5 ms·

Processors change, compilers change. If you care about such details, best to ship multiple variants and pick the fastest one at runtime. As I've mentioned here

• dist-epoch 2y ago
  Funnily enough, this is sort of what the NVIDIA drivers do: they intercept game shaders and replace them by custom ones optimized by NVIDIA. Which is why you see stuff like this in NVIDIA drivers changelog: "optimized game X, runs 40% faster"
  • snicker7 2y ago
    Imagine being the dev competing game Y and seeing the changelog.
    • surajrmal 2y ago
      It wouldn't be surprising to find out Nvidia talks directly with game developers to give them hints as to how to optimize their games
      • account42 2y ago
        It wouldn't be surprising at all seeing as they are even in the credits for most games.
  • esperent 2y ago
    I'd love to read more about this, what kind of changes they make and how many games they do it for. Do they ever release technical articles about it?
    • sigmoid10 2y ago
      The other commenter makes it sound a bit more crazy than it is. "Intercept shaders" sounds super hacky, but in reality, games simply don't ship with compiled shaders. Instead they are compiled by your driver for your exact hardware. Naturally that allows the compiler to perform more or less aggressive optimisations, similar to how you might be able to optimise CPU programs by shipping C code and only compiling everything on the target machine once you know the exact feature sets.
      • crazygringo 2y ago
        I don't know -- if that other commenter is correct, it does sound pretty crazy. Improving your compiler for everybody's code is one thing. But saying, if the shader that comes in is exactly this code from this specific game, then use this specific precompiled binary, or even just apply these specific hand-tuned optimizations that aren't normally applied, that does seem pretty crazy to me. But I don't know which it is?
        • Cieric 2y ago
          Finger printing based on shaders is quite rare, really most of the time we detect things like the exe name calling us or sometime, very rarely they will give us a better name through an extension. (unreal engine does this automatically). From there all the options are simple, but full shader replacements are one. In the api I work on the shaders have a built in hash value, so that along with the game identified means we know exsactly what shader it is. Most of the replacements aren't complicated though, it's just replacing slow things with faster things for our specific hardware. In the end we are the final compiler so us tweaking things to work better should be expected to a degree.
          • chrisjj 2y ago
            > most of the time we detect things like the exe name calling us What could possibly go wrong? :)
  • crazygringo 2y ago
    Wow, how did they pick which games to optimize? Does the studio pay them to do it? Because Nvidia wouldn't care otherwise? Does Nvidia do it unasked, for competitive reasons? To maximize how much faster their GPU's perform than competitors' on the same games? And therefore decide purely by game popularity? Or is it some kinda of alliance thing between Nvidia and studios, in exchange for something like the studios optimizing for Nvidia in the first place, to further benefit Nvidia's competitive lead?
    • flohofwoe 2y ago
      AFAIK NVIDIA and AMD do this unasked for popular game releases because it gives them a competitive advantage if 'popular game X' runs better on NVIDIA than AMD (and vice versa). If you're an AAA studio you typically also have a 'technical liason' at the GPU vendors though. It's basically an arms race. This is also the reason why graphics drivers for Windows are so frigging big (also AFAIK).
      • diegoperini 2y ago
        Double "AFAIK" makes me trust you more :) I think this is very accurate. The exception is probably those block buster games. Those probably get direct consultancy from NVIDIA during the development to make them NVIDIA-ready from day 1.
        • account42 2y ago
          Even smaller games often get some amount of consultation from Nvidia/AMD - they are typically in the credits in some form.
