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The gold standard of optimization: A look under the hood of RollerCoaster Tycoon

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Re: The gold standard of optimization: A look under the hood of RollerCoaster Tycoon

#31

Post by sublinear »

lukan wrote: post #29
sublinear wrote: post #24

Earlier quoted context omitted.

This way of thinking has caused at least a few prominent recurring bugs I can think of. Texture resolution mismatches causing blurriness/aliasing, floating point errors and bad level design causing collision detection problems (getting stuck in the walls), frame rate and other update rates not being synced causing stutter and lag (and more collision detection problems), bad illumination parameters ruining the look th…

Game designer != game engine designer (But it definitely helps if the game designer knows of the technical limits)

Sorry, I'm not super familiar with professional game dev, but I am familiar with professional web dev. The problems seem similar, as evidenced by the constant complaining here on HN about the state of the web.

Who formats or cleans up the assets and at least oversees that things are done according to a consistent spec, process, and guidelines? Is that not a game designer or someone under their leadership?

I think in all the cases I gave, what might be completely delegated to "engine design" really should be teamwork with game design and art direction too. This is what the top-level comment was talking about. Even when a game is "well made", they just adopted someone else's standards and that sucks all the soul out of it. This is a common problem in all creative work.

(adding this due to reply depth): Coordination is a big aspect of design and can often be the most impactful to the result.

Re: The gold standard of optimization: A look under the hood of RollerCoaster Tycoon

#32

Post by lukan »

sublinear wrote: post #31
lukan wrote: post #29

Earlier quoted context omitted.

Game designer != game engine designer (But it definitely helps if the game designer knows of the technical limits)

Sorry, I'm not super familiar with professional game dev, but I am familiar with professional web dev. The problems seem similar, as evidenced by the constant complaining here on HN about the state of the web. Who formats or cleans up the assets and at least oversees that things are done according to a consistent spec, process, and guidelines? Is that not a game designer or someone under their leadership? I think in…

It depends how big the studio is, but a job of a game designer is usually not cleaning up assets. It is to well, design the game. The big picture.

Re: The gold standard of optimization: A look under the hood of RollerCoaster Tycoon

#35

Post by shaggie76 »

shakow wrote: post #15
fweimer wrote: post #4

What language is this article talking where compilers don't optimize multiplication and division by powers of two? Even for division of signed integers, current compilers emit inline code that handles positive and negative values separately, still avoiding the division instruction (unless when optimizing for size, of course).

That's what I would have thought as well, but looks like that on x86, both clang and gcc use variations of LEA. But if they're doing it this way, I'm pretty sure it must be faster, because even if you change the ×4 for a https://godbolt.org/z/EKj58dx9T

Not only is LEA more flexible I believe it's preferred to SHL even for simple operations because it doesn't modify the flags register which can make it easier to schedule.

Re: The gold standard of optimization: A look under the hood of RollerCoaster Tycoon

#36

Post by adrian_b »

lefty2 wrote: post #8

> The same trick can also be used for the other direction to save a division: > NewValue = OldValue >> 3; You need to be careful, because this doesn't work if the value is negative. A

It works fine when the value is negative.

However, there is a quirk of the hardware of most CPUs that has been inherited by the C language and by other languages.

There are multiple ways of defining integer division when the dividend is not a multiple of the divisor, depending on the rounding rule used for the quotient.

The 2 most frequently used definitions is to have a positive remainder, which corresponds to rounding the quotient by using the floor function, and to have a remainder of the same sign with the quotient, which corresponds to rounding the quotient by truncation.

In most CPUs, the hardware is designed such that for signed integers the division instruction uses the second definition, while the right shift uses the first definition.

This means that when the dividend is a multiple of the divisor, division and right shift are the same, but otherwise the quotient may differ by one unit due to different rounding rules.

Because of this, compilers will not replace automatically divisions with right shifts, because there are operands where the result is different.

Nevertheless, the programmer can always replace a division by a power of two with a right shift. In all the programs that I have ever seen, either the rounding rule for the quotient does not matter or the desired definition for the division is the one with positive remainder, i.e. the definition implemented by right shift.

