7 ms·

Lookup tables were indeed a common technique used by games in the past.

asveikau 9y ago
And present, too, right? It's not the same reason as it would have been in the 80s, but today in performance critical code it is not uncommon to reduce the number of conditionals for better CPU pipelining, and lookup tables are a very common tool for this.
- derefr 9y ago
  I have a strong feeling that Super Mario Maker always has the same physics engine going, but just has four different lookup tables that it switches between depending on the level theme. Anyone want to partially disassemble it for comparison?
  - ageitgey 9y ago
    Super Mario Maker uses "New SMB" physics for all themes. It doesn't actually mimic the exact physics of each game (aside from small tweaks like not letting you wall-jump or carry shells depending on the theme). They did this because newer players found it super confusing to go back to the older physics. From Takashi Tezuka: > “In the end we used the New Super Mario Bros. U system for all of the game styles. There was quite a lot of discussion about this within the team. Staff who had strong attachment to the original games expressed a strong desire to see implemented the same system they remembered. However, when players who are used to the modern Mario physics tried playing with the old physics, they found it much more difficult than they remembered." People on reddit have done some pretty extensive breakdowns of the physics of different Mario versions vs. SMM: - https://imgur.com/XC38rcX https://imgur.com/XC38rcX - https://www.reddit.com/r/MarioMaker/comments/4iqa5s/super_mario_world_physics_another_reason_why/d310ay5/ https://www.reddit.com/r/MarioMaker/comments/4iqa5s/super_ma...
- vardump 9y ago
  > it is not uncommon to reduce the number of conditionals for better CPU pipelining, and lookup tables are a very common tool for this. On modern CPUs, data dependency, such as lookup tables often cause pipeline stalls — worse pipelining. L1 cache is at a premium as well, you rarely want to waste it to access LUTs. You can compute a lot in 12 cycles caused by L2 hit (L1 miss). In theory up to 32 * 12 = 384 floating point operations.
  - BeeOnRope 9y ago
    To be fair, replacing a series of ALU ops with a lookup table doesn't usually add a "data dependency" - the data dependency probably already existed, but perhaps flowed through registers rather than memory. What adding a lookup table can do is to add the load-latency to the dependency chain involving the calculation, which seems to be what you are talking about here. For an L1 hit that's usually 4 or 5 cycles, and for L2 hits and beyond it's worse, as you point out. How much that actually matters depends on whether the code is latency-bound and the involved lookup is on the critical path: in many cases where there is enough ILP it won't be (an general rule is that in most code most instructions are not on a critical dependency-chain). If the involved method isn't that hot then L1 misses (like your example) or worse are definitely a possibility. On the other hand, in that case performance isn't that critical by definition. If the method is really hot, e.g., in a tight(ish) loop, then you are mostly going to be getting L1 hits. The comparison with 384 FOPs seems a bit off: I guess you are talking about about some 32-FOP per cycle SIMD implementation (AVX512?) - but the assumption of data dependencies kind of rules that out: one would assume it's scalar code here. If it's vectorization, then the whole equation changes!
    - vardump 9y ago
