Intel publish a very thick optimization manual, which is a good help.

Compilers aren't great at using the real parameters of the chip (i.e. LLVM knows how wide the reader buffer is but I'm not sure if it actually can use the information), but knowing latencies for ISel and things like that is very helpful. To get those details you do need to rely on people like (the god amongst men) agner fog.

That should say reorder buffer above.

Intel contributes optimizations to gcc and llvm.

Apple also contributes plenty to LLVM, more than Intel actually, naively based on commit counts of @apple.com and @intel.com git committer email addresses. This isn't very surprising given that Chris Lattner worked at Apple for over a decade.

LLVM is Apple's baby, beyond its genesis under Lattner. They hate the GPL, that's it.

The thing about their contributions is that they upstream stuff they want other people to standardize on but aren't doing it out of love, as far as I can tell e.g. Valve have a direct business interest in having Linux kicking ass, Apple actively loses out (psychologically at least) if a non-apple toolchain is as good as theirs.

Apple used to contribute plenty to LLVM.

Nowadays less so, for them C++ support is good enough for what they make out of it (MSL is based on C++14 and IO/Driver Kit use an embedded subset).

The main focus is how well it is churning Objective-C, Swift and their own special flavour of bitcode.

None of them end up upstream as one might wish for.

They do now; I think I remember the time when they didn't.