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by mathisfun123 1119 days ago
>and expecting the optimizer to make a better decision next time. It doesn't work that way

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.
2 comments
mfranc42 1119 days ago
I don't remember using PGO like ever. And even if you do use it, isn't it like breaking compilation into two phases that are themselves deterministic?

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.

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slavapestov 1119 days ago
> 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.

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mathisfun123 1119 days ago
>PGO is an edge case and not used a whole lot in practice. Many compilers do not support it at all, even production compilers.

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.

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slavapestov 1119 days ago
> 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.

How many binaries in a typical Linux distro are built with PGO for example? The answer is approximately zero.

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mathisfun123 1119 days ago
Your point? Because your first goalpost was "most compilers don't even support PGO". And now the goalpost is Linux distros? How about this one - how many binaries at FB/G are built with PGO? Answer: a great deal (llvm/bolt is a FB project and MLGO is a G project).
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slavapestov 1119 days ago
My point is that the view of a compiler as a series of transformation passes from a source program to machine code, implemented as pure functions, is an entirely appropriate way to describe how they work in a random introductory blog post?

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.

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mathisfun123 1119 days ago
>My point is that the view of a compiler...

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

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