      > To be fair, replacing a series of ALU ops with a lookup table doesn't usually add a "data dependency" If it's not vectorizable, LUT result is often used for indirect jump/call (like large switch statement) or memory access (say, a histogram etc.). > What adding a lookup table can do is to add the load-latency to the dependency chain involving the calculation, which seems to be what you are talking about here. Yeah, used a bit sloppy terminology. Loads can affect performance system wide. To be a win, LUT function needs to be something pretty heavy, while LUT itself needs to be small (at least <16 kB, preferably <1 kB). > If the method is really hot, e.g., in a tight(ish) loop, then you are mostly going to be getting L1 hits. That really depends. It's generally good to keep L1 footprint small. There are just 512 of 64-byte L1 cache lines. Hyperthread shares L1 as well. There can be other hot loops nearby that could also benefit from hot L1. It's very easy to start to spill to L2 (and further). Microbenchmarks often miss "system" level issues. > The comparison with 384 FOPs seems a bit off 384 was for the extreme vectorization case, 12 x 2 x 8 FMACs (AVX). Most vendors count FMACs nowadays as two FOPs... > If it's vectorization, then the whole equation changes Well, isn't that where the performance wins are and what you need to do to extract maximum performance from that hot loop? A good truly parallel vector gather implementation could make (small) LUTs very interesting performance wise.
- white-flame 9y ago
  I think the biggest difference is that modern games tend to be written for variable frame rates, stuffing floating point time deltas through equations. A lookup table meshes much better with fixed frame rate gameplay, either with one entry per frame, or quantizing countdown timers of how many frames to wait to go to the next state.
  - usernam 9y ago
    Actually, modern (and not so modern) physics engines as used in games generally use a fixed time delta for each step, and just iterate faster/slower to keep the simulation in sync[1]. This is done for many reasons, but predominantly numerical stability. [1] not the full story
- simooooo 9y ago
  Yes I recall a Forza motorsport physics guy saying they used a simple lookup table for the chart which holds the curve for the limit if grip on a tyre. Beats the crab out if calculating it every time.
katastic 9y ago
Thank you. I wrote a long post but then I decided not to post it... Always remember to view past "Tricks" from the era they were written in. Lookup tables, in the 90's, for example, were used EVERYWHERE. People always used for sin/cos/tan functions. Fixed-point math was very common as well. Nowadays, it may seem like "magic" but it's not. It was just "the way" to get things done. I'm not downplaying the skill of the programmers of those eras, it's just keep it in perspective. Many "tricks" weren't invented for that one game, they were common place for all programmers using those platforms. I used to use compiled sprites all the time in the 90's. They seem like magic today, but they were just another kind of drawing to us. In Allegro 4, you could even draw one by simply calling a "make compiled sprite" function and making sure to not draw it outside of the clipping rectangle (since they can't be clipped). That was it. You build it with a builder function, and then you call draw on it. But it "seems" insane nowadays to convert the bits of a bitmap into raw machine code that draws those bits to save some CPU cycles.
- mercer 9y ago
  I was going to respond to an earlier comment of yours, decided I couldn't quite figure out what to ask, and so kept reading. But after this comment I think my question is a bit clearer: please write longer posts or start a blog with stories of the kind of work you were doing > 10 years ago. It's fascinating, and you're a good writer!

