e11094b28b
This patch fixes a subtle bug that occurred in the MachBuffer branch optimization: in tracking labels at the current buffer tail using a sorted-by-offset array, the code did not update this array properly when redirecting labels. As a result, the dead-branch removal was unsafe, because not every label pointing to a branch is guaranteed to be redirected properly first. Discovered while doing performance testing: bz2 silently took a wrong branch and exited compression early. (Eek!) To address this problem, this patch adopts a slightly simpler data structure: we only track the labels *at the current buffer tail*, and *at the start of each branch*, and we're careful to update these appropriately to maintain the invariants. I'm pretty confident that this is correct now, but we should (still) fuzz it a bunch, because wrong control flow scares me a nonzero amount. I should probably also actually write out a formal proof that these data-structure updates are correct. The optimizations are important for performance (removing useless empty blocks, and taking advantage of any fallthrough opportunities at all), so I don't think we would want to drop them entirely.
This crate contains the core Cranelift code generator. It translates code from an intermediate representation into executable machine code.