4 ms·

Right, everything you've said is consistent with my "I'd expect skilled human optimization to make the most difference at the smallest scales: less hot code, ch

Right, everything you've said is consistent with my "I'd expect skilled human optimization to make the most difference at the smallest scales: less hot code, changing infrequently, targeting fewer critical (micro)architectures". In other situations, many folks (including me) get 15% improvements from PGO, and it's hard to take seriously any argument that suggests that can't happen.

ncmncm 6y ago
In other words, you didn't read what you were replying to: "That [optimizers] achieve any positive result, on average, with or without profile guidance, is impressive". What you call "skilled human optimization" appears to refer to the noted more or less accidental optimizations cited. We may congratulate ourselves when we find one that turns out can be applied elsewhere, and even program it into our compiler, but these are benefits more often of luck than skill, and are often much less durable than we hoped, washed away in the next microarchitectural generation, and sometimes even come to degrade performance. In the second example cited, recent Gcc versions deliberately intersperse other instructions between those that could be fused, to fill in for pipeline delays in a previous generation; but the benchmarks referenced for current chips were compiled with a previous compiler that didn't. So, current compilers improve performance for obsolete hardware and pessimize current hardware. Potential improvements in the next microarchitectural generation are often held hostage by optimizations performed in old compilers, used in reference benchmarks, that would be pessimizations in the better microarchitecture (therefore) not shipped.
- scottlamb 6y ago
  You led with "my experiments with profile-guided optimization have been thus far disappointing". My point is that in many cases people get 15% improvements. Do you consider that a disappointing result? Are you disregarding other people's results? or are you disappointed that they don't apply in your specific situations? I originally gave you the benefit of the doubt by thinking it was the last of these, and speculated about how your situation differs. Now I don't know what you want, except to show off how smart you are vs how poor a reader I am, regardless of accuracy. I'm not interested. Bye.
  - ncmncm 6y ago
    Yes: 15% is rarely achieved, yet still disappointing. As I actually came out and said, microarchitectures are now so complicated that we cannot program optimizers to do better. Instead we resign ourselves to (more usually) 10%, and congratulate ourselves on getting that far.

ncmncm 6y ago

In other words, you didn't read what you were replying to: "That [optimizers] achieve any positive result, on average, with or without profile guidance, is impressive". What you call "skilled human optimization" appears to refer to the noted more or less accidental optimizations cited. We may congratulate ourselves when we find one that turns out can be applied elsewhere, and even program it into our compiler, but these are benefits more often of luck than skill, and are often much less durable than we hoped, washed away in the next microarchitectural generation, and sometimes even come to degrade performance. In the second example cited, recent Gcc versions deliberately intersperse other instructions between those that could be fused, to fill in for pipeline delays in a previous generation; but the benchmarks referenced for current chips were compiled with a previous compiler that didn't. So, current compilers improve performance for obsolete hardware and pessimize current hardware. Potential improvements in the next microarchitectural generation are often held hostage by optimizations performed in old compilers, used in reference benchmarks, that would be pessimizations in the better microarchitecture (therefore) not shipped.

scottlamb 6y ago
You led with "my experiments with profile-guided optimization have been thus far disappointing". My point is that in many cases people get 15% improvements. Do you consider that a disappointing result? Are you disregarding other people's results? or are you disappointed that they don't apply in your specific situations? I originally gave you the benefit of the doubt by thinking it was the last of these, and speculated about how your situation differs. Now I don't know what you want, except to show off how smart you are vs how poor a reader I am, regardless of accuracy. I'm not interested. Bye.
- ncmncm 6y ago
  Yes: 15% is rarely achieved, yet still disappointing. As I actually came out and said, microarchitectures are now so complicated that we cannot program optimizers to do better. Instead we resign ourselves to (more usually) 10%, and congratulate ourselves on getting that far.