7 ms·

Is this really C++ specific though? It seems like the optimisations are happening on a lower level, and so would 'infect' other languages too. Whatever the lan

Is this really C++ specific though? It seems like the optimisations are happening on a lower level, and so would 'infect' other languages too. Whatever the language, at some point in performance tweaking you will end up having to look at the assembly produced by your compiler, and discovering all kinds of surprises.

tialaramex 2y ago
LLVM isn't perfect, but the problem here is that there's a C++ compiler flag (-ffast-math) which says OK, disregard how arithmetic actually works, we're going to promise across the entire codebase that we don't actually care and that's fine. This is nonsense, but it's really common, distressingly common, for C and C++ programmers to use this sort of inappropriate global modelling. It's something which cannot scale, it works OK for one man projects, "Oh, I use the Special Goose Mode to make routine A better, so even though normal Elephants can't Teleport I need to remember that in Special Goose Mode the Elephants in routine B might Teleport". In practice you'll screw this up, but it feels like you'll get it right often enough to be valuable. In a large project where we're doing software engineering this is complete nonsense, now Jenny, the newest member of the team working on routine A, will see that obviously Special Goose Mode is a great idea, and turn it on, whereupon the entirely different team handling routine B find that their fucking Elephants can now Teleport. WTF. The need to never do this is why I was glad to see Rust stabilize (e.g) u32::unchecked_add fairly recently. This (unsafe obviously) method says no, I don't want checked arithmetic, or wrapping, or saturating, I want you to assume this cannot overflow. I am formally promising that this addition is never going to overflow, in order to squeeze out the last drops of performance. Notice that's not a global flag. I can write let a = unsafe { b.unchecked_add(c) }; in just one place in a 50MLOC system, and for just that one place the compiler can go absolutely wild optimising for the promise that overflows never happen - and yet right next door, even on the next line, I can write let x = y + z; and that still gets the kid gloves, if it overflows nothing catches on fire. That's how granular this needs to be to be useful, unlike C++ -ffast-math.
- gpderetta 2y ago
  You can set fast math (or a subset of it) on a translation unit basis.
  - tialaramex 2y ago
    Because the language works by textual inclusion and so a "translation unit" isn't really just your code this is much more likely to result in nasty surprises, up to and including ODR violations.
    - gpderetta 2y ago
      Yes, if you try hard enough I'm sure you can find ways to screw up. From a practical point of view it is fine.
  - cozzyd 2y ago
    Or even on a per function basis, at least with gcc (no clue about clang...)
    - gpderetta 2y ago
      In principle yes, you can use Attribute optimize, but I wouldn't rely on it. Too many bugs open against it.
      - cozzyd 2y ago
        It's worked for me in the past but maybe I got lucky.
  - pdw 2y ago
    That's not really true. If you link a shared library that was compiled with -ffast-math, that will affect the entire program. https://moyix.blogspot.com/2022/09/someones-been-messing-with-my-subnormals.html https://moyix.blogspot.com/2022/09/someones-been-messing-wit...

tialaramex 2y ago

LLVM isn't perfect, but the problem here is that there's a C++ compiler flag (-ffast-math) which says OK, disregard how arithmetic actually works, we're going to promise across the entire codebase that we don't actually care and that's fine. This is nonsense, but it's really common, distressingly common, for C and C++ programmers to use this sort of inappropriate global modelling. It's something which cannot scale, it works OK for one man projects, "Oh, I use the Special Goose Mode to make routine A better, so even though normal Elephants can't Teleport I need to remember that in Special Goose Mode the Elephants in routine B might Teleport". In practice you'll screw this up, but it feels like you'll get it right often enough to be valuable. In a large project where we're doing software engineering this is complete nonsense, now Jenny, the newest member of the team working on routine A, will see that obviously Special Goose Mode is a great idea, and turn it on, whereupon the entirely different team handling routine B find that their fucking Elephants can now Teleport. WTF. The need to never do this is why I was glad to see Rust stabilize (e.g) u32::unchecked_add fairly recently. This (unsafe obviously) method says no, I don't want checked arithmetic, or wrapping, or saturating, I want you to assume this cannot overflow. I am formally promising that this addition is never going to overflow, in order to squeeze out the last drops of performance. Notice that's not a global flag. I can write let a = unsafe { b.unchecked_add(c) }; in just one place in a 50MLOC system, and for just that one place the compiler can go absolutely wild optimising for the promise that overflows never happen - and yet right next door, even on the next line, I can write let x = y + z; and that still gets the kid gloves, if it overflows nothing catches on fire. That's how granular this needs to be to be useful, unlike C++ -ffast-math.

gpderetta 2y ago
You can set fast math (or a subset of it) on a translation unit basis.
- tialaramex 2y ago
  Because the language works by textual inclusion and so a "translation unit" isn't really just your code this is much more likely to result in nasty surprises, up to and including ODR violations.
  - gpderetta 2y ago
    Yes, if you try hard enough I'm sure you can find ways to screw up. From a practical point of view it is fine.
- cozzyd 2y ago
  Or even on a per function basis, at least with gcc (no clue about clang...)
  - gpderetta 2y ago
    In principle yes, you can use Attribute optimize, but I wouldn't rely on it. Too many bugs open against it.
    - cozzyd 2y ago
      It's worked for me in the past but maybe I got lucky.
- pdw 2y ago
  That's not really true. If you link a shared library that was compiled with -ffast-math, that will affect the entire program. https://moyix.blogspot.com/2022/09/someones-been-messing-with-my-subnormals.html https://moyix.blogspot.com/2022/09/someones-been-messing-wit...