4 ms·

`eachindex` is — in quite a few situations — faster than `1:n`. We've also been trying to promote a culture of not blindly putting `@inbounds` notations on thi

`eachindex` is — in quite a few situations — faster than `1:n`. We've also been trying to promote a culture of not blindly putting `@inbounds` notations on things as the compiler gets smarter. `@inbounds` is a hack around a dumb compiler, especially when the loop is as simple as many of these examples. It's not needed there anymore (but was 5 years ago).

kazinator 4y ago
The question is: is it at least as fast in all situations? Was it always that way? The 1 to length loop just has to initialize a local variable and step it; it cannot do anything else. It doesn't worry about the kinds of array that A may be, with its particular configuration of indexing, right? You may promote a culture of not doing certain things, but that by itself won't make those things disappear from existing code. Say you're trying to ship some product and you receive a bulletin from the language mailing list encouraging you, "try not to use @inbounds, it's a hack around a dumb compiler". You know you have that in numerous places; but you're not going to stop what you're doing and start removing @inbounds from the code base. If you're remarkably conscientious, you might open a ticket for that, which someone will look into in another season.
- mbauman 4y ago
  > The question is: is it at least as fast in all situations? Was it always that way? Yes and yes.
bjourne 4y ago
Perhaps that is part of the point of the article? If you accept things like @inbounds, which is a horrible hack and was a horrible hack five years ago, then perhaps the culture is a little too tolerant towards horrible hacks. Because many of the bugs the author enumerates are of the "fixes the problem for now, let's deal with the consequences later" type.
- grayclhn 4y ago
  Yes and no... Julia's been focused on high performance numerical computing from the beginning (and other related scientific applications). Using macros to get good performance from relatively generic code was (from my outside perspective) a really effective way to support real applications early on and also give time for the compiler to get "sufficiently smart" to make the macros less necessary.
- kristofferc 4y ago
  I wouldn't say that @inbounds is a "horrible hack". Just like the `unsafe` part of Rust is not a "horrible hack". There are cases where it is impossible for a compiler to statically verify that an index access is in bounds and in those cases it will need to emit a check and an exception. This prevents many other optimizations (for example SIMD). So for a language that is intended for people to write low-level numerical routines there has to be a way to opt out of these checks or people would have to write their numerical routines in a completely different language. But the important part is that index access is memory safe by default (as opposed to e.g. C) and you can also force boundschecking to be turned on (to override @inbounds) with a command-line flag (--check-bounds=yes). So if you want, you could pretend "@inbounds" doesn't exist by just aliasing your julia executable to apply that command line flag.

kazinator 4y ago

The question is: is it at least as fast in all situations? Was it always that way? The 1 to length loop just has to initialize a local variable and step it; it cannot do anything else. It doesn't worry about the kinds of array that A may be, with its particular configuration of indexing, right? You may promote a culture of not doing certain things, but that by itself won't make those things disappear from existing code. Say you're trying to ship some product and you receive a bulletin from the language mailing list encouraging you, "try not to use @inbounds, it's a hack around a dumb compiler". You know you have that in numerous places; but you're not going to stop what you're doing and start removing @inbounds from the code base. If you're remarkably conscientious, you might open a ticket for that, which someone will look into in another season.

mbauman 4y ago
> The question is: is it at least as fast in all situations? Was it always that way? Yes and yes.

bjourne 4y ago

Perhaps that is part of the point of the article? If you accept things like @inbounds, which is a horrible hack and was a horrible hack five years ago, then perhaps the culture is a little too tolerant towards horrible hacks. Because many of the bugs the author enumerates are of the "fixes the problem for now, let's deal with the consequences later" type.

grayclhn 4y ago
Yes and no... Julia's been focused on high performance numerical computing from the beginning (and other related scientific applications). Using macros to get good performance from relatively generic code was (from my outside perspective) a really effective way to support real applications early on and also give time for the compiler to get "sufficiently smart" to make the macros less necessary.
kristofferc 4y ago
I wouldn't say that @inbounds is a "horrible hack". Just like the `unsafe` part of Rust is not a "horrible hack". There are cases where it is impossible for a compiler to statically verify that an index access is in bounds and in those cases it will need to emit a check and an exception. This prevents many other optimizations (for example SIMD). So for a language that is intended for people to write low-level numerical routines there has to be a way to opt out of these checks or people would have to write their numerical routines in a completely different language. But the important part is that index access is memory safe by default (as opposed to e.g. C) and you can also force boundschecking to be turned on (to override @inbounds) with a command-line flag (--check-bounds=yes). So if you want, you could pretend "@inbounds" doesn't exist by just aliasing your julia executable to apply that command line flag.