# LLVM 3.1 Release Notes

1. Introduction
2. Sub-project Status Update
3. External Projects Using LLVM 3.1
4. What's New in LLVM?
5. Installation Instructions
6. Known Problems
7. Additional Information

Written by the LLVM Team

# These are in-progress notes for the upcoming LLVM 3.1 release.
You may prefer the LLVM 3.0 Release Notes.

## Introduction

This document contains the release notes for the LLVM Compiler Infrastructure, release 3.1. Here we describe the status of LLVM, including major improvements from the previous release, improvements in various subprojects of LLVM, and some of the current users of the code. All LLVM releases may be downloaded from the LLVM releases web site.

For more information about LLVM, including information about the latest release, please check out the main LLVM web site. If you have questions or comments, the LLVM Developer's Mailing List is a good place to send them.

Note that if you are reading this file from a Subversion checkout or the main LLVM web page, this document applies to the next release, not the current one. To see the release notes for a specific release, please see the releases page.

## Sub-project Status Update

The LLVM 3.1 distribution currently consists of code from the core LLVM repository (which roughly includes the LLVM optimizers, code generators and supporting tools), and the Clang repository. In addition to this code, the LLVM Project includes other sub-projects that are in development. Here we include updates on these subprojects.

### Clang: C/C++/Objective-C Frontend Toolkit

Clang is an LLVM front end for the C, C++, and Objective-C languages. Clang aims to provide a better user experience through expressive diagnostics, a high level of conformance to language standards, fast compilation, and low memory use. Like LLVM, Clang provides a modular, library-based architecture that makes it suitable for creating or integrating with other development tools. Clang is considered a production-quality compiler for C, Objective-C, C++ and Objective-C++ on x86 (32- and 64-bit), and for Darwin/ARM targets.

In the LLVM 3.1 time-frame, the Clang team has made many improvements:

- C++11 support is greatly expanded including lambdas, initializer lists, constexpr, user-defined literals, and atomics.
- ...

For more details about the changes to Clang since the 2.9 release, see the Clang release notes

If Clang rejects your code but another compiler accepts it, please take a look at the language compatibility guide to make sure this is not intentional or a known issue.

### DragonEgg: GCC front-ends, LLVM back-end

DragonEgg is a gcc plugin that replaces GCC's optimizers and code generators with LLVM's. It works with gcc-4.5 and gcc-4.6 (and partially with gcc-4.7), can target the x86-32/x86-64 and ARM processor families, and has been successfully used on the Darwin, FreeBSD, KFreeBSD, Linux and OpenBSD platforms. It fully supports Ada, C, C++ and Fortran. It has partial support for Go, Java, Obj-C and Obj-C++.

The 3.1 release has the following notable changes:

- Partial support for gcc-4.7. Ada support is poor, but other languages work fairly well.
- Support for ARM processors. Some essential gcc headers that are needed to build DragonEgg for ARM are not installed by gcc. To work around this, copy the missing headers from the gcc source tree.
- Better optimization for Fortran by exploiting the fact that Fortran scalar arguments have 'restrict' semantics.
- Better optimization for all languages by passing information about type aliasing and type ranges to the LLVM optimizers.
- A regression test-suite was added.

### compiler-rt: Compiler Runtime Library

The new LLVM compiler-rt project is a simple library that provides an implementation of the low-level target-specific hooks required by code generation and other runtime components. For example, when compiling for a 32-bit target, converting a double to a 64-bit unsigned integer is compiled into a runtime call to the "__fixunsdfdi" function. The compiler-rt library provides highly optimized implementations of this and other low-level routines (some are 3x faster than the equivalent libgcc routines).

....

### LLDB: Low Level Debugger

LLDB is a ground-up implementation of a command line debugger, as well as a debugger API that can be used from other applications. LLDB makes use of the Clang parser to provide high-fidelity expression parsing (particularly for C++) and uses the LLVM JIT for target support.

...

### libc++: C++ Standard Library

Like compiler_rt, libc++ is now dual licensed under the MIT and UIUC license, allowing it to be used more permissively.

...

### VMKit

The VMKit project is an implementation of a Java Virtual Machine (Java VM or JVM) that uses LLVM for static and just-in-time compilation.

In the LLVM 3.1 time-frame, VMKit has had significant improvements on both runtime and startup performance:

- ...

### Polly: Polyhedral Optimizer

Polly is an experimental optimizer for data locality and parallelism. It currently provides high-level loop optimizations and automatic parallelisation (using the OpenMP run time). Work in the area of automatic SIMD and accelerator code generation was started.

Within the LLVM 3.1 time-frame there were the following highlights:

- Polly became an official LLVM project
- Polly can be loaded directly into clang (Enabled by '-O3 -mllvm -polly' )
- An automatic scheduling optimizer (derived from Pluto) was integrated. It performs loop transformations to optimize for data-locality and parallelism. The transformations include, but are not limited to interchange, fusion, fission, skewing and tiling.

