4 ms·

Yes, but also, UB is a "feature" to work around the fact that C is a relatively un-expressive language with a simple type system. It's great that the compiler c

by geofft 6y ago

Yes, but also, UB is a "feature" to work around the fact that C is a relatively un-expressive language with a simple type system. It's great that the compiler can optimize for (i = 0; i < *len; i++) array[i] = 0; without having to check on each iteration whether len secretly points inside the array and got modified by the loop. It would be better if this were not a heuristic and there were clear compile-time information saying for sure that this optimization is safe.

sharpneli 6y ago
If len and array are both of the same type then the compiler must assume they can alias (or one of them is char*). One has to explicitly state with restrict that they cannot alias. In case of similar but where it’s float array[N] etc then compiler can assume that they do not alias.
- geofft 6y ago
  Right, that's a heuristic, and the consequence of that heuristic is that a bunch of things where you intentionally have pointers of different types to the same memory - the sort of low-level data manipulation that C ought to be good for - is UB unless you are careful about it and you find one of the exceptions to this rule.
  - sharpneli 6y ago
    Not a heuristic. Well defined, even if it could be defined better. I'd personally make all types not alias by default and then have a separate keyword 'alias' to specify things that must alias. One of the reasons why Fortran was faster for decades was the default aliasing rules, restrict finally got us out of that trap. If we'd always assume everything alias plenty of code would become way slower. Also C has well defined ways to access memory via different types. Make an union type and access via that. It makes it explicit.
    - AlotOfReading 6y ago
      Is it really fair to call type punning "well defined"? To my knowledge, it was considered implementation defined from C89 all the way through C11.
      - sharpneli 6y ago
        Before C11 char* was the only way to do type punning, badly. So it was basically forbidden, but still well defined. Since then they specified that unions are the way to go.
    - geofft 6y ago
      It's deterministic, but it's still a heuristic in the sense that it's a guess at the underlying intent of the code. You can come up with different, perhaps simpler guesses, but they're still guesses. It's quite common in existing real-world C code to rely on the ability to fill in a variable via a pointer of one type and then access it via a pointer of another type, and you expect the language to properly return the data you wrote to that pointer. The C committee determined that, just about always, that happens when one of the pointers is a char pointer (e.g., you call read() or write() on a struct), and they said that char pointers are allowed to alias by default but others can't. That's their guess, which is mostly accurate. There are much nicer answers here if you're not obligated to be compatible with existing C code - but in that case, especially today, I'd go with one of several other languages that can accurately express intention at the source code level instead of making backwards-incompatible changes to C. :)
jzwinck 6y ago
"restrict" does exactly what you're looking for.
- rurban 6y ago
  If it wouldn't be broken it would indeed be useful. With gcc restrict is still broken, you have define it away. clang had fixed it just recently I think. https://github.com/autoconf-archive/autoconf-archive/pull/209 https://github.com/autoconf-archive/autoconf-archive/pull/20...

by geofft 6y ago

sharpneli 6y ago

If len and array are both of the same type then the compiler must assume they can alias (or one of them is char*). One has to explicitly state with restrict that they cannot alias. In case of similar but where it’s float array[N] etc then compiler can assume that they do not alias.

geofft 6y ago
Right, that's a heuristic, and the consequence of that heuristic is that a bunch of things where you intentionally have pointers of different types to the same memory - the sort of low-level data manipulation that C ought to be good for - is UB unless you are careful about it and you find one of the exceptions to this rule.
- sharpneli 6y ago
  Not a heuristic. Well defined, even if it could be defined better. I'd personally make all types not alias by default and then have a separate keyword 'alias' to specify things that must alias. One of the reasons why Fortran was faster for decades was the default aliasing rules, restrict finally got us out of that trap. If we'd always assume everything alias plenty of code would become way slower. Also C has well defined ways to access memory via different types. Make an union type and access via that. It makes it explicit.
  - AlotOfReading 6y ago
    Is it really fair to call type punning "well defined"? To my knowledge, it was considered implementation defined from C89 all the way through C11.
    - sharpneli 6y ago
      Before C11 char* was the only way to do type punning, badly. So it was basically forbidden, but still well defined. Since then they specified that unions are the way to go.
  - geofft 6y ago
    It's deterministic, but it's still a heuristic in the sense that it's a guess at the underlying intent of the code. You can come up with different, perhaps simpler guesses, but they're still guesses. It's quite common in existing real-world C code to rely on the ability to fill in a variable via a pointer of one type and then access it via a pointer of another type, and you expect the language to properly return the data you wrote to that pointer. The C committee determined that, just about always, that happens when one of the pointers is a char pointer (e.g., you call read() or write() on a struct), and they said that char pointers are allowed to alias by default but others can't. That's their guess, which is mostly accurate. There are much nicer answers here if you're not obligated to be compatible with existing C code - but in that case, especially today, I'd go with one of several other languages that can accurately express intention at the source code level instead of making backwards-incompatible changes to C. :)

jzwinck 6y ago

"restrict" does exactly what you're looking for.

rurban 6y ago
If it wouldn't be broken it would indeed be useful. With gcc restrict is still broken, you have define it away. clang had fixed it just recently I think. https://github.com/autoconf-archive/autoconf-archive/pull/209 https://github.com/autoconf-archive/autoconf-archive/pull/20...