LLVM ist a mess

Posted Mar 17, 2024 17:49 UTC (Sun) by farnz (subscriber, #17727)
In reply to: LLVM ist a mess by tialaramex
Parent article: Cranelift code generation comes to Rust

And even when they didn't assume C++, there's often been bugs that boil down to "Clang doesn't use this much, therefore it's not routinely tested and there's lot of lurking bugs"; see the fun Rust has had trying to use noalias on references, where because the matching Clang feature (the C99 restrict type qualifier) is rarely used correctly, miscompilations by LLVM traceable to noalias in LLVM IR kept blocking Rust from using it for Rust references (which definitionally can't alias each other).

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LLVM ist a mess

Posted Mar 17, 2024 19:09 UTC (Sun) by Wol (subscriber, #4433) [Link] (1 responses)

Is LLVM written in C++? Should they re-write it in Rust?

:-))

Cheers,
Wol

LLVM ist a mess

Posted Mar 17, 2024 19:53 UTC (Sun) by farnz (subscriber, #17727) [Link]

Implementation language isn't at the root of this; the underlying issue is that LLVM IR's semantics aren't (yet) formally defined, but instead rely on informal reasoning throughout LLVM. As a consequence, it's intractable to verify that LLVM actually implements the claimed semantics, and it's not reasonable to write test cases that validate the LLVM IR semantics are met in edge cases, since we don't actually know what the edge cases are.

There's efforts afoot to fully define LLVM IR semantics formally, and one of the biggest outputs those efforts are having (at the moment) is finding bugs in existing LLVM functionality, where existing LLVM code assumes opposing meanings (that both fit the informally defined semantics) for the same LLVM IR construct in different places.