3 ms·

> Furthermore, a very large subset of optimizations won't have any path dependence in the optimization that it choses Hot code should be: optimized for speed,

> Furthermore, a very large subset of optimizations won't have any path dependence in the optimization that it choses Hot code should be: optimized for speed, inlined, unrolled (where it helps), kept local with other hot code, and register-allocated with other hot code. Cold code should be: optimized for size, not inlined, not unrolled, kept away from hot code, and should never be accommodated in register allocation for any reason (ie. should never reduce availability of registers needed by hot paths). So I don't agree that optimizations should be performed in a path-independent way. > But that's the thing, measurement of your input is a heuristic. By that standard, everything is heuristic. The assumption that your cost model matches the actual CPU the code will run on is a heuristic.

barrkel 11y ago
If we followed your advice, the differential between worst case and best case performance would be large. There are good reasons for preferring a less extreme difference in potential performance for general computing with AOT compiled languages. In particular, we should ensure the worst case performance isn't awful. We wouldn't want e.g. an occasional exception getting thrown to act like a DoS attack on a web server.
- haberman 11y ago
  Nothing I was suggesting would create "awful" performance. Inlining and unrolling are not necessary to get reasonable performance -- just try gcc or Clang with -Os. For an occasional exception to "act like a DoS attack", the cold path would have to be thousands or millions of times slower than the hot paths. But compiler optimizations don't create anywhere near these kinds of constant factors. Even from -O0 to -O3 is more like a 5-10x difference, maybe 100x in extreme cases. The difference between -Os and -O3 is much more modest, more like 0-50%. Nothing that is going to cause anything remotely resembling a DoS.

barrkel 11y ago

If we followed your advice, the differential between worst case and best case performance would be large. There are good reasons for preferring a less extreme difference in potential performance for general computing with AOT compiled languages. In particular, we should ensure the worst case performance isn't awful. We wouldn't want e.g. an occasional exception getting thrown to act like a DoS attack on a web server.

haberman 11y ago
Nothing I was suggesting would create "awful" performance. Inlining and unrolling are not necessary to get reasonable performance -- just try gcc or Clang with -Os. For an occasional exception to "act like a DoS attack", the cold path would have to be thousands or millions of times slower than the hot paths. But compiler optimizations don't create anywhere near these kinds of constant factors. Even from -O0 to -O3 is more like a 5-10x difference, maybe 100x in extreme cases. The difference between -Os and -O3 is much more modest, more like 0-50%. Nothing that is going to cause anything remotely resembling a DoS.