## External Open Source Projects Using LLVM 3.1

An exciting aspect of LLVM is that it is used as an enabling technology for a lot of other language and tools projects. This section lists some of the projects that have already been updated to work with LLVM 3.1.

### Glasgow Haskell Compiler (GHC)

GHC is an open source compiler and programming suite for Haskell, a lazy functional programming language. It includes an optimizing static compiler generating good code for a variety of platforms, together with an interactive system for convenient, quick development.

GHC 7.0 and onwards include an LLVM code generator, supporting LLVM 2.8 and later.

### Pure

Pure is an algebraic/functional programming language based on term rewriting. Programs are collections of equations which are used to evaluate expressions in a symbolic fashion. The interpreter uses LLVM as a backend to JIT-compile Pure programs to fast native code. Pure offers dynamic typing, eager and lazy evaluation, lexical closures, a hygienic macro system (also based on term rewriting), built-in list and matrix support (including list and matrix comprehensions) and an easy-to-use interface to C and other programming languages (including the ability to load LLVM bitcode modules, and inline C, C++, Fortran and Faust code in Pure programs if the corresponding LLVM-enabled compilers are installed).

Pure version 0.54 has been tested and is known to work with LLVM 3.1 (and continues to work with older LLVM releases >= 2.5).

## What's New in LLVM 3.1?

This release includes a huge number of bug fixes, performance tweaks and minor improvements. Some of the major improvements and new features are listed in this section.

### Major New Features

LLVM 3.1 includes several major changes and big features:

- AddressSanitizer, a fast memory error detector.
- MachineInstr Bundles, Support to model instruction bundling / packing.
- ARM Integrated Assembler, A full featured assembler and direct-to-object support for ARM.
- Basic Block Placement Probability driven basic block placement.
- ....

### LLVM IR and Core Improvements

LLVM IR has several new features for better support of new targets and that expose new optimization opportunities:

- IR support for half float
- IR support for vectors of pointers, including vector GEPs.
- Module flags have been introduced. They convey information about the module as a whole to LLVM subsystems.
- Loads can now have range metadata attached to them to describe the possible values being loaded.
- Inline cost heuristics have been completely overhauled and now closely model constant propagation through call sites, disregard trivially dead code costs, and can model C++ STL iterator patterns.
- ....

### Optimizer Improvements

In addition to many minor performance tweaks and bug fixes, this release includes a few major enhancements and additions to the optimizers:

- The loop unroll pass now is able to unroll loops with run-time trip counts. This feature is turned off by default, and is enabled with the `-unroll-runtime` flag.
- A new basic-block autovectorization pass is available. Pass `-vectorize` to run this pass along with some associated post-vectorization cleanup passes. For more information, see the EuroLLVM 2012 slides: Autovectorization with LLVM.
- ....

### MC Level Improvements

The LLVM Machine Code (aka MC) subsystem was created to solve a number of problems in the realm of assembly, disassembly, object file format handling, and a number of other related areas that CPU instruction-set level tools work in. For more information, please see the Intro to the LLVM MC Project Blog Post.

- ....

### Target Independent Code Generator Improvements

We have changed the way that the Type Legalizer legalizes vectors. The type legalizer now attempts to promote integer elements. This enabled the implementation of vector-select. Additionally, we see a performance boost on workloads which use vectors of chars and shorts, since they are now promoted to 32-bit types, which are better supported by the SIMD instruction set. Floating point types are still widened as before.

We have put a significant amount of work into the code generator infrastructure, which allows us to implement more aggressive algorithms and make it run faster:

- TableGen can now synthesize register classes that are only needed to represent combinations of constraints from instructions and sub-registers. The synthetic register classes inherit most of their properties form their closest user-defined super-class.
- `MachineRegisterInfo` now allows the reserved registers to be frozen when register allocation starts. Target hooks should use the `MRI->canReserveReg(FramePtr)` method to avoid accidentally disabling frame pointer elimination during register allocation.
- A new kind of `MachineOperand` provides a compact representation of large clobber lists on call instructions. The register mask operand references a bit mask of preserved registers. Everything else is clobbered.

We added new TableGen infrastructure to support bundling for Very Long Instruction Word (VLIW) architectures. TableGen can now automatically generate a deterministic finite automaton from a VLIW target's schedule description which can be queried to determine legal groupings of instructions in a bundle.

We have added a new target independent VLIW packetizer based on the DFA infrastructure to group machine instructions into bundles.

#### Basic Block Placement

A probability based block placement and code layout algorithm was added to LLVM's code generator. This layout pass supports probabilities derived from static heuristics as well as source code annotations such as `__builtin_expect`.

