I am also still exploring Swift, so forgive me if I got this wrong, but isn't the problem here the following:
Your RenderGradient() takes a RenderBuffer struct. Structs are passed by value. So two things happen in your RenderBuffer function:
1) You get a copy of the RenderBuffer, which means it and its contents (a lot of Pixel instances - Arrays are passed by value too!) will be copied every iteration
2) The work you do on the pixels in the RenderBuffer copy has no effect: you change the values of a copy and then ignore the copy. You don't return the modified RenderBuffer it from RenderGradient() or do anything with the result within RenderGradient().
So I think the reason you see such a big difference in this test between optimization and no-optimization is that in the first case, with optimization, the compiler knows that the work you do in RenderBuffer is a waste of time and simply does not do it.
While in the debug build the compiler does not apply optimization so your useless code is still executed.
You can confirm this by looking at the generated code. Or maybe simpler, render the RenderBuffer in an actual buffer or image for a visual inspection.
I think you are correct, and it (as expected) changes the results. I ran owensd code on my machine, in a completely non scientific and error prone setup. For release builds:
Objective-C: 0.0310617s
Safe Swift (incorrect): 0.0641295s (106.5% slower)
Safe Swift (correct): 0.0355441s (14.4% slower)
So the "safe" Swift version is running about 14.4% slower than the Objective-C version. It's not bad at all for a language that is so young! In debug mode, however, I didn't even have the patience to wait for the test run to finish...
Yes, the var should have been inout. It had an affect on the release build performance (twice as fast), but not the debug build. I've updated the blog post.
The performance problem is regarding all retain/releases though; not the copy of the data. That's why the debug builds are not affected.
I can understand the frustration. However I think it's a little premature to cast aspersions on the whole language because it isn't good at this particular test right now.
We still have ObjC. The two languages work together. (we also have C and asm). Pick the right tool for the job, and have reasonable expectations about the maturity and progress of Swift.
I wonder if creating a wrapper struct (let's call it Buffer) which combines an UnsafeMutablePointer and an UnsafeMutableBufferPointer would improve the language ergonomics while keeping the performance of the 'Swift*' version.
Buffer would have a constructor taking the number of items to store, and would upon creation create a buffer using UnsafeMutablePointer.alloc(). It would then use the UnsafeMutablePointer to build an UnsafeMutableBufferPointer. The latter type allows subscript indexing (e.g. myBuffer[10]) and iteration using for-in, which we could support by implementing a pass-through subscript() and generate() function.
Then, when using the struct, we'd pass it using inout parameters to ensure that it doesn't get copied.
This wouldn't fix the problem of Arrays being ludicrously unperformant, but it would at least make using the C-style buffer less painful.
He always runs 30 iterations (Search for "NUMBER_OF_ITERATIONS = 30"), so for example ObjC -O0 takes 0.099769s to render 30 frames, or about 0,0033256s per single frame.
The Swift safe version has the benefit of not corrupting memory if you run off the end of the array. In release builds that might account for the small perf difference.
It is not accurate to claim this is comparing the Objective-C version to Swift. It's comparing the C version.
One thing that i want to add is that this whole exercise is actually a straw man. You yourself start your post with
"Problem Statement : Design an algorithm that fills in a buffer of pixel data"
If that is the problem, then do that. Do not fill an array with Pixel object references. It is not the same problem.
I am not denying that Swift has performance issues. I do think your test and proof is artificial: nobody would use the 'Swift Safe' method to manipulate pixel data. This is exactly why the unsafe features exists.
I think what you need to discover is how to use structs, classes and raw buffers. None of those is a golden hammer. If you have performance problems with one of those then that does not mean that it is flawed. It is likely simply a bad choice for the problem at hand.
First off, a struct is not an object. Structs in Swift, like many languages, are value types. They have the same packing structure in Swift as in C, especially in the example that I gave.
I can write the same problem with components of the array as bytes if you prefer, but then performance will be even worse. The root cause of the performance issues is the array access and set. The more the do, the worse the performance.
So what you are saying, is that no person would ever create an array of data and expect to do anything with that data? And they would never do that in a processing loop? Ok...
If you actually read the blog post, you can see that I already gave an example using raw buffers (the UnsafeMutablePointer<Pixel>). Which means that every index into the pointer moves 4 bytes. Again, using UnsafeMutablePointer<Byte> is not doing anything other than creating a harder API surface to work with.
Also, the UnsafePointer types exist because without them, there would be no way to interact with C code. And if I need to use the UnsafePointer types everywhere I have arrays so that I can have reasonable performance in debug and release builds, then there is a bigger problem that needs to be addressed.
> So what you are saying, is that no person would ever create an array of data and expect to do anything with that data? And they would never do that in a processing loop? Ok...
No. I'm saying the memory layout of Array<Pixel> is undefined. You cannot assume that it is a contiguous chunk of memory for example.
Therefore it is the wrong solution for a render buffer.
Your RenderGradient() takes a RenderBuffer struct. Structs are passed by value. So two things happen in your RenderBuffer function:
1) You get a copy of the RenderBuffer, which means it and its contents (a lot of Pixel instances - Arrays are passed by value too!) will be copied every iteration
2) The work you do on the pixels in the RenderBuffer copy has no effect: you change the values of a copy and then ignore the copy. You don't return the modified RenderBuffer it from RenderGradient() or do anything with the result within RenderGradient().
So I think the reason you see such a big difference in this test between optimization and no-optimization is that in the first case, with optimization, the compiler knows that the work you do in RenderBuffer is a waste of time and simply does not do it.
While in the debug build the compiler does not apply optimization so your useless code is still executed.
You can confirm this by looking at the generated code. Or maybe simpler, render the RenderBuffer in an actual buffer or image for a visual inspection.