LLVM 3.0 Exception Handling Redesign

One of the biggest IR changes in the LLVM 3.0 release is a redesign and reimplementation of the LLVM IR exception handling model. The old model, while it worked for most cases, fell over in some key situations, leading to obscure miscompilations, missed optimizations, and poor compile time. This post talks about the changes in LLVM 3.0 and how to move an existing LLVM front-end to the new design. It assumes some familiarity with the Itanium C++ ABI for exception handling.

LLVM 3.0 Type System Rewrite

One of the most pervasive IR (and thus compiler API) changes in LLVM 3.0 was a complete reimplementation of the LLVM IR type system. This change was long overdue (the original type system lasted from LLVM 1.0!) and made the compiler faster, greatly simplified a critical subsystem of VMCore, and eliminated some design points of IR that were frequently confusing and inconvenient. This post explains why the change was made along with how the new system works.

Greedy Register Allocation in LLVM 3.0

LLVM has two new register allocators: Basic and Greedy. When LLVM 3.0 is released, the default optimizing register allocator will no longer be linear scan, but the new greedy register allocator.
With its global live range splitting, the greedy algorithm generates code that is 1-2% smaller, and up to 10% faster than code produced by linear scan.