> Seems bad for rust that safe code is 25x slower.
Most of the optimizations listed have nothing to do with unsafe, and have to do with algorithmic changes that you could apply in any language, and could apply to safe code just as well as to unsafe code. Even if we pretend all unsafe used there is necessary, and interpret the source in the worst possible light for rust - that the optimization steps would be impossible without unsafe - I think we'd only be talking 4x to 5x slower from this article, not 25x.
But - I strongly suspect not all that unsafe is necessary, and the post certainly doesn't claim it is.
The first code snippet uses unsafe to skip bounds checks. There are plenty of other ways to get the optimizer to skip them for you in safe code - or at the very least, hoist them out of the loop instead. This seems like a good read showing some concrete examples:
Explicitly using SIMD code like the second code snippet does might require unsafe for the transmuted slicing under the hood (implemented as raw pointer derefs in this case), but it's possible to build safe abstractions on top of that. That blog post links this, for example, although it didn't end up using it (EDIT: Actually, it initially didn't, but now does): https://github.com/AdamNiederer/faster . It does use unsafe internally: https://github.com/AdamNiederer/faster/search?q=unsafe&unsco...
Most of the optimizations listed have nothing to do with unsafe, and have to do with algorithmic changes that you could apply in any language, and could apply to safe code just as well as to unsafe code. Even if we pretend all unsafe used there is necessary, and interpret the source in the worst possible light for rust - that the optimization steps would be impossible without unsafe - I think we'd only be talking 4x to 5x slower from this article, not 25x.
But - I strongly suspect not all that unsafe is necessary, and the post certainly doesn't claim it is.
The first code snippet uses unsafe to skip bounds checks. There are plenty of other ways to get the optimizer to skip them for you in safe code - or at the very least, hoist them out of the loop instead. This seems like a good read showing some concrete examples:
https://coaxion.net/blog/2018/01/speeding-up-rgb-to-grayscal...
Explicitly using SIMD code like the second code snippet does might require unsafe for the transmuted slicing under the hood (implemented as raw pointer derefs in this case), but it's possible to build safe abstractions on top of that. That blog post links this, for example, although it didn't end up using it (EDIT: Actually, it initially didn't, but now does): https://github.com/AdamNiederer/faster . It does use unsafe internally: https://github.com/AdamNiederer/faster/search?q=unsafe&unsco...