LLVM 3.0 Release Notes

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 3.0 time-frame, the Clang team has made many improvements:

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 is a gcc plugin that replaces GCC's optimizers and code generators with LLVM's. Currently it requires a patched version of gcc-4.5. The plugin can target the x86-32 and x86-64 processor families and has been used successfully on the Darwin, FreeBSD and Linux platforms. The Ada, C, C++ and Fortran languages work well. The plugin is capable of compiling plenty of Obj-C, Obj-C++ and Java but it is not known whether the compiled code actually works or not!

The 3.0 release has the following notable changes:

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).

In the LLVM 3.0 timeframe,

LLDB is a brand new member of the LLVM umbrella of projects. LLDB is a next generation, high-performance debugger. It is built as a set of reusable components which highly leverage existing libraries in the larger LLVM Project, such as the Clang expression parser, the LLVM disassembler and the LLVM JIT.

LLDB is has advanced by leaps and bounds in the 3.0 timeframe. It is dramatically more stable and useful, and includes both a new tutorial and a side-by-side comparison with GDB.

libc++ is another new member of the LLVM family. It is an implementation of the C++ standard library, written from the ground up to specifically target the forthcoming C++'0X standard and focus on delivering great performance.

In the LLVM 3.0 timeframe,

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

LLBrowse is an interactive viewer for LLVM modules. It can load any LLVM module and displays its contents as an expandable tree view, facilitating an easy way to inspect types, functions, global variables, or metadata nodes. It is fully cross-platform, being based on the popular wxWidgets GUI toolkit.

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. As of LLVM 3.0, VMKit now supports generational garbage collectors. The garbage collectors are provided by the MMTk framework, and VMKit can be configured to use one of the numerous implemented collectors of MMTk.

Crack aims to provide the ease of development of a scripting language with the performance of a compiled language. The language derives concepts from C++, Java and Python, incorporating object-oriented programming, operator overloading and strong typing.

TCE is a toolset for designing application-specific processors (ASP) based on the Transport triggered architecture (TTA). The toolset provides a complete co-design flow from C/C++ programs down to synthesizable VHDL 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++ 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.

PinaVM is an open source, SystemC front-end. Unlike many other front-ends, PinaVM actually executes the elaboration of the program analyzed using LLVM's JIT infrastructure. It later enriches the bitcode with SystemC-specific information.

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 0.47 has been tested and is known to work with LLVM 3.0 (and continues to work with older LLVM releases >= 2.5).

IcedTea provides a harness to build OpenJDK using only free software build tools and to provide replacements for the not-yet free parts of OpenJDK. One of the extensions that IcedTea provides is a new JIT compiler named Shark which uses LLVM to provide native code generation without introducing processor-dependent code.

OpenJDK 7 b112, IcedTea6 1.9 and IcedTea7 1.13 and later have been tested and are known to work with LLVM 3.0 (and continue to work with older LLVM releases >= 2.6 as well).

GHC is an open source, state-of-the-art programming suite for Haskell, a standard 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.

In addition to the existing C and native code generators, GHC 7.0 now supports an LLVM code generator. GHC supports LLVM 2.7 and later.

Polly is a project that aims to provide advanced memory access optimizations to better take advantage of SIMD units, cache hierarchies, multiple cores or even vector accelerators for LLVM. Built around an abstract mathematical description based on Z-polyhedra, it provides the infrastructure to develop advanced optimizations in LLVM and to connect complex external optimizers. In its first year of existence Polly already provides an exact value-based dependency analysis as well as basic SIMD and OpenMP code generation support. Furthermore, Polly can use PoCC(Pluto) an advanced optimizer for data-locality and parallelism.

Rubinius is an environment for running Ruby code which strives to write as much of the implementation in Ruby as possible. Combined with a bytecode interpreting VM, it uses LLVM to optimize and compile ruby code down to machine code. Techniques such as type feedback, method inlining, and deoptimization are all used to remove dynamism from ruby execution and increase performance.

FAUST is a compiled language for real-time audio signal processing. The name FAUST stands for Functional AUdio STream. Its programming model combines two approaches: functional programming and block diagram composition. In addition with the C, C++, JAVA output formats, the Faust compiler can now generate LLVM bitcode, and works with LLVM 2.7-3.0.

LLVM 3.0 includes several major new capabilities:

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

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

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.

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:

New features and major changes in the X86 target include:

• The CRC32 intrinsics have been renamed. The intrinsics were previously @llvm.x86.sse42.crc32.[8|16|32] and @llvm.x86.sse42.crc64.[8|64]. They have been renamed to @llvm.x86.sse42.crc32.32.[8|16|32] and @llvm.x86.sse42.crc32.64.[8|64].

New features of the ARM target include:

If you're already an LLVM user or developer with out-of-tree changes based on LLVM 2.9, this section lists some "gotchas" that you may run into upgrading from the previous release.

• The LowerSetJmp pass wasn't used effectively by any target and has been removed.
• The syntax of volatile loads and stores in IR has been changed to "load volatile"/"store volatile". The old syntax ("volatile load"/"volatile store") is still accepted, but is now considered deprecated.