In those cases when the rounding rule matters, the worrisome case is when you must use division not when you can use right shift, so you must correct the result to correspond to rounding by floor, instead of the rounding by truncation provided by the hardware. For this, you must not use the "/" operator of the C language, but one of the "div" functions from "stdlib.h", or you may use "/" but divide the absolute values of the operands, after which you compute the correct signed results.

Re: The gold standard of optimization: A look under the hood of RollerCoaster Tycoon

#37

Post by cjbgkagh »

rawling wrote: post #23
cjbgkagh wrote: post #10

Earlier quoted context omitted.

It was written in assembly so goes through an assembler instead of a compiler.

I assume GP is talking about the bit in the article that goes > RCT does this trick all the time, and even in its OpenRCT2 version, this syntax hasn’t been changed, since compilers won’t do this optimization for you.

That makes more sense, I second their sentiment, modern compilers will do this. I guess the trick is knowing to use numbers that have these options.

Re: The gold standard of optimization: A look under the hood of RollerCoaster Tycoon

#38

Post by bombcar »

edflsafoiewq wrote: post #9
applfanboysbgon wrote: post #7

> Imagine a programmer asking a game designer if they could change their formula to use an 8 instead of a 9.5 because it is a number that the CPU prefers to calculate with. There is a very good argument to be made that a game designer should never have to worry about the runtime performance characteristics of binary arithmetic in their life, that’s a fate reserved for programmers Numeric characteristics are absolutel…

Examples?

Read all of the Factorio Friday Facts https://factorio.com/blog/ - a number of the more obscure bug/performance issues come down to making something fit naturally into a value the CPU can handle.

Re: The gold standard of optimization: A look under the hood of RollerCoaster Tycoon

#39

Post by bombcar »

cjbgkagh wrote: post #37
rawling wrote: post #23

Earlier quoted context omitted.

I assume GP is talking about the bit in the article that goes > RCT does this trick all the time, and even in its OpenRCT2 version, this syntax hasn’t been changed, since compilers won’t do this optimization for you.

That makes more sense, I second their sentiment, modern compilers will do this. I guess the trick is knowing to use numbers that have these options.

There was a recent article on HN about which compiler optimizations would occur and which wouldn't and it was surprising in two ways - first, it would make some that you might not expect, and it would not make others that you would - because in some obscure calling method, it wouldn't work. Fixing that path would usually get the expected optimization.

Re: The gold standard of optimization: A look under the hood of RollerCoaster Tycoon

#40

Post by ryandrake »

timschmidt wrote: post #11
applfanboysbgon wrote: post #7

> Imagine a programmer asking a game designer if they could change their formula to use an 8 instead of a 9.5 because it is a number that the CPU prefers to calculate with. There is a very good argument to be made that a game designer should never have to worry about the runtime performance characteristics of binary arithmetic in their life, that’s a fate reserved for programmers Numeric characteristics are absolutel…

Absolutely. I have written a small but growing CAD kernel which is seeing use in some games and realtime visualization tools ( https://github.com/timschmidt/csgrs ) and can say that computing with numbers isn't really even a solved problem yet. All possible numerical representations come with inherent trade-offs around speed, accuracy, storage size, complexity, and even the kinds of questions one can ask (it's often…

Back in the early, early days, the game designer was the graphic designer, who also was the programmer. So, naturally, the game's rules and logic aligned closely with the processor's native types, memory layout, addressing, arithmetic capabilities, even cache size. Now we have different people doing different roles, and only one of them (the programmer) might have an appreciation for the computer's limits and happy-paths. The game designers and artists? They might not even know what the CPU does or what a 32 bit word even means.

Today, I imagine we have conversations like this happening:

Game designer: We will have 300 different enemy types in the game.

Programmer: Things could be really, really faster if you could limit it to 256 types.

Game designer: ?????

That ????? is the sign of someone who is designing a computer program who doesn't understand the basics of computers.

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