The Glasgow Haskell Compiler and LLVM

If you read the LLVM 2.7 release notes carefully you would have noticed that one of the new external users is the Glasgow Haskell Compiler (GHC). As the author of the LLVM backend for GHC, I have been invited to write a post detailing the design of the backend and my experiences with using LLVM. This is that post :).

Extensible Metadata in LLVM IR

A common request by front-end authors is to be able to add some sort of metadata to LLVM IR. This metadata could be used to influence language-specific optimization passes (for example, Type Based Alias Analysis in C), tag information for a custom code generator, or pass through information to link time optimization. LLVM 2.7 provides first-class support for this, and has switched debug information over to use it (improving debug info!).

While the details of this feature can be found in the LLVM Language Reference manual, sometimes it is hard to distill the big picture from the low-level details. This post tries to fill the gap by explaining some history, motivation and example use cases for this new LLVM 2.7 feature.

This post was written by Devang Patel and myself.

The x86 Disassembler

Disassemblers make binary analysis work. With a reliable disassembler, you can solve high-level problems like tracing back through a program's call stack or analyzing sample-based profiles to low-level problems like figuring out how your compiler unrolled a tight floating-point loop or what advantages declaring a variable const actually had at the other end of the optimization chain. A reliable disassembler, which takes sequences of bytes and prints human-readable instruction mnemonics, is a crucial part of any development platform. You're about to go on a whirlwind tour of the LLVM disassembler: why one should exist, what's great about this one, and how you can use it.

Address of Label and Indirect Branches in LLVM IR

The GCC Compiler supports a useful "Label as Values" extension, which allows code to take the address of a label and then later do an unconditional branch to an address specified as a void*. This extension is particularly useful for building efficient interpreters.

LLVM has long supported this extension by lowering it to a "correct" but extremely inefficient form. New in LLVM 2.7 is IR support for taking the address of a label and jumping to it later, which allows implementing this extension much more efficiently. This post describes this new LLVM IR feature and how it works.

Advanced Topics in Redundant Load Elimination with a Focus on PHI Translation

In our previous post on GVN we introduced some basics of load elimination.  This post describes some advanced topics and focuses on PHI translation: what it is, why it is important, shows some nice things it can do, and describes the implementation in LLVM.