4 ms·

How is strict aliasing magic? How could you possibly define the meaning of accessing an object through an incompatible type since the type of an object determin

How is strict aliasing magic? How could you possibly define the meaning of accessing an object through an incompatible type since the type of an object determines how the compiler interprets its value? The bits stored in the object do not even necessarily correspond to a value for every type, after all. And that's not even considering the fact that there is a great deal of freedom wrt how implementations can represent floating point numbers, etc. so how can the standard impose a requirement on what happens when you, say, access a double as an int? What do you mean by weak typing? That can you cast away basically anything? >It's easier to predict OCaml assembly output than the GCC's one Surely you are kidding. If that were true, why doesn't OCaml curb-stomp C in benchmarks? IMO GCC's output isn't even a little surprising, esp. once you've gained experience with its favorite optimizations.

wwright 7y ago
Not only can you cast away anything, but: - casting away types unsafely is required for many common patterns in C, including any form of runtime polymorphism - basically anything involving numerics has a good chance of happily casting for you without asking you or telling you
- madmax96 7y ago
  >- casting away types unsafely is required for many common patterns in C, including any form of runtime polymorphism Which patterns? I'm thinking of stuff like the Berkeley Sockets API, which you can absolutely implement safely. re numerics: fortunately, that's 100% amenable to static analysis (see -Wconversion, -Wsign-conversion in gcc/clang and coverity's warnings about potentially unsafe casts - e.g. promoting an int to a long is always safe, but an int to a float not necessarily) and follows very straightforward rules.
atilaneves 7y ago
> Surely you are kidding. If that were true, why doesn't OCaml curb-stomp C in benchmarks? Exactly because nobody (except backend writers) can predict what C compiler backends emit, especially when abusing undefined behaviour.
- madmax96 7y ago
  OCaml: https://godbolt.org/z/8bWDXy https://godbolt.org/z/8bWDXy C: https://godbolt.org/z/WAjsvk https://godbolt.org/z/WAjsvk I've never actually looked at the output of an OCaml compiler, I have to say I'm surprised by how clean it is. But I wouldn't call it more predictable than that C's output.
  - ernst_klim 7y ago
    >C You forgot about -O2/3.
ernst_klim 7y ago
> Surely you are kidding. If that were true, why doesn't OCaml curb-stomp C in benchmarks? You are confusing predictability with performance. OCaml is way more predictable exactly because it generates way more straightforward code. On the other hand, GCC could do all kind of stuff, generating any sort of assembly. It can even rewrite fairly complex math functions, simplifying them. Parent was talking about predictable ASM, not about performant ASM.

wwright 7y ago

Not only can you cast away anything, but: - casting away types unsafely is required for many common patterns in C, including any form of runtime polymorphism - basically anything involving numerics has a good chance of happily casting for you without asking you or telling you

madmax96 7y ago
>- casting away types unsafely is required for many common patterns in C, including any form of runtime polymorphism Which patterns? I'm thinking of stuff like the Berkeley Sockets API, which you can absolutely implement safely. re numerics: fortunately, that's 100% amenable to static analysis (see -Wconversion, -Wsign-conversion in gcc/clang and coverity's warnings about potentially unsafe casts - e.g. promoting an int to a long is always safe, but an int to a float not necessarily) and follows very straightforward rules.

atilaneves 7y ago

> Surely you are kidding. If that were true, why doesn't OCaml curb-stomp C in benchmarks? Exactly because nobody (except backend writers) can predict what C compiler backends emit, especially when abusing undefined behaviour.

madmax96 7y ago
OCaml: https://godbolt.org/z/8bWDXy https://godbolt.org/z/8bWDXy C: https://godbolt.org/z/WAjsvk https://godbolt.org/z/WAjsvk I've never actually looked at the output of an OCaml compiler, I have to say I'm surprised by how clean it is. But I wouldn't call it more predictable than that C's output.
- ernst_klim 7y ago
  >C You forgot about -O2/3.

ernst_klim 7y ago

> Surely you are kidding. If that were true, why doesn't OCaml curb-stomp C in benchmarks? You are confusing predictability with performance. OCaml is way more predictable exactly because it generates way more straightforward code. On the other hand, GCC could do all kind of stuff, generating any sort of assembly. It can even rewrite fairly complex math functions, simplifying them. Parent was talking about predictable ASM, not about performant ASM.