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Look at how often folks on HN complain that the Web platform is useless because JS is slow. I haven't actually seen anyone complaining about that. Do you have

Look at how often folks on HN complain that the Web platform is useless because JS is slow. I haven't actually seen anyone complaining about that. Do you have any links? There are some specific complaints like: JS can't do 64-bit arithmetic or SIMD; but that's only really needed for games and scientific computing, which don't need to use JS. Or that JS is single-threaded; that's a fundamental feature of its design, nothing to do with optimisation. C without optimisations is just as slow if not slower. Nobody's talking about taking away all optimisations, just not trying to do extreme optimisations that exploit undefined behavior (or rather, assume it can never occur). Plenty of C compilers worked that way in the 90s and performance was perfectly acceptable (on hardware with a fraction of the speed and memory of today's computers and phones). Modern C++ probably relies on a higher level of optimisation, but that's another story.

pcwalton 9y ago
> Nobody's talking about taking away all optimisations, just not trying to do extreme optimisations that exploit undefined behavior (or rather, assume it can never occur). Those "extreme" optimizations are usually just surprising behavior that emerges from perfectly reasonable optimizations. For example, assuming that code that follows null dereference is dead is important. > Plenty of C compilers worked that way in the 90s and performance was perfectly acceptable (on hardware with a fraction of the speed and memory of today's computers and phones). You can get that experience with GCC -O0. Do you want to run code like that? I don't, and neither do customers. People who don't work on modern compilers often think that there is a subset of "simple" optimizations that catches "obvious" things, and optimizations beyond that are weird esoteric things nobody cares about. That isn't how it works. Tons of "obvious" optimizations require assumptions about undefined behavior.
- iainmerrick 9y ago
  If I could get that on -O1 or -O2, or maybe -Os, I would very happily do so. (But I don't actually know what optimisations those entail without poring over the manuals.) You're implying that 90s compilers had no optimisations, which is incorrect. Why is this a hot topic now, and why was it not a hot topic 10 or 15 years ago? I suggest that something changed in the interim, and that what changed is the addition of dangerous optimisations. I'm not sure where it all started but strict aliasing in GCC is a potential candidate. As others have pointed out, GCC and Clang seem to have by far the most horror stories, even though they don't actually generate the fastest code. I imagine that's mostly because GCC and Clang are so widely used, though.

pcwalton 9y ago

> Nobody's talking about taking away all optimisations, just not trying to do extreme optimisations that exploit undefined behavior (or rather, assume it can never occur). Those "extreme" optimizations are usually just surprising behavior that emerges from perfectly reasonable optimizations. For example, assuming that code that follows null dereference is dead is important. > Plenty of C compilers worked that way in the 90s and performance was perfectly acceptable (on hardware with a fraction of the speed and memory of today's computers and phones). You can get that experience with GCC -O0. Do you want to run code like that? I don't, and neither do customers. People who don't work on modern compilers often think that there is a subset of "simple" optimizations that catches "obvious" things, and optimizations beyond that are weird esoteric things nobody cares about. That isn't how it works. Tons of "obvious" optimizations require assumptions about undefined behavior.

iainmerrick 9y ago
If I could get that on -O1 or -O2, or maybe -Os, I would very happily do so. (But I don't actually know what optimisations those entail without poring over the manuals.) You're implying that 90s compilers had no optimisations, which is incorrect. Why is this a hot topic now, and why was it not a hot topic 10 or 15 years ago? I suggest that something changed in the interim, and that what changed is the addition of dangerous optimisations. I'm not sure where it all started but strict aliasing in GCC is a potential candidate. As others have pointed out, GCC and Clang seem to have by far the most horror stories, even though they don't actually generate the fastest code. I imagine that's mostly because GCC and Clang are so widely used, though.