Windows (32-bit)

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

• The biggest and most pervasive change is that llvm::Type's are no longer returned or accepted as 'const' values. Instead, just pass around non-const Type's.
• PHINode::reserveOperandSpace has been removed. Instead, you must specify how many operands to reserve space for when you create the PHINode, by passing an extra argument into PHINode::Create.
• PHINodes no longer store their incoming BasicBlocks as operands. Instead, the list of incoming BasicBlocks is stored separately, and can be accessed with new functions PHINode::block_begin and PHINode::block_end.
• Various functions now take an ArrayRef instead of either a pair of pointers (or iterators) to the beginning and end of a range, or a pointer and a length. Others now return an ArrayRef instead of a reference to a SmallVector or std::vector. These include:
  • CallInst::Create
  • ComputeLinearIndex (in llvm/CodeGen/Analysis.h)
  • ConstantArray::get
  • ConstantExpr::getExtractElement
  • ConstantExpr::getGetElementPtr
  • ConstantExpr::getInBoundsGetElementPtr
  • ConstantExpr::getIndices
  • ConstantExpr::getInsertElement
  • ConstantExpr::getWithOperands
  • ConstantFoldCall (in llvm/Analysis/ConstantFolding.h)
  • ConstantFoldInstOperands (in llvm/Analysis/ConstantFolding.h)
  • ConstantVector::get
  • DIBuilder::createComplexVariable
  • DIBuilder::getOrCreateArray
  • ExtractValueInst::Create
  • ExtractValueInst::getIndexedType
  • ExtractValueInst::getIndices
  • FindInsertedValue (in llvm/Analysis/ValueTracking.h)
  • gep_type_begin (in llvm/Support/GetElementPtrTypeIterator.h)
  • gep_type_end (in llvm/Support/GetElementPtrTypeIterator.h)
  • GetElementPtrInst::Create
  • GetElementPtrInst::CreateInBounds
  • GetElementPtrInst::getIndexedType
  • InsertValueInst::Create
  • InsertValueInst::getIndices
  • InvokeInst::Create
  • IRBuilder::CreateCall
  • IRBuilder::CreateExtractValue
  • IRBuilder::CreateGEP
  • IRBuilder::CreateInBoundsGEP
  • IRBuilder::CreateInsertValue
  • IRBuilder::CreateInvoke
  • MDNode::get
  • MDNode::getIfExists
  • MDNode::getTemporary
  • MDNode::getWhenValsUnresolved
  • SimplifyGEPInst (in llvm/Analysis/InstructionSimplify.h)
  • TargetData::getIndexedOffset
• All forms of StringMap::getOrCreateValue have been remove except for the one which takes a StringRef.
• The LLVMBuildUnwind function from the C API was removed. The LLVM unwind instruction has been deprecated for a long time and isn't used by the current front-ends. So this was removed during the exception handling rewrite.
• The LLVMAddLowerSetJmpPass function from the C API was removed because the LowerSetJmp pass was removed.
• The DIBuilder interface used by front ends to encode debugging information in the LLVM IR now expects clients to use DIBuilder::finalize() at the end of translation unit to complete debugging information encoding.

The following components of this LLVM release are either untested, known to be broken or unreliable, or are in early development. These components should not be relied on, and bugs should not be filed against them, but they may be useful to some people. In particular, if you would like to work on one of these components, please contact us on the LLVMdev list.

• The Alpha, Blackfin, CellSPU, MicroBlaze, MSP430, MIPS, PTX, SystemZ and XCore backends are experimental.
• llc "-filetype=obj" is experimental on all targets other than darwin and ELF X86 systems.

• The X86 backend does not yet support all inline assembly that uses the X86 floating point stack. It supports the 'f' and 't' constraints, but not 'u'.
• The X86-64 backend does not yet support the LLVM IR instruction va_arg. Currently, front-ends support variadic argument constructs on X86-64 by lowering them manually.
• Windows x64 (aka Win64) code generator has a few issues.
  • llvm-gcc cannot build the mingw-w64 runtime currently due to lack of support for the 'u' inline assembly constraint and for X87 floating point inline assembly.
  • On mingw-w64, you will see unresolved symbol __chkstk due to Bug 8919. It is fixed in r128206.
  • Miss-aligned MOVDQA might crash your program. It is due to Bug 9483, lack of handling aligned internal globals.

• The Linux PPC32/ABI support needs testing for the interpreter and static compilation, and lacks support for debug information.

• Thumb mode works only on ARMv6 or higher processors. On sub-ARMv6 processors, thumb programs can crash or produce wrong results (PR1388).
• Compilation for ARM Linux OABI (old ABI) is supported but not fully tested.

• The SPARC backend only supports the 32-bit SPARC ABI (-m32); it does not support the 64-bit SPARC ABI (-m64).

• 64-bit MIPS targets are not supported yet.

• On 21164s, some rare FP arithmetic sequences which may trap do not have the appropriate nops inserted to ensure restartability.

The C backend has numerous problems and is not being actively maintained. Depending on it for anything serious is not advised.

• The C backend has only basic support for inline assembly code.
• The C backend violates the ABI of common C++ programs, preventing intermixing between C++ compiled by the CBE and C++ code compiled with llc or native compilers.
• The C backend does not support all exception handling constructs.
• The C backend does not support arbitrary precision integers.

LLVM 3.0 will be the last release of llvm-gcc.

llvm-gcc is generally very stable for the C family of languages. The only major language feature of GCC not supported by llvm-gcc is the __builtin_apply family of builtins. However, some extensions are only supported on some targets. For example, trampolines are only supported on some targets (these are used when you take the address of a nested function).

Fortran support generally works, but there are still several unresolved bugs in Bugzilla. Please see the tools/gfortran component for details. Note that llvm-gcc is missing major Fortran performance work in the frontend and library that went into GCC after 4.2. If you are interested in Fortran, we recommend that you consider using dragonegg instead.

The llvm-gcc 4.2 Ada compiler has basic functionality, but is no longer being actively maintained. If you are interested in Ada, we recommend that you consider using dragonegg instead.