Performance Tips for Frontend Authors

Abstract

The intended audience of this document is developers of language frontends targeting LLVM IR. This document is home to a collection of tips on how to generate IR that optimizes well.

IR Best Practices

As with any optimizer, LLVM has its strengths and weaknesses. In some cases, surprisingly small changes in the source IR can have a large effect on the generated code.

Beyond the specific items on the list below, it’s worth noting that the most mature frontend for LLVM is Clang. As a result, the further your IR gets from what Clang might emit, the less likely it is to be effectively optimized. It can often be useful to write a quick C program with the semantics you’re trying to model and see what decisions Clang’s IRGen makes about what IR to emit. Studying Clang’s CodeGen directory can also be a good source of ideas. Note that Clang and LLVM are explicitly version locked so you’ll need to make sure you’re using a Clang built from the same git revision or release as the LLVM library you’re using. As always, it’s strongly recommended that you track tip of tree development, particularly during bring up of a new project.

The Basics

1. Make sure that your Modules contain both a data layout specification and target triple. Without these pieces, non of the target-specific optimization will be enabled. This can have a major effect on the generated code quality.