And present, too, right? It's not the same reason as it would have been in the 80s, but today in performance critical code it is not uncommon to reduce the number of conditionals for better CPU pipelining, and lookup tables are a very common tool for this.

derefr 9y ago
I have a strong feeling that Super Mario Maker always has the same physics engine going, but just has four different lookup tables that it switches between depending on the level theme. Anyone want to partially disassemble it for comparison?
- ageitgey 9y ago
  Super Mario Maker uses "New SMB" physics for all themes. It doesn't actually mimic the exact physics of each game (aside from small tweaks like not letting you wall-jump or carry shells depending on the theme). They did this because newer players found it super confusing to go back to the older physics. From Takashi Tezuka: > “In the end we used the New Super Mario Bros. U system for all of the game styles. There was quite a lot of discussion about this within the team. Staff who had strong attachment to the original games expressed a strong desire to see implemented the same system they remembered. However, when players who are used to the modern Mario physics tried playing with the old physics, they found it much more difficult than they remembered." People on reddit have done some pretty extensive breakdowns of the physics of different Mario versions vs. SMM: - https://imgur.com/XC38rcX https://imgur.com/XC38rcX - https://www.reddit.com/r/MarioMaker/comments/4iqa5s/super_mario_world_physics_another_reason_why/d310ay5/ https://www.reddit.com/r/MarioMaker/comments/4iqa5s/super_ma...
vardump 9y ago
> it is not uncommon to reduce the number of conditionals for better CPU pipelining, and lookup tables are a very common tool for this. On modern CPUs, data dependency, such as lookup tables often cause pipeline stalls — worse pipelining. L1 cache is at a premium as well, you rarely want to waste it to access LUTs. You can compute a lot in 12 cycles caused by L2 hit (L1 miss). In theory up to 32 * 12 = 384 floating point operations.
- BeeOnRope 9y ago
  To be fair, replacing a series of ALU ops with a lookup table doesn't usually add a "data dependency" - the data dependency probably already existed, but perhaps flowed through registers rather than memory. What adding a lookup table can do is to add the load-latency to the dependency chain involving the calculation, which seems to be what you are talking about here. For an L1 hit that's usually 4 or 5 cycles, and for L2 hits and beyond it's worse, as you point out. How much that actually matters depends on whether the code is latency-bound and the involved lookup is on the critical path: in many cases where there is enough ILP it won't be (an general rule is that in most code most instructions are not on a critical dependency-chain). If the involved method isn't that hot then L1 misses (like your example) or worse are definitely a possibility. On the other hand, in that case performance isn't that critical by definition. If the method is really hot, e.g., in a tight(ish) loop, then you are mostly going to be getting L1 hits. The comparison with 384 FOPs seems a bit off: I guess you are talking about about some 32-FOP per cycle SIMD implementation (AVX512?) - but the assumption of data dependencies kind of rules that out: one would assume it's scalar code here. If it's vectorization, then the whole equation changes!
  - vardump 9y ago
    > To be fair, replacing a series of ALU ops with a lookup table doesn't usually add a "data dependency" If it's not vectorizable, LUT result is often used for indirect jump/call (like large switch statement) or memory access (say, a histogram etc.). > What adding a lookup table can do is to add the load-latency to the dependency chain involving the calculation, which seems to be what you are talking about here. Yeah, used a bit sloppy terminology. Loads can affect performance system wide. To be a win, LUT function needs to be something pretty heavy, while LUT itself needs to be small (at least <16 kB, preferably <1 kB). > If the method is really hot, e.g., in a tight(ish) loop, then you are mostly going to be getting L1 hits. That really depends. It's generally good to keep L1 footprint small. There are just 512 of 64-byte L1 cache lines. Hyperthread shares L1 as well. There can be other hot loops nearby that could also benefit from hot L1. It's very easy to start to spill to L2 (and further). Microbenchmarks often miss "system" level issues. > The comparison with 384 FOPs seems a bit off 384 was for the extreme vectorization case, 12 x 2 x 8 FMACs (AVX). Most vendors count FMACs nowadays as two FOPs... > If it's vectorization, then the whole equation changes Well, isn't that where the performance wins are and what you need to do to extract maximum performance from that hot loop? A good truly parallel vector gather implementation could make (small) LUTs very interesting performance wise.
white-flame 9y ago
I think the biggest difference is that modern games tend to be written for variable frame rates, stuffing floating point time deltas through equations. A lookup table meshes much better with fixed frame rate gameplay, either with one entry per frame, or quantizing countdown timers of how many frames to wait to go to the next state.
- usernam 9y ago
  Actually, modern (and not so modern) physics engines as used in games generally use a fixed time delta for each step, and just iterate faster/slower to keep the simulation in sync[1]. This is done for many reasons, but predominantly numerical stability. [1] not the full story
simooooo 9y ago
Yes I recall a Forza motorsport physics guy saying they used a simple lookup table for the chart which holds the curve for the limit if grip on a tyre. Beats the crab out if calculating it every time.

katastic 9y ago

Thank you. I wrote a long post but then I decided not to post it... Always remember to view past "Tricks" from the era they were written in. Lookup tables, in the 90's, for example, were used EVERYWHERE. People always used for sin/cos/tan functions. Fixed-point math was very common as well. Nowadays, it may seem like "magic" but it's not. It was just "the way" to get things done. I'm not downplaying the skill of the programmers of those eras, it's just keep it in perspective. Many "tricks" weren't invented for that one game, they were common place for all programmers using those platforms. I used to use compiled sprites all the time in the 90's. They seem like magic today, but they were just another kind of drawing to us. In Allegro 4, you could even draw one by simply calling a "make compiled sprite" function and making sure to not draw it outside of the clipping rectangle (since they can't be clipped). That was it. You build it with a builder function, and then you call draw on it. But it "seems" insane nowadays to convert the bits of a bitmap into raw machine code that draws those bits to save some CPU cycles.

mercer 9y ago
I was going to respond to an earlier comment of yours, decided I couldn't quite figure out what to ask, and so kept reading. But after this comment I think my question is a bit clearer: please write longer posts or start a blog with stories of the kind of work you were doing > 10 years ago. It's fascinating, and you're a good writer!