4 ms·

I wish there was more explicit support for fixed point decimal out there.

by taeric 4mo ago

koolala 4mo ago
Speed wise no hardware can do it though.
- taeric 4mo ago
  This depends entirely on the sizes needed. For a surprising number of things people would use fixed point for, I would be surprised if you couldn't get good speed with surprisingly little effort.
  - vardump 4mo ago
    I would be surprised if you could get even one third of performance of floats. So why bother?
    - taeric 4mo ago
      If you don't have to do trig, I'd be surprised if you aren't faster by default, oddly. Indeed, if you are just adding and subtracting, it is just a number. If you are doing multiplication, it is a multiply and shift. So long as you don't try and support massive numbers of different fixed sizes, that shift is almost certainly still cheaper than float hardware. (Indeed, a lot of multiplications wouldn't even need the shift...) Again, I do not mean this as a criticism of floats. For simulations and for numbers where you do have to support completely arbitrary values, there is a reason floats are a thing.
      - jcranmer 4mo ago
        An integer add, sub, or shift is 1 cycle of latency; integer mul is generally 3 cycles; integer div is lol-that-is-slow. Floating-point adds, subs, muls, and fmas are generally 4 cycles, with div being lol-that-is-slow (but generally faster than integer division because your divisor and dividend have fewer bits). So fixed-point addition and subtraction are definitely faster, multiplication is a wash if you're doing binary-based fixed point (but slower if you're doing decimal-based fixed point), and fixed-point division is definitely slower than floating-point division.
        taeric 4mo ago
        Kudos on providing the numbers. I wasn't confident in the numbers that I remembered and with how pipelined everything is, I didn't know how much to lean on them. Not exactly my standard workflow to care about this level of speed. My gut would still be that it is typically a wash for most everyone as far as speeds go? If default libraries supported it more directly, I would think it would largely be a win for a lot of reasoning. In particular, silly stuff like 1e32 + 1e1 would not be nearly as surprising to most people. And the entire class of bugs around stuff like doing something until it reaches 0.9 would almost certainly go away if we guaranteed precision to a set number decimal places. Alas, default libraries do not support this, though. So the above is admittedly wishful thinking on my end. And I could as easily describe a world where people insist on arbitrary numbers of fixed point values and how that would be its own set of landmines.
        DarkUranium 4mo ago
        I very commonly see fixed-point libs be slower than floating-point (assuming floating-point hardware is available --- soft-float is slow as hell compared to fixed-point, of course). Commonly enough that I think it's some fundamental reason (given available/current hardware as opposed to hypothetical). Two reasons I can think of, though granted, they only apply in certain niches: 1) Floating-point SIMD extensions are far more common than integer ones. This means you can compute N (often N=4 or 8) float operations in one instruction, vs 1 integer operation. 2) For any GPGPU processing: GPUs far prefer floats, to the point where you didn't even use to have integers available (somewhat ironically, the platforms that prefer float much more strongly to int are mobile/embedded ones nowadays --- which is the exact opposite of the CPU situation). To this day, you have a `mul24` intrinsic for integer multiplication in some languages ... which converts two integers to floats, multiplies them as floats, and then converts back to integers. Yes, that was faster than direct multiplication. I'm sure many GPUs do it directly nowadays though. It's also worth considering that a typical fixed-point multiply (as opposed to integer) is an integer multiply followed by a shift; often to a 2×-bit intermediate, if you want to preserve precision. That's a cost.
        3mo ago
bee_rider 4mo ago
Do you mean fixed point using integers, or actually decimal (base-10)? My gut would be to use fixed-point integers with a power of two in the denominator. On the other hand, I’m not sure if modern compilers even bother converting division/multiplications by powers of two to shifts these days, so maybe it isn’t worth it…
- taeric 4mo ago
  Fair that relying on decimal is an interesting choice. My gut is that it would help with a lot of reasoning for folks. This comes up all of the time with stuff like lat/lng values. People are convinced you have to use doubles because of the inaccuracy of floats. Completely skipping over the fact that you could have fixed point accuracy to 6 decimal digits with the same number of bits as a float. You just reduce your max/min value that you can represent. Which, for lat/lng, you are already heavily bounded. I think it is fair to argue that basic libraries don't support trig on common fixed point sizes. But that is ultimately my lament. Floats are amazing for what they need to do. For what many people need, though, it feels overkill because it is overkill.
  - addaon 4mo ago
    > This comes up all of the time with stuff like lat/lng values. People are convinced you have to use doubles because of the inaccuracy of floats. Using any floating point representation for lat/long makes no sense at all to me. Floating point is designed for representing values where expected error scales with magnitude, not for values where error is constant with magnitude; the latter requires fixed point. I truly don't understand why someone would optimize their position representation for maximum accuracy off the coast of Africa, instead of having it uniform across the world.
    - taeric 4mo ago
      Agreed. But I'm assuming we are in the minority on this one? Bad assumption?
      - 4mo ago

