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How does your programming language handle “minus zero” (-0.0)?

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Re: How does your programming language handle “minus zero” (-0.0)?

#151

Post by quelsolaar »

neurocline wrote: post #136
quelsolaar wrote: post #66

Earlier quoted context omitted.

I dont think this is true. Integer math is used in some game engines because they need to be deterministic, between networked computers (and different CPUs rounds floating point numbers differently). I have written an engine like that, and i know that Starcraft 2 is all integer for the same reason. No one does it because its faster or easier. Its a pain.

Starcraft II uses floats, but for the game simulation portion it uses an internal software floating-point library to ensure consistency.

You are right, all the graphics are full of floats. Writing "all" was a bit of an overreach.

Re: How does your programming language handle “minus zero” (-0.0)?

#152

Post by throwaway8581 »

TacticalCoder wrote: post #96
TorKlingberg wrote: post #60

I feel like floating point is a completely separate branch of programming that a lot of coders never use. When they do it's often a mistake, like for currency. Yet floating point is very useful for scientific calculations and simulations, which is what computers were all about for the first couple of decades. I have a suspicion that on a fundamental level, floating point isn't actually good for games or machine learn…

> When they do it's often a mistake, like for currency. The most beautiful FP bug I remember was a denial of service in some webservers where by setting the header "Accepted Language: en-gb;q=0.3 en;q=0.8 ..." to a specific value you could send the official Java floating-point parser in an infinite loop (and this affected several Java webservers). So at each webpage request, you were sending one CPU core of the webse…

There is nothing wrong with the protocol. The protocol has nothing to do with what representation is used by software that speaks the protocol. Software could just as well parse those values as integers. Just drop the decimal point and pad with zeroes. Or use a decimal type (but there is really no need to).

Re: How does your programming language handle “minus zero” (-0.0)?

#153

Post by Qwertious »

adwn wrote: post #106
linkdd wrote: post #102

Earlier quoted context omitted.

Yes, that's exactly why I wrote: > some game engines and > simple games A 3D game is not a simple game.

Ugh, do you really have to be this nitpicky? Fine, you wrote: > the approximation is generally good enough for a game No, it's not generally good enough for a game – only for some games, and not good enough for any game with 3D graphics.

Speaking of nitpicky, I thought the gamecube's GPU was integer-driven? And it has plenty of 3D games.

Re: How does your programming language handle “minus zero” (-0.0)?

#154

Post by boomlinde »

bendiksolheim wrote: post #2

Does anyone have a good example of a programming problem where we _need_ signed zero? Or where it makes things significantly simpler? As far as I know, there is no distinction between -0 and +0 in math, so I have never really understood why this is a thing in computers.

> Does anyone have a good example of a programming problem where we _need_ signed zero? Or where it makes things significantly simpler?

Maybe when you're implementing floating point numbers in a way that's simple and widely applicable?

I'm only half joking, too, though I can't tell you what exactly having a distinct sign bit simplifies.

I can say off the bat that it is probably useful that a very small quantity that might otherwise lose enough precision to round to zero maintains its sign regardless.

Re: How does your programming language handle “minus zero” (-0.0)?

#155

Post by zokier »

quelsolaar wrote: post #123
bjourne wrote: post #115

Earlier quoted context omitted.

Which modern general-purpose CPUs do not follow IEEE 754? I don't think what you're saying is true.

They all follow IEEE 754. But that defines the representation of the bits in a floating point number, not what happens when you operate on them. Different implementations do rounding differently, and there are arguments about how to do this correctly. There can be multiple ways to represent the same number even (this is a reason not to ever do a == compare on floating point number that have been operated on). Not onl…

IEEE 754 does also specify operations and their behavior. E.g. section 5.1 (from 2008 edition):

> All conforming implementations of this standard shall provide the operations listed in this clause for all supported arithmetic formats, except as stated below. Each of the computational operations that return a numeric result specified by this standard shall be performed as if it first produced an intermediate result correct to infinite precision and with unbounded range, and then rounded that intermediate result, if necessary, to fit in the destination’s format (see 4 and 7).

Re: How does your programming language handle “minus zero” (-0.0)?

#156

Post by retrac »

As an aside, historically many of the early binary computers used ones complement. So they had both -0 and +0 integers as well. While ones complement is probably more obvious than twos complement, it was near-universally considered a terrible mistake by the mid-1960s, and everything of note since has been twos complement.

