5 ms·

I don't believe it is possible to hand tune every program to beat a compiler.

by pests 8y ago

MaxBarraclough 8y ago
More a question of practicality, no? Programmers are creative, compilers are well-tuned machines. Sometimes it's tough to beat a compiler's code, but I'm not sure that 'impossible' is meaningful. Monkeys with typewriters could function as a highly parallel superoptimiser, after all.
jeremiep 8y ago
When you realize the compiler's optimizations only account for about 10% of the total program's performance you find that the other 90% is entirely up to the programmer. Architecture, data structures, batching operations, memory locality, and a bunch of other metrics are all concepts the compiler can't really help you with whatsoever and they have a much larger impact on performance than the 10% the compiler is actually able to optimize. The problem is that either programmers don't care, or they can't make the distinction between premature optimizations and architecture planning.
- steveklabnik 8y ago
  Where do you get that 90/10 split from? Just curious.
  - jeremiep 8y ago
    Talks from Mike Acton and Scott Meyers, specifically "Data-Driven Development" and "CPU Caches and why you should care" respectively. I forgot exactly where I got that number, but it's been a pretty good metric so far. In a nutshell; the compiler is great a micro-optimizations and absolutely terrible at macro-optimizations. The former will get you a few percent of perf boosts while the later usually results in orders of magnitudes of performance gains. Its near impossible to apply macro-optimizations at the end of a project without massive refactors.
    - steveklabnik 8y ago
      Cool, I'll check those out, thank you!
      - Narishma 8y ago
        I believe the 10/90 number may be from this talk, though I don't have time to rewatch it to confirm: https://www.youtube.com/watch?v=rX0ItVEVjHc https://www.youtube.com/watch?v=rX0ItVEVjHc
  - deleted 8y ago
    [deleted]
- emodendroket 8y ago
  In a sense you're right, but hand-tuning assembly is kind of an orthogonal problem to determining whether you're using the right algorithms and data structures.
- MaxBarraclough 8y ago
  You're right to emphasise good data-structures and algorithms (also concurrency, parallelism, etc), but compiler optimisation is nothing to sneeze at. '10%' is laughably off-base. From a quick google: compiler optimisation can accelerate CPU-bound code to over 5x the unoptimised performance. https://www.phoronix.com/scan.php?page=article&item=clang-gcc-opts&num=3 https://www.phoronix.com/scan.php?page=article&item=clang-gc...
  - jeremiep 8y ago
    They seem to be benchmarking very specific things and not actual applications. These numbers do not hold in the real world.
mhh__ 8y ago
How so? Compilers aren't magic, you can - by definition (Assuming the current state machine style (no AI or nuthin') of compilers ) - you can just manually compile it. Compilers are clever, but humans can also be really clever: They know how to think - You can change the algorithm entirely. For practical reasons however (Premature optimisation also), trust your compiler.
kbenson 8y ago
But it should be possible to hand tune to match a compiler, right? And that would mean the compiler was not faster, which is the general claim I was calling into question.

I don't believe it is possible to hand tune every program to beat a compiler.

by pests 8y ago

I don't believe it is possible to hand tune every program to beat a compiler.

More a question of practicality, no? Programmers are creative, compilers are well-tuned machines. Sometimes it's tough to beat a compiler's code, but I'm not sure that 'impossible' is meaningful. Monkeys with typewriters could function as a highly parallel superoptimiser, after all.

jeremiep 8y ago

When you realize the compiler's optimizations only account for about 10% of the total program's performance you find that the other 90% is entirely up to the programmer. Architecture, data structures, batching operations, memory locality, and a bunch of other metrics are all concepts the compiler can't really help you with whatsoever and they have a much larger impact on performance than the 10% the compiler is actually able to optimize. The problem is that either programmers don't care, or they can't make the distinction between premature optimizations and architecture planning.

steveklabnik 8y ago
Where do you get that 90/10 split from? Just curious.
- jeremiep 8y ago
  Talks from Mike Acton and Scott Meyers, specifically "Data-Driven Development" and "CPU Caches and why you should care" respectively. I forgot exactly where I got that number, but it's been a pretty good metric so far. In a nutshell; the compiler is great a micro-optimizations and absolutely terrible at macro-optimizations. The former will get you a few percent of perf boosts while the later usually results in orders of magnitudes of performance gains. Its near impossible to apply macro-optimizations at the end of a project without massive refactors.
  - steveklabnik 8y ago
    Cool, I'll check those out, thank you!
    - Narishma 8y ago
      I believe the 10/90 number may be from this talk, though I don't have time to rewatch it to confirm: https://www.youtube.com/watch?v=rX0ItVEVjHc https://www.youtube.com/watch?v=rX0ItVEVjHc
- deleted 8y ago
  [deleted]
emodendroket 8y ago
In a sense you're right, but hand-tuning assembly is kind of an orthogonal problem to determining whether you're using the right algorithms and data structures.
MaxBarraclough 8y ago
You're right to emphasise good data-structures and algorithms (also concurrency, parallelism, etc), but compiler optimisation is nothing to sneeze at. '10%' is laughably off-base. From a quick google: compiler optimisation can accelerate CPU-bound code to over 5x the unoptimised performance. https://www.phoronix.com/scan.php?page=article&item=clang-gcc-opts&num=3 https://www.phoronix.com/scan.php?page=article&item=clang-gc...
- jeremiep 8y ago
  They seem to be benchmarking very specific things and not actual applications. These numbers do not hold in the real world.

mhh__ 8y ago

How so? Compilers aren't magic, you can - by definition (Assuming the current state machine style (no AI or nuthin') of compilers ) - you can just manually compile it. Compilers are clever, but humans can also be really clever: They know how to think - You can change the algorithm entirely. For practical reasons however (Premature optimisation also), trust your compiler.

kbenson 8y ago

But it should be possible to hand tune to match a compiler, right? And that would mean the compiler was not faster, which is the general claim I was calling into question.