Float and integer arithmetic follow two different paradigms

Float and integer arithmetic follow two different paradigms(blog.pkh.me)

41 points by ibobev 3 days ago | 9 comments

a_e_k 3 minutes ago |

For the float stuff, beware of `-ffinite-math-only`. If enabled, `isfinite()` and may compile out as assumed true (with similar assumptions around `isnan()` and `isinf()`).

And `-ffinite-math-only` is enabled by `-ffast-math` which in turn is enabled by `-Ofast`.

weinzierl 2 hours ago |

The "integer arithmetic paradigm" would mean either checking after (almost) every operation or living with potentially incorrect results.

The first is terribly inefficient, the second is just wrong (even if insanely common).

This is sad because there is no reason at all why integers couldn’t follow what OP calls the "float paradigm". It doesn’t even have to be slow. Most modern processors (if we ignore x86) support some form of sticky arithmetic flags. Unfortunately, programming languages don’t support them, so they aren’t used.

There is also something to be said about the special treatment division by zero gets.

dnautics 2 hours ago | |

Pretty sure zig has safe integer operations: integers aren't allowed to overflow with the standard operators, there are overflowing and saturating operators if that's what you want.

kibwen 1 hour ago | |

> Most modern processors (if we ignore x86) support some form of sticky arithmetic flags. Unfortunately, programming languages don’t support them, so they aren’t used.

I'd say the reason that programming languages don't have built-in support for checking the CPU's sticky status flags is precisely because of a lack of x86 support. Plenty of languages do have support for checking overflow on each individual operation, e.g. Rust's `overflowing_foo` methods, which return a tuple whose second member is a boolean indicating overflow: https://doc.rust-lang.org/std/primitive.i32.html#method.over...

jcranmer 36 minutes ago | | |

Sticky flags tend to be really annoying for a compiler to support for various reasons, but principally it makes every operation have a hidden dependency to shared global state that is almost never read. Note that IEEE 754 standardized support for sticky flags 40 years ago, but support for these sticky bits is still poor to nonexistent in most programming languages, and sticky bit stuff for floating-point operations is less problematic than integer operations because FP math is already far more black box in practice.

dooglius 2 hours ago |

FWIW one can configure floating point exceptions at runtime at no overhead, see `man 3 fenv`

jcranmer 50 minutes ago | |

It's no overhead in the same sense that zero-cost exception-handling is zero-cost: turning them on properly (e.g., #pragma STDC FENV_ACCESS ON or equivalent command-line flags) disables a fair amount of optimizations which incurs an overhead in and of itself.

dzaima 56 minutes ago | |

Except gcc doesn't actually support it to any sane extent (it ignores and warns on "#pragma STDC FENV_ACCESS ON", which C requires for fenv.h to actually function; and as such gcc makes a bunch invalid optimizations); clang supports it, but only as of somewhat-recently.

westurner 1 hour ago | |

fenv: https://manpages.debian.org/testing/manpages-dev/fenv.3.en.h... :

> These eleven functions were defined in C99, and describe the handling of floating-point rounding and exceptions (overflow, zero-divide, etc.).