LLVM 3.3 Release Notes

Introduction

This document contains the release notes for the LLVM Compiler Infrastructure, release 3.3. 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.

Non-comprehensive list of changes in this release

• The CellSPU port has been removed. It can still be found in older versions.
• The IR-level extended linker APIs (for example, to link bitcode files out of archives) have been removed. Any existing clients of these features should move to using a linker with integrated LTO support.
• LLVM and Clang’s documentation has been migrated to the Sphinx documentation generation system which uses easy-to-write reStructuredText. See llvm/docs/README.txt for more information.
• TargetTransformInfo (TTI) is a new interface that can be used by IR-level passes to obtain target-specific information, such as the costs of instructions. Only “Lowering” passes such as LSR and the vectorizer are allowed to use the TTI infrastructure.
• We’ve improved the X86 and ARM cost model.
• The Attributes classes have been completely rewritten and expanded. They now support not only enumerated attributes and alignments, but “string” attributes, which are useful for passing information to code generation. See How To Use Attributes for more details.
• TableGen’s syntax for instruction selection patterns has been simplified. Instead of specifying types indirectly with register classes, you should now specify types directly in the input patterns. See SparcInstrInfo.td for examples of the new syntax. The old syntax using register classes still works, but it will be removed in a future LLVM release.
• MCJIT now supports exception handling. Support for it in the old jit will be removed in the 3.4 release.
• Command line options can now be grouped into categories which are shown in the output of -help. See Grouping options into categories.
• The appearance of command line options in -help that are inherited by linking with libraries that use the LLVM Command line support library can now be modified at runtime. See The cl::getRegisteredOptions function.

AArch64 target

We’ve added support for AArch64, ARM’s 64-bit architecture. Development is still in fairly early stages, but we expect successful compilation when:

• compiling standard compliant C99 and C++03 with Clang;
• using Linux as a target platform;
• where code + static data doesn’t exceed 4GB in size (heap allocated data has no limitation).

Some additional functionality is also implemented, notably DWARF debugging, GNU-style thread local storage and inline assembly.

Hexagon Target

Removed support for legacy hexagonv2 and hexagonv3 processor architectures which are no longer in use. Currently supported architectures are hexagonv4 and hexagonv5.

Mips target

New features and improvements:

• Clang driver

• Support for Sourcery CodeBench Mips toolchain directories tree.
• Support for new command line options including:

• -mxgot/-mno-xgot
• -EL / -EB
• -mmicromips / -mno-micromips
• -msingle-float / -mdouble-float
• -mabi=32 (o32 abi) and -mabi=64 (n64 abi)

• Previously, options such as -mips16, -mmicromips, -mdsp and -mdspr2 were not passed to the assembler. This issue has been fixed.

• A number of changes have been made to improve the quality of DSP-ASE code generation.

• Multiply and multiply-accumulate instructions can now use all four accumulators.
• Instruction selection patterns have been added so that DSP instructions are emitted without having to use builtins.

• Delay slot filler pass can now search successor blocks for instructions to fill delay slots (use option -disable-mips-df-succbb-search=false).

PowerPC Target

New features and improvements:

• PowerPC now supports an assembly parser.
• Support added for thread-local storage. 64-bit ELF subtarget only.
• Support added for medium and large code model (-mcmodel=medium,large). Medium code model is now the default. 64-bit ELF subtarget only.
• Improved register allocation (fewer reserved registers).
• 64-bit atomic load and store are now supported.
• Improved code generation for unaligned memory accesses of scalar types.
• Improved performance of floating-point divide and square root with -ffast-math.
• Support for predicated returns.
• Improved code generation for comparisons.
• Support added for inline setjmp and longjmp.
• Support added for many instructions introduced in PowerISA 2.04, 2.05, and 2.06.
• Improved spill code for vector registers.
• Support added for -mno-altivec.
• ABI compatibility fixes for complex parameters, 128-bit integer parameters, and varargs functions. 64-bit ELF subtarget only.

Loop Vectorizer

We’ve continued the work on the loop vectorizer. The loop vectorizer now has the following features:

• Loops with unknown trip counts.
• Runtime checks of pointers.
• Reductions, Inductions.
• Min/Max reductions of integers.
• If Conversion.
• Pointer induction variables.
• Reverse iterators.
• Vectorization of mixed types.
• Vectorization of function calls.
• Partial unrolling during vectorization.

