Why does FORTRAN, which is older than C, has no trouble with auto-vectorization?
Mostly because of a very different memory model, of course. Arrays are first-class things, not hacks on top of pointers, and can't alias each other.
Mostly because of a very different memory model, of course. Arrays are first-class things, not hacks on top of pointers, and can't alias each other.
Anyway auto-vectorization will never compete with intrinsics for performance, so this all seems rather silly. Try expressing pshub in auto-vectorizer. Or a bunch of optimal load/swizzles to out perform gather.
And of course Python has a memory model. While that model is not as well understood as C's model, it is the key to Python's success and popularity as a generic programming language and as a numeric/scientific programming language. Python's memory model unlike C's or Fortran's, isn't designed for high performance. It's designed for rich abstractions, high ergonomics and high interoperability with those performant languages. For most people, the processing time lost executing python code is an acceptable tradeoff for the highly expressive control that Python gives them over the scheduling of lower level operations.
This does not mean Fortran is bad (obligatory disclaimer for Fortran fans).
One of the best improvements to Python's memory model was extending it to allow sensible access to blobs of memory containing regular structures such as vectors and matrices, making it relatively easy to interface with and control already-available, well-tried, highly-optimised algorithms.
There is an interesting writeup by dvyukov that shows the potential cost of "thin veneers" :) https://github.com/dvyukov/perf-load
"Hacks" feels like the wrong term to use here. Comparing managed objects surrounded by runtime to memory isn't a fair comparison.
A proper array would at least know its length. It does not require a lot of runtime to do, and most C code require libc anyway for basic stuff like malloc().
An array member of a struct will have its data allocated contiguously with other struct members (subject to compiler padding). An int* would point outside the struct. This is possible even with variable length arrays. Specifically, you can declare the final member of a struct as an int[] which makes it a "flexible array member". Then you have to malloc the struct to be the sizeof the struct + whatever size you want the array to be.
https://en.wikipedia.org/wiki/Flexible_array_member
This is used in struct sockaddr for generic APIs where the actual struct can be of different sizes.
https://lwn.net/Articles/997094/
This is rather pedantic but the memory model for arrays is important here. If you're iterating over multiple struct members that are arrays, the fact the arrays are stored contiguously in memory is going to matter to SIMD.
Ultra pedantic, but you actually do not need to alloc struct+size; you want something like:
// A flexible array member may have lower alignment requirements than the struct
// overall - in that case, it can overlap with the trailing padding of the rest
// of the struct, and a naive sizeof(base) + sizeof(flex) * count calculation
// will not take into account that overlap, and allocate more than is required.
#define SIZEOF_FLEX(type, member, count) MAX(sizeof(type), offsetof(type, member[count]))
As an example, C++'s std::array can be implemented as a struct containing a fixed-size C-style array. This can be passed-by-value. This means that returning a std::array can be more performant than returning a std::vector (which might be implemented as an int* that is reallocated when you add too many elements), because a std::array is returned on the stack while a std::vector is returned on the heap.
I was bit by this once when returning a std::unordered_map because I was doing a heap-allocation in a performance-critical section.
> a structure is a type consisting of a sequence of members, whose storage is allocated in an ordered sequence, (emphasis added)
sizeof(int[n]) vs sizeof(int*). Then there's: 'int getint(int *I){return *I;}' which wont let me pass '&int[n]' but I can 'int* = int[n]' then pass &int*. It's not so much a hack but a very primitive implementation on top of pointers (alright maybe it is a bit hacky.)
> A proper array would at least know its length. It does not require a lot of runtime to do, and most C code require libc anyway for basic stuff like malloc().
Something like a fat pointer 'typedef struct {long len, void *data} MyArray;' then make a libMyArray to operate on MyArray objects. But now you have a libMyArray dependency. You also loose type information unless you add fields to MyArray with an enum or have a MyArray for each type which gets messy. Then you cant do shit like just use an identifier in a conditional e.g. 'while(MyArray[n++])' unless you change how C works which makes it notC (uh, maybe pick another name.)
I feel that C is fine as it's an old machine oriented language. Leave it alone. I would look to reach for another tool when the primitive design of C makes life difficult.