It didn't die quickly though. The UNIVAC is still with us (in emulation) and that is likely the reason why the C standard addresses the question of negative zero integers. (Their handling is implementation specific, of course.)

Re: How does your programming language handle “minus zero” (-0.0)?

#157

Post by brandmeyer »

rav wrote: post #98
brandmeyer wrote: post #54

Earlier quoted context omitted.

> You shouldn't really use equality on floating point numbers, except on very special circumstances. This is very common advice, so common that it gets cargo-culted into situations where it is really quite poor. Information storage, retrieval, and transmission systems should faithfully deliver floating-point values that are good to the last bit. Round-trips through databases, transmission over network protocols, etc…

By "the last bit" do you mean the 32nd bit or the 64th bit? :-) Many times I've tracked down the place in our stack where a double-precision value from user input accidentally goes through a single-precision variable in some C code somewhere and crashes some Python code later on because the values don't match "to the last bit" in the way that the programmer thought... But that's a bug in the C code - I agree complete…

This is actually exactly what I mean. Its probably the most common bug I've come across in this class. I don't expect unit tests to capture all rounding bugs (say, due to serialization and de-serialization through text). But I do expect to capture gross errors, such as an inadvertent cast to lower-precision somewhere in the pipeline.

I've worked with highly experienced and accomplished software engineers that expected interchange through protobuf or sql to be inaccurate due to rounding. No! If you stick a finite number in, you should get the exact same finite number back out again. Direct equality is fine for most cases. The sign bit of zero and NaN should also be returned faithfully and tested using memcmp when required.

IMO, the payload bits of NaN should also be also returned faithfully, but too many systems in common practice drop them.

Re: How does your programming language handle “minus zero” (-0.0)?

#158

Post by lylecheatham »

legulere wrote: post #80
TorKlingberg wrote: post #60

I feel like floating point is a completely separate branch of programming that a lot of coders never use. When they do it's often a mistake, like for currency. Yet floating point is very useful for scientific calculations and simulations, which is what computers were all about for the first couple of decades. I have a suspicion that on a fundamental level, floating point isn't actually good for games or machine learn…

One example where floating point does not match the problem field is GIS. Coordinates are bound by the earth's circumference. Fixed point would make much more sense, but neither hardware nor software support is there.

I mean most GIS software is pretty aware of precision models. Take GEOS for example, it's got a variety of ways to specify the precision in number of decimal places for a geometry. It still uses doubles underneath, but that's to avoid unneeded code complexity.

Re: How does your programming language handle “minus zero” (-0.0)?

#159

Post by edflsafoiewq »

linkdd wrote: post #83
quelsolaar wrote: post #78

Earlier quoted context omitted.

>And computing integers IS faster than computing floats, at least on a CPU. In the abstract yes. In reality not so much. A lot of what floats are used for in games is vector math (you are often in a 2D/3D world), and vector math in fixed point requires a lot more work in integer math since you constantly need to shift down things in order to avoid overflow. Overflow bugs are a constant problem when doing dot products…

I never use the square root in games, I always compare squared distances (since the properties are the same). If you try to do with Fixed Point Arithmetic what was intended to be done with Floating Point, you're the problem. A 32 bits integer can hold values up to 4 billions, if I have that kind of value in a simple game, then yes i will switch to Floating Point Arithmetic, but when does that use case happen if you'r…

The biggest problem with fixed point is reciprocals behave very poorly; ie. x·(1/x)=1 and 1/(1/x)=x fail very badly. In order for these to work you obviously need the same amount of numbers above 1 as below 1, like in floating point.

Using fixed point with 11 bits after the decimal, with numbers as low as 100 (18 bits) we already have errors as large as 100·(1/100)=0.977.

Re: How does your programming language handle “minus zero” (-0.0)?

#160

Post by PaulHoule »

TorKlingberg wrote: post #60

I feel like floating point is a completely separate branch of programming that a lot of coders never use. When they do it's often a mistake, like for currency. Yet floating point is very useful for scientific calculations and simulations, which is what computers were all about for the first couple of decades. I have a suspicion that on a fundamental level, floating point isn't actually good for games or machine learn…

It is the opposite.

From Microsoft BASIC to Javascript, many programmers have worked in languages that only have floats. Financial calculations involve exponential/log math much like scientific problems. There are issues with rounding and representation there (e.g. there is no such thing as $0.01 in floating point, only $0.50, $0.25, $0.125 and some sum of 1/2 fractions that comes very close to $0.01 and even reads in and writes out as $0.01 even though it isn't.)

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