Advice on Packaging LLVM

Overview

LLVM sets certain default configure options to make sure our developers don't break things for constrained platforms. These settings are not optimal for most desktop systems, and we hope that packagers (e.g., Redhat, Debian, MacPorts, etc.) will tweak them. This document lists settings we suggest you tweak.

LLVM's API changes with each release, so users are likely to want, for example, both LLVM-2.6 and LLVM-2.7 installed at the same time to support apps developed against each.

Compile Flags

LLVM runs much more quickly when it's optimized and assertions are removed. However, such a build is currently incompatible with users who build without defining NDEBUG, and the lack of assertions makes it hard to debug problems in user code. We recommend allowing users to install both optimized and debug versions of LLVM in parallel. The following configure flags are relevant:

--disable-assertions

Builds LLVM with NDEBUG defined. Changes the LLVM ABI. Also available by setting DISABLE_ASSERTIONS=0|1 in make's environment. This defaults to enabled regardless of the optimization setting, but it slows things down.

--enable-debug-symbols

Builds LLVM with -g. Also available by setting DEBUG_SYMBOLS=0|1 in make's environment. This defaults to disabled when optimizing, so you should turn it back on to let users debug their programs.

--enable-optimized

(For git checkouts) Builds LLVM with -O2 and, by default, turns off debug symbols. Also available by setting ENABLE_OPTIMIZED=0|1 in make's environment. This defaults to enabled when not in a checkout.

C++ Features

RTTI

LLVM disables RTTI by default. Add REQUIRES_RTTI=1 to your environment while running make to re-enable it. This will allow users to build with RTTI enabled and still inherit from LLVM classes.

Shared Library

Configure with --enable-shared to build libLLVM-<major>.<minor>.(so|dylib) and link the tools against it. This saves lots of binary size at the cost of some startup time.

Dependencies

--enable-libffi

Depend on libffi to allow the LLVM interpreter to call external functions.

--with-oprofile

Depend on libopagent (>=version 0.9.4) to let the LLVM JIT tell oprofile about function addresses and line numbers.