[zozbot234]

> Is this understandable, or does this tell us something bad about llvm?

LLVM is a large project that's mostly written in pre-modern C++, and "noalias" is a highly non-trivial feature that affects many parts of the compiler in 'cross-cutting' ways. It would be surprising if it did not turn up some initial bugs.

[galangalalgol]

Initial, yes, but this was first uncovered in Oct 2015. That seems like long enough to fix it.

[masklinn]

It’s not a single bug, it’s a bunch of different bugs in the interactions between noalias and various analysis and optimisation passes.

[saagarjha]

Aliasing analysis is a complicated part of the compiler, and it underpins a lot of optimization passes. It’s not an easy thing to bolt on.

[masklinn]

TBF the internal API could be designed more pessimistically, as in llvm could drop noalias annotations unless they’re explicitly maintained (/ converted).

This means optimisation phases would need to explicitly opt-in and aliasing optimisations would commonly be missed by default, but it would avoid miscompilations.