I wish there was more explicit support for fixed point decimal out there.

by taeric 4mo ago

I wish there was more explicit support for fixed point decimal out there.

koolala 4mo ago

Speed wise no hardware can do it though.

taeric 4mo ago
This depends entirely on the sizes needed. For a surprising number of things people would use fixed point for, I would be surprised if you couldn't get good speed with surprisingly little effort.
- vardump 4mo ago
  I would be surprised if you could get even one third of performance of floats. So why bother?
  - taeric 4mo ago
    If you don't have to do trig, I'd be surprised if you aren't faster by default, oddly. Indeed, if you are just adding and subtracting, it is just a number. If you are doing multiplication, it is a multiply and shift. So long as you don't try and support massive numbers of different fixed sizes, that shift is almost certainly still cheaper than float hardware. (Indeed, a lot of multiplications wouldn't even need the shift...) Again, I do not mean this as a criticism of floats. For simulations and for numbers where you do have to support completely arbitrary values, there is a reason floats are a thing.
    - jcranmer 4mo ago
      An integer add, sub, or shift is 1 cycle of latency; integer mul is generally 3 cycles; integer div is lol-that-is-slow. Floating-point adds, subs, muls, and fmas are generally 4 cycles, with div being lol-that-is-slow (but generally faster than integer division because your divisor and dividend have fewer bits). So fixed-point addition and subtraction are definitely faster, multiplication is a wash if you're doing binary-based fixed point (but slower if you're doing decimal-based fixed point), and fixed-point division is definitely slower than floating-point division.
      - taeric 4mo ago
        Kudos on providing the numbers. I wasn't confident in the numbers that I remembered and with how pipelined everything is, I didn't know how much to lean on them. Not exactly my standard workflow to care about this level of speed. My gut would still be that it is typically a wash for most everyone as far as speeds go? If default libraries supported it more directly, I would think it would largely be a win for a lot of reasoning. In particular, silly stuff like 1e32 + 1e1 would not be nearly as surprising to most people. And the entire class of bugs around stuff like doing something until it reaches 0.9 would almost certainly go away if we guaranteed precision to a set number decimal places. Alas, default libraries do not support this, though. So the above is admittedly wishful thinking on my end. And I could as easily describe a world where people insist on arbitrary numbers of fixed point values and how that would be its own set of landmines.
        DarkUranium 4mo ago
        I very commonly see fixed-point libs be slower than floating-point (assuming floating-point hardware is available --- soft-float is slow as hell compared to fixed-point, of course). Commonly enough that I think it's some fundamental reason (given available/current hardware as opposed to hypothetical). Two reasons I can think of, though granted, they only apply in certain niches: 1) Floating-point SIMD extensions are far more common than integer ones. This means you can compute N (often N=4 or 8) float operations in one instruction, vs 1 integer operation. 2) For any GPGPU processing: GPUs far prefer floats, to the point where you didn't even use to have integers available (somewhat ironically, the platforms that prefer float much more strongly to int are mobile/embedded ones nowadays --- which is the exact opposite of the CPU situation). To this day, you have a `mul24` intrinsic for integer multiplication in some languages ... which converts two integers to floats, multiplies them as floats, and then converts back to integers. Yes, that was faster than direct multiplication. I'm sure many GPUs do it directly nowadays though. It's also worth considering that a typical fixed-point multiply (as opposed to integer) is an integer multiply followed by a shift; often to a 2×-bit intermediate, if you want to preserve precision. That's a cost.
        3mo ago

bee_rider 4mo ago

Do you mean fixed point using integers, or actually decimal (base-10)? My gut would be to use fixed-point integers with a power of two in the denominator. On the other hand, I’m not sure if modern compilers even bother converting division/multiplications by powers of two to shifts these days, so maybe it isn’t worth it…

taeric 4mo ago
Fair that relying on decimal is an interesting choice. My gut is that it would help with a lot of reasoning for folks. This comes up all of the time with stuff like lat/lng values. People are convinced you have to use doubles because of the inaccuracy of floats. Completely skipping over the fact that you could have fixed point accuracy to 6 decimal digits with the same number of bits as a float. You just reduce your max/min value that you can represent. Which, for lat/lng, you are already heavily bounded. I think it is fair to argue that basic libraries don't support trig on common fixed point sizes. But that is ultimately my lament. Floats are amazing for what they need to do. For what many people need, though, it feels overkill because it is overkill.
- addaon 4mo ago
  > This comes up all of the time with stuff like lat/lng values. People are convinced you have to use doubles because of the inaccuracy of floats. Using any floating point representation for lat/long makes no sense at all to me. Floating point is designed for representing values where expected error scales with magnitude, not for values where error is constant with magnitude; the latter requires fixed point. I truly don't understand why someone would optimize their position representation for maximum accuracy off the coast of Africa, instead of having it uniform across the world.
  - taeric 4mo ago
    Agreed. But I'm assuming we are in the minority on this one? Bad assumption?
    - 4mo ago