### X86-32 and X86-64 Target Improvements

New features and major changes in the X86 target include:

- Bug fixes and improved support for AVX1
- Support for AVX2 (still incomplete at this point)
- Call instructions use the new register mask operands for faster compile times and better support for different calling conventions. The old WINCALL instructions are no longer needed.
- DW2 Exception Handling is enabled on Cygwin and MinGW.
- Support for implicit TLS model used with MS VC runtime

### ARM Target Improvements

New features of the ARM target include:

- The constant island pass now supports basic block and constant pool entry alignments greater than 4 bytes.
- On Darwin, the ARM target now has a full-featured integrated assembler.

#### ARM Integrated Assembler

The ARM target now includes a full featured macro assembler, including direct-to-object module support for clang. The assembler is currently enabled by default for Darwin only pending testing and any additional necessary platform specific support for Linux.

Full support is included for Thumb1, Thumb2 and ARM modes, along with subtarget and CPU specific extensions for VFP2, VFP3 and NEON.

The assembler is Unified Syntax only (see ARM Architecural Reference Manual for details). While there is some, and growing, support for pre-unfied (divided) syntax, there are still significant gaps in that support.

### MIPS Target Improvements

This release has seen major new work on just about every aspect of the MIPS backend. Some of the major new features include:

- ....

### Other Target Specific Improvements

Support for Qualcomm's Hexagon VLIW processor has been added.

- ....

### Major Changes and Removed Features

If you're already an LLVM user or developer with out-of-tree changes based on LLVM 3.1, this section lists some "gotchas" that you may run into upgrading from the previous release.

- LLVM 3.1 removes support for reading LLVM 2.9 bitcode files. Going forward, we aim for all future versions of LLVM to read bitcode files and `.ll` files produced by LLVM 3.0 and later.
- The `unwind` instruction is now gone. With the introduction of the new exception handling system in LLVM 3.0, the `unwind` instruction became obsolete.
- LLVM 3.0 and earlier automatically added the returns_twice fo functions like setjmp based on the name. This functionality was removed in 3.1. This affects Clang users, if -ffreestanding is used.
- ....

### Internal API Changes

In addition, many APIs have changed in this release. Some of the major LLVM API changes are:

- Target specific options have been moved from global variables to members on the new `TargetOptions` class, which is local to each `TargetMachine`. As a consequence, the associated flags will no longer be accepted by `clang -mllvm`. This includes:
  - `llvm::PrintMachineCode`
  - `llvm::NoFramePointerElim`
  - `llvm::NoFramePointerElimNonLeaf`
  - `llvm::DisableFramePointerElim(const MachineFunction &)`
  - `llvm::LessPreciseFPMADOption`
  - `llvm::LessPrecideFPMAD()`
  - `llvm::NoExcessFPPrecision`
  - `llvm::UnsafeFPMath`
  - `llvm::NoInfsFPMath`
  - `llvm::NoNaNsFPMath`
  - `llvm::HonorSignDependentRoundingFPMathOption`
  - `llvm::HonorSignDependentRoundingFPMath()`
  - `llvm::UseSoftFloat`
  - `llvm::FloatABIType`
  - `llvm::NoZerosInBSS`
  - `llvm::JITExceptionHandling`
  - `llvm::JITEmitDebugInfo`
  - `llvm::JITEmitDebugInfoToDisk`
  - `llvm::GuaranteedTailCallOpt`
  - `llvm::StackAlignmentOverride`
  - `llvm::RealignStack`
  - `llvm::DisableJumpTables`
  - `llvm::EnableFastISel`
  - `llvm::getTrapFunctionName()`
  - `llvm::EnableSegmentedStacks`
- The MDBuilder class has been added to simplify the creation of metadata.
- ....

### Tools Changes

In addition, some tools have changed in this release. Some of the changes are:

- llvm-stress is a command line tool for generating random .ll files to fuzz different LLVM components.
- llvm-ld has been removed. Use llvm-link or Clang instead.
- ....

- ....

## Known Problems

LLVM is generally a production quality compiler, and is used by a broad range of applications and shipping in many products. That said, not every subsystem is as mature as the aggregate, particularly the more obscure targets. If you run into a problem, please check the LLVM bug database and submit a bug if there isn't already one or ask on the LLVMdev list.

Known problem areas include:

- The Alpha, Blackfin, CellSPU, MSP430, PTX, SystemZ and XCore backends are experimental, and the Alpha, Blackfin and SystemZ targets have already been removed from mainline.
- The integrated assembler, disassembler, and JIT is not supported by several targets. If an integrated assembler is not supported, then a system assembler is required. For more details, see the Target Features Matrix.
- The C backend has numerous problems and is not being actively maintained. Depending on it for anything serious is not advised.

## Additional Information

A wide variety of additional information is available on the LLVM web page, in particular in the documentation section. The web page also contains versions of the API documentation which is up-to-date with the Subversion version of the source code. You can access versions of these documents specific to this release by going into the "`llvm/doc/`" directory in the LLVM tree.

If you have any questions or comments about LLVM, please feel free to contact us via the mailing lists.