The loop vectorizer is now enabled by default for -O3.

SLP Vectorizer

LLVM now has a new SLP vectorizer. The new SLP vectorizer is not enabled by default but can be enabled using the clang flag -fslp-vectorize. The BB-vectorizer can also be enabled using the command line flag -fslp-vectorize-aggressive.

R600 Backend

The R600 backend was added in this release, it supports AMD GPUs (HD2XXX - HD7XXX). This backend is used in AMD’s Open Source graphics / compute drivers which are developed as part of the Mesa3D project.

SystemZ/s390x Backend

LLVM and clang now support IBM’s z/Architecture. At present this support is restricted to GNU/Linux (GNU triplet s390x-linux-gnu) and requires z10 or greater.

Sub-project Status Update

In addition to the core LLVM 3.3 distribution of production-quality compiler infrastructure, the LLVM project includes sub-projects that use the LLVM core and share the same distribution license. This section provides updates on these sub-projects.

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, 4.6, 4.7 and 4.8, 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++. Note that gcc-4.6 is the best supported version, and that Ada in particular doesn’t work well with gcc-4.7 and newer.

The 3.3 release has the following notable changes.

• supports gcc-4.8 (requires gcc-4.8.1 or newer)
• object files can be written directly using LLVM’s integrated assembler
• produces saner debug info
• bitfields can now contain arbitrary scalar types (useful for Ada)

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 scripts and other applications. LLDB uses the following components of the LLVM core distribution to support the latest language features and target support:

• the Clang parser for high-quality parsing of C, C++ and Objective C
• the LLVM disassembler
• the LLVM JIT compiler (MCJIT) for expression evaluation

The 3.3 release has the following notable changes.

Linux Features:

• Support for watchpoints
• vim integration for lldb commands and program status using a vim plug-in
• Improved register support including vector registers
• Builds with cmake/ninja/auto-tools/clang 3.3/gcc 4.6

Linux Improvements:

• Debugging multi-threaded programs
• Debugging i386 programs
• Process list, attach and fork
• Expression evaluation

External Open Source Projects Using LLVM 3.3

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 3.3.

Portable Computing Language (pocl)

In addition to producing an easily portable open source OpenCL implementation, another major goal of pocl is improving performance portability of OpenCL programs with compiler optimizations, reducing the need for target-dependent manual optimizations. An important part of pocl is a set of LLVM passes used to statically parallelize multiple work-items with the kernel compiler, even in the presence of work-group barriers. This enables static parallelization of the fine-grained static concurrency in the work groups in multiple ways.

TTA-based Co-design Environment (TCE)

TCE is a toolset for designing new processors based on the Transport triggered architecture (TTA). The toolset provides a complete co-design flow from C/C++ programs down to synthesizable VHDL/Verilog and parallel program binaries. Processor customization points include the register files, function units, supported operations, and the interconnection network.

TCE uses Clang and LLVM for C/C++/OpenCL C language support, target independent optimizations and also for parts of code generation. It generates new LLVM-based code generators “on the fly” for the designed TTA processors and loads them in to the compiler backend as runtime libraries to avoid per-target recompilation of larger parts of the compiler chain.

Just-in-time Adaptive Decoder Engine (Jade)

Jade (Just-in-time Adaptive Decoder Engine) is a generic video decoder engine using LLVM for just-in-time compilation of video decoder configurations. Those configurations are designed by MPEG Reconfigurable Video Coding (RVC) committee. MPEG RVC standard is built on a stream-based dataflow representation of decoders. It is composed of a standard library of coding tools written in RVC-CAL language and a dataflow configuration — block diagram — of a decoder.

Jade project is hosted as part of the Open RVC-CAL Compiler (Orcc) and requires it to translate the RVC-CAL standard library of video coding tools into an LLVM assembly code.

LDC - the LLVM-based D compiler

D is a language with C-like syntax and static typing. It pragmatically combines efficiency, control, and modeling power, with safety and programmer productivity. D supports powerful concepts like Compile-Time Function Execution (CTFE) and Template Meta-Programming, provides an innovative approach to concurrency and offers many classical paradigms.

LDC uses the frontend from the reference compiler combined with LLVM as backend to produce efficient native code. LDC targets x86/x86_64 systems like Linux, OS X and Windows and also Linux/PPC64. Ports to other architectures like ARM are underway.

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/docs/ directory in the LLVM tree.

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