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CPU makers add so many complicated features to let users optimize their programs that it takes too much knowledge for mere mortals to optimize their programs -

CPU makers add so many complicated features to let users optimize their programs that it takes too much knowledge for mere mortals to optimize their programs - That you have to rely on heuristics and measures, as it is usually and wisely advised, is a bit unsatisfying. That's sort of ironic, in a way. I used to do assembly and count cycles, but now I wouldn't dare; it's hardcore compiler and library makers stuff. It's like "don't do your own crypto (optimization)". Everyone knows why it is so, though - we cannot solve the problem by throwing more Gigahertz at it.

cout 5y ago
I have hand-written asm for use in production code, and I would do it again. I also knew exactly what cpu my code would be running on. If I were publishing the code to run on a variety of hardware, I would be very cautious. Writing your own crypto is very different; the stakes are higher if you get it wrong.
- pjmlp 5y ago
  Including the microcode version currently installed into the CPU and gone through the release notes, alongside a profile like V-Tune?
WalterBright 5y ago
A good heuristic is simply minimize the number of instructions.
- mhh__ 5y ago
  I doubt it'd be an issue for memcpy but minimizing the size of a loop body can lead to very counterintuitive speedups now processors have LSDs (loop stream detectors).
  - jeffbee 5y ago
    If you make them too small it can lead to counterintuitive slowdowns.
kierank 5y ago
This is definitely not correct. For many SIMDable workloads it's possible to achieve order of magnitude improvements over the compiler.