But in practice it does work exactly that way because of PGO and if we're precluding PGO then the observation is as banal as "programs that have no side-effects are pure". Like I get that the post is trying to paint some beautiful picture about how compiler passes are abstract beautiful transformations of representations of programs but it's not a useful picture at all.
It is actually a very useful picture especially to a beginner.
PGO is an edge case and not used a whole lot in practice. Many compilers do not support it at all, even production compilers. Someone learning to write a compiler does not need to think about PGO. And besides, a compiler with PGO is still a pure function of source + flags + profile.
Both clang and gcc support `-fprofile-generate`. Beyond that generic infrastructure, in my area (DL compilers) if you're not doing PGO/autotuning you're not serious.
>Someone learning to write a compiler does not need to think about PGO.
That's like saying someone learning to write a compiler doesn't need to think about optimization at all - sure maybe the first time. But then every time after that it should be at the forefront of your mind.
How many binaries in a typical Linux distro are built with PGO for example? The answer is approximately zero.
Congratulations on your recent PhD in quantum machine learning or whatever -- I hope you find some way to speed up FB/G's ad serving algorithms by 1% and land that promotion. I'm sure you're very smart.
Your point in bringing up Linux was to reiterate your original point? Interesting.
> I hope you find some way to speed up FB/G's ad serving algorithms by 1% and land that promotion. I'm sure you're very smart.
lol
It is pretty useful picture in my eyes, because that's what I've been observing since like forever. Non-deterministic compiler would be pretty hard to test or reason about.
It might be a banal observation, but an important one if you want to contrast a compiler with something like an OS kernel or most modern programming projects that interact heavily with outside world. When you throw hardware, network traffic, or users into the mix, it gets crazy.
It's bit of a stretch, sure, but I don't expect my performance problems to go away by recompiling stuff over and over again without changing either the code or flags and expecting the optimizer to make a better decision next time. It doesn't work that way.