LLVM ist a mess

Posted Mar 17, 2024 19:09 UTC (Sun) by Wol (subscriber, #4433)
In reply to: LLVM ist a mess by farnz
Parent article: Cranelift code generation comes to Rust

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.