pascal: Round machine dependent
Modula 2			round implementation defined.
Perl			does not have a round function
PHP	round	via optional argument	other modes available (toward zero, to odd)
OpenGL		roundEven	round is up to implementation (presumably whatever is fastest).
OpenCL	round	rint (regardless of rounding mode)
Ada	Rounding	Unbiased_Rounding	Machine_Rounding: whatever is most efficient.
APL			No round?
AppleScript	via rounding as taught in school option	round
SAS	ROUND	ROUNDE
PostScript	round
COBOL	ROUNDED clause	via options
D	round (changed in dlang/phobos@91c38b4)
Forth		ISO: FROUND
Tcl	round (clarified in bug report)
Smalltalk	rounded
Prolog		SICStus: round	ISO: round ties to +Inf
Delphi		Round
Xpath		round-half-to-even	round: ties to +Inf
SQL	ROUND (mostly, though varies)

My summary:

• Older languages typically use (a).
• Those without a focus on floating point either copy C or don't implement it.
• Lisps and functional languages seem to prefer (b).
• Java and JavaScript are bizarre (particularly Java pre-v7).

[davidssmith]

FWIW, I suspect most scientific users will expect (a) to happen.

It would be nice if iround and round were consistent.

[simonbyrne]

@davidssmith I guess if you define "scientific users" as C/Fortran/Matlab. Mathematica/R/Python 3 users might disagree.

[davidssmith]

Wow, interesting that Python is changing their rounding behavior.

I guess I meant people who learned rounding in math class, not CS class. I think round should keep doing (a), and we should retain intrinsics to do (b), preferably using a native LLVM instruction should it become available. I often have inner loops where rounding is occurring and could use either behavior, especially if one was much faster.

[timholy]

@simonbyrne: first, that table is amazing. Really nice work. Second, when it comes to floating point, I'd just do whatever @simonbyrne told me to do...oh, right, that's you. Seriously, I'd put a lot of faith in whatever you think is best, but I tend to generally think Julia should usually follow whatever is implemented by the hardware (unless there are strong reasons to do otherwise).

[ViralBShah]

That table really is amazing. It should perhaps be added to the wikipedia entry on Rounding. I would have found (a) to be more intuitive, and wouldn't have guessed that so many systems choose (b). I also would have naively expected that (a) and not (b) would have a native CPU instruction.

FWIW, the R page says the following - Note that for rounding off a 5, the IEC 60559 standard is expected to be used, ‘go to the even digit’.

Cc: @alanedelman @ArchRobison

[simonbyrne]

It would be great to know why Python decided to change. I had a look through the python-dev archives, but couldn't find any discussion as to how it was decided. If anyone knows more, I would certainly be interested to hear it.

The classic argument for (b) is that it is unbiased: if you're summing or averaging a bunch of rounded numbers, you won't introduce an unexpected bias. It's typical in stats (I was taught this way in my first-year undergrad stats class), which is probably why R does it that way.

[ViralBShah]

Apparently, the ISO 60559 standard is identical to IEEE 754.

[ViralBShah]

@simonbyrne That does sound intuitive. The R manual claims that the IEEE standard requires (b). The Wikipedia page on IEEE seems to be consistent with what you say - provide both modes, but no names or defaults are discussed.

[simonbyrne]

I tried to create a benchmark here:
https://gist.github.com/simonbyrne/1e4f5234eaa70725c30f

This compares the native instruction rint_llvm, doing (b) with two competing implementations of round, which does (a) (see discussion of #8364). I'm using LLVM 3.3, and it didn't appear to use the vectorised instructions. The TLDR: (a) can be done basically as fast as (b), at least on my setup.

[simonbyrne]

And to answer my own question about LLVM support: looking at the source, it seems they don't currently expose ties-to-even instruction in a way that doesn't depend on rounding mode (if they did, the appropriate code would be 0x0).

[simonbyrne]

I emailed Jeffrey Yasskin asking about the reason for the Python change, he responded:

Here's the email that decided it for Python:
https://mail.python.org/pipermail/python-dev/2008-January/075910.html
Here's a paper that discusses it a bit in the context of C++:
http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2012/n3352.html#basic_operations

[StefanKarpinski]

Can't we just have an optional rounding mode argument to round that defaults to RoundFromZero? I find the numerical arguments for breaking ties to even a bit questionable and both of the motivations given in that Python email seem to be the same reason to me.

[simonbyrne]

I don't find these numerical arguments terribly convincing either. They do make sense in the context of ordinary floating point operations (which is what the C++ link was referring to), but when you call round you're intentionally losing precision, so I don't think it really makes sense to talk about accuracy. While the statistician in me appreciates unbiasedness, it's clear that there are valid reasons for other behaviour: I even came across a reasonable argument for Prolog/Java/JavaScript-style rounding involving grid interpolation.

Now we can dispatch on rounding mode, it should be fairly easy to implement them all via optional arguments to round, so the main question is what should be the default?

The main reasons for (a):

• For most people, it's how they learnt it in school (cf AppleScript).
• It's what C--and all languages which use the C libm--does.

The main reasons for (b):

• It's faster on any SSE4.1 (i.e. Intel post-2008, AMD post-2011) processor (see [1]).
• It's conceptually elegant, in that "round to nearest integer" means the same thing as "round to nearest floating point value".

I would lean toward (b) purely for performance reasons, but am not going to be disheartened if we go the other way.

[1] The "prevfloat" trick I used for the fast benchmark for (a) isn't strictly correct according to the IEEE standard, as it will misbehave under different global rounding modes, and incorrectly raise Inexact floating point exceptions, which I suspect is why it isn't used in any library. Also, the copysign seemed to prevent it from vectorizing, though I imagine that this should be fixable by LLVM.

[vchuravy]

Isn't (b) also defined as the standard rounding behaviour in IEEE754-2008 (at least according to Wikipedia it is)
I would stick to IEEE754 as close as possible and expose the other rounding behaviours as options and round-to-even as default

[simonbyrne]

@vchuravy No, in section 5.9 (or for free, see this late draft) it clearly says to offer:

• roundToIntegralTiesToEven: (b) above
• roundToIntegralTowardZero: trunc in Julia
• roundToIntegralTowardPositive: ceil
• roundToIntegralTowardNegative: floor
• roundToIntegralTiesToAway: (a) above
• roundToIntegralExact: not exported, but available via Base.Math.rint (v0.4 only)