    • Cieric 2y ago
      I can't give details on how we do our selections (not nvidia but another gpu manufacturer). But we do have direct contacts into a lot of studios and we do try and help them fix their game if possible before ever putting something in the driver to fix it. Studios don't pay us, it's mutually benefital for us to improve the performance of the games. It also help the game run better on our cards by avoiding some of the really slow stuff. In general if our logo is in the game, we helped them by actually writing code for them, if it's not then we might have only given them directions on how to fix issues in their game or put something in the driver to tweak how things execute. From an outside perspective (but still inside on the gpu space) nvidia does give advice to keep their competitive advantage. In my experience so far ignoring barriers that are needed as per the spec, defaulting to massive numbers when the gpu isn't known ("batman and tessellation" should be enough to find that), and then doing out right weird stuff that doesn't look like something any sane person would do in writing shaders (I have a thought in my head for that one, but it's not considered public knowledge. )
  • Cieric 2y ago
    I don't work on the nvidia side of things but it's likely to be the same. Shader replacement is only one of a whole host of things we can do to make games run faster. It's actually kind of rare for use to do them since it boats the size of the driver so much. A lot of our options do change how shaders work though, like forcing a shader to use double precision floats instead of the single it was compiled with.
    • SpaghettiCthulu 2y ago
      > A lot of our options do change how shaders work though, like forcing a shader to use double precision floats instead of the single it was compiled with. What benefit would that give? Is double precision faster than single on modern hardware?
      • kevingadd 2y ago
        I would guess it's done for compatibility or quality reasons
      • dahart 2y ago
        Only more precision. But no, doubles are not faster. At best they’re the same instruction latency & throughput as singles, and that’s only on a few expensive pro/datacenter GPUs. Even if they are technically the same instruction speed, they’re still 2x the memory & register usage, which can compromise perf in other ways. Doubles on consumer GPUs are typically anywhere from 16 to 64 times slower than singles. FWIW, I’ve never heard of shader replacement to force doubles. It’d be interesting to hear when that’s been used and why, and surprising to me if it was ever done for a popular game.
      • Cieric 2y ago
        That's specifically because gpus aren't IEEE compliant, and calculations will drift differently on different gpus. Double precision can help avoid divide by zero errors in some shaders because most don't guard against that and NANs propagate easily and show up as visual corruption.
        • 2y ago
  • chrisjj 2y ago
    > "optimized game X, runs 40% faster" ... and looks 4O% crappier? E.g. stuttery, because the driver does not get to see the code ahead of time.
• Amadiro 2y ago
  I think figuring out the fastest version of a shader at runtime is very non-trivial, I'm not aware of any game or engine that can do this. I think it'd be possible in principle, because most APIs (D3D, GL, Vulkan etc) expose performance counters (which may or may not be reliable depending on the vendor), and you could in principle construct a representative test scene that you replay a couple times to measure different optimizations. But a lot of games are quite dynamic, having dynamically generated scenes and also dynamically generated shaders, so the number of combinations you might have to test seems like an obstacle. Also you might have to ask the user to spend time waiting on the benchmark to finish. You could probably just do this ahead of time with a bunch of different GPU generations from each vendor if you have the hardware, and then hard-code the most important decision. So not saying it'd be impossible, but yeah I'm not aware of any existing infrastructure for this.
  • hansvm 2y ago
    The last time I did anything like this (it was for CPU linear algebra code designed to run in very heterogeneous clusters), I first came up with a parameterization that approximated how I'd expect an algorithm to perform. Then, once for each hardware combination, you sweep through the possible parameterization space. I used log-scaled quantization to make it cheap to index into an array of function pointers based on input specifics. The important thing to note is that you can do that computation just once, like when you install the game, and it isn't that slow. Your parameterization won't be perfect, but it's not bad to create routines that are much faster than any one implementation on nearly every architecture.
  • ijustlovemath 2y ago
    you'd only have to test worst/median case scenes, which you could find with a bit of profiling!
• alexvitkov 2y ago
  This would be acceptable if it meant adding one more shader, but with "modern" graphics APIs forcing us to sometimes have thousands of permutations for the same shader, every variant you add multiplies that count by 2x. We also don't have an infinite amount of time to work on each shader. You profile on the hardware you care about, and if the choice you've made is slower on some imaginary future processor, so be it - hopefully that processor is faster enough that this doesn't matter.
  • account42 2y ago
    Graphics APIs don't force your to have thousands of shaders. The abstraction in your engine might.