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Dan Luu demonstrates that even when optimizing, compilers often produce very slow code as compared to very basic source that is easily accessible to every assembly code programmer: Hand coded assembly beats intrinsics in speed and simplicity:
Every once in a while, I hear how intrinsics have improved enough that it's safe to use them for high performance code. That would be nice. The promise of intrinsics is that you can write optimized code by calling out to functions (intrinsics) that correspond to particular assembly instructions. Since intrinsics act like normal functions, they can be cross platform. And since your compiler has access to more computational power than your brain, as well as a detailed model of every CPU, the compiler should be able to do a better job of micro-optimizations. Despite decade old claims that intrinsics can make your life easier, it never seems to work out.
The last time I tried intrinsics was around 2007; for more on why they were hopeless then (see this exploration by the author of VirtualDub). I gave them another shot recently, and while they've improved, they're still not worth the effort. The problem is that intrinsics are so unreliable that you have to manually check the result on every platform and every compiler you expect your code to be run on, and then tweak the intrinsics until you get a reasonable result. That's more work than just writing the assembly by hand. If you don't check the results by hand, it's easy to get bad results.
For example, as of this writing, the first two Google hits for popcnt benchmark (and 2 out of the top 3 bing hits) claim that Intel's hardware popcnt instruction is slower than a software implementation that counts the number of bits set in a buffer, via a table lookup using the SSSE3 pshufb instruction. This turns out to be untrue, but it must not be obvious, or this claim wouldn't be so persistent. Let's see why someone might have come to the conclusion that the popcnt instruction is slow if they coded up a solution using intrinsics.
In my own experience, I have yet to find an optimizing compiler that generates code as fast or as compact as I am able to with hand-optimized code.
Dan Luu's entire website is a treasure trove of education for experienced and novice coders alike. I look forward to studying the whole thing. His refreshingly simple HTML 1.0 design is obviously intended to educate, and is an example of my assertion that the true experts all have austere websites.
(Score: 1, Insightful) by Anonymous Coward on Saturday September 10 2016, @04:39PM
Yes, and the hand-crafted assembly usually has to be CPU-specific to get the best performance (pipelines and all that).
About a million years ago (in 1999) I bought myself an AMD K6-2/400 because it had floating-point SIMD (3DNow) and I figured it would be cool to write some code for it, and I was keen to improve my very feeble coding skills, and I needed a hobby to keep me out of the pub of an evening. (There were loads of posers over on the green site raving about the AltiVec on their Power Macs in those days.)
So I had this "great idea" of a library of code, cross-platform, with C and SIMD implementations of various simple bits and pieces so that you could get a bit of a performance boost if you had the hardware. Being young and stupid, I had no idea how much work it would be, or how bad my programming skills really were. I also was completely underwhelmed by the amount of interest the was in that sort of thing in the world in general. And anyway, I eventually found myself in situations where I had no time to work on it, instead having to focus on evil things like C++ and Perl.
It rots away on sourceforge [sourceforge.net]. Every so often I decide to do some more to it, but I'm always thwarted by life. This time it was Java :-( The last thing I put into it was a rudimentary C unit testing framework of my own Quioxtic devising. That's probably more use than the rest of it, which doesn't actually do very much at all.
(Score: 0) by Anonymous Coward on Saturday September 10 2016, @07:03PM
My project: http://sourceforge.net/projects/chaosesqueanthology/ [sourceforge.net]
(Score: 0) by Anonymous Coward on Saturday September 10 2016, @07:35PM
Cool.