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:

• Greatly improved support for building C++ applications, with greater stability and better diagnostics.
• Improved support for the C++ 2011 standard, including implementations of non-static data member initializers, alias templates, delegating constructors, the range-based for loop, and implicitly-generated move constructors and move assignment operators, among others.
• Implemented support for some features of the upcoming C1x standard, including static assertions and generic selections.
• Better detection of include and linking paths for system headers and libraries, especially for Linux distributions.
• Implemented support for Automatic Reference Counting for Objective-C.
• Implemented a number of optimizations in libclang, the Clang C interface, to improve the performance of code completion and the mapping from source locations to abstract syntax tree nodes.

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 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.

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.

AddressSanitizer uses compiler instrumentation and a specialized malloc library to find C/C++ bugs such as use-after-free and out-of-bound accesses to heap, stack, and globals. The key feature of the tool is speed: the average slowdown introduced by AddressSanitizer is less than 2x.

Clam AntiVirus is an open source (GPL) anti-virus toolkit for UNIX, designed especially for e-mail scanning on mail gateways.

Since version 0.96 it has bytecode signatures that allow writing detections for complex malware.

It uses LLVM's JIT to speed up the execution of bytecode on X86, X86-64, PPC32/64, falling back to its own interpreter otherwise. The git version was updated to work with LLVM 3.0.

clReflect is a C++ parser that uses clang/LLVM to derive a light-weight reflection database suitable for use in game development. It comes with a very simple runtime library for loading and querying the database, requiring no external dependencies (including CRT), and an additional utility library for object management and serialisation.

Cling is an interactive compiler interface (aka C++ interpreter). It uses LLVM's JIT and clang; it currently supports C++ and C. It has a prompt interface, runs source files, calls into shared libraries, prints the value of expressions, even does runtime lookup of identifiers (dynamic scopes). And it just behaves like one would expect from an interpreter.

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.

Eero is a fully header-and-binary-compatible dialect of Objective-C 2.0, implemented with a patched version of the Clang/LLVM compiler. It features a streamlined syntax, Python-like indentation, and new operators, for improved readability and reduced code clutter. It also has new features such as limited forms of operator overloading and namespaces, and strict (type-and-operator-safe) enumerations. It is inspired by languages such as Smalltalk, Python, and Ruby.

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.

GHC 7.0 and onwards include an LLVM code generator, supporting LLVM 2.8 and later. Since LLVM 2.9, GHC now includes experimental support for the ARM platform with LLVM 3.0.

gwXscript is an object oriented, aspect oriented programming language which can create both executables (ELF, EXE) and shared libraries (DLL, SO, DYNLIB). The compiler is implemented in its own language and translates scripts into LLVM-IR which can be optimized and translated into native code by the LLVM framework. Source code in gwScript contains definitions that expand the namespaces. So you can build your project and simply 'plug out' features by removing a file. The remaining project does not leave scars since you directly separate concerns by the 'template' feature of gwX. It is also possible to add new features to a project by just adding files and without editing the original project. This language is used for example to create games or content management systems that should be extendable.

gwXscript is strongly typed and offers comfort with its native types string, hash and array. You can easily write new libraries in gwXscript or native code. gwXscript is type safe and users should not be able to crash your program or execute malicious code except code that is eating CPU time.

include-what-you-use is a tool to ensure that a file directly #includes all .h files that provide a symbol that the file uses. It also removes superfluous #includes from source files.

ispc is a compiler for "single program, multiple data" (SPMD) programs. It compiles a C-based SPMD programming language to run on the SIMD units of CPUs; it often delivers 5-6x speedups on a single core of a CPU with an 8-wide SIMD unit compared to serial code, while still providing a clean and easy-to-understand programming model. For an introduction to the language and its performance, see the walkthrough of a short example program. ispc is licensed under the BSD license.

LanguageKit is a framework for implementing dynamic languages sharing an object model with Objective-C. It provides static and JIT compilation using LLVM along with its own interpreter. Pragmatic Smalltalk is a dialect of Smalltalk, built on top of LanguageKit, that interfaces directly with Objective-C, sharing the same object representation and message sending behaviour. These projects are developed as part of the Étoié desktop environment.

LuaAV is a real-time audiovisual scripting environment based around the Lua language and a collection of libraries for sound, graphics, and other media protocols. LuaAV uses LLVM and Clang to JIT compile efficient user-defined audio synthesis routines specified in a declarative syntax.

An open source, cross-platform implementation of C# and the CLR that is binary compatible with Microsoft.NET. Has an optional, dynamically-loaded LLVM code generation backend in Mini, the JIT compiler.

Note that we use a Git mirror of LLVM with some patches. See: https://github.com/mono/llvm

Portable OpenCL is an open source implementation of the OpenCL standard which can be easily adapted for new targets. One of the goals of the project is improving performance portability of OpenCL programs, avoiding the need for target-dependent manual optimizations. A "native" target is included, which allows running OpenCL kernels on the host (CPU).

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

Renderscript is Android's advanced 3D graphics rendering and compute API. It provides a portable C99-based language with extensions to facilitate common use cases for enhancing graphics and thread level parallelism. The Renderscript compiler frontend is based on Clang/LLVM. It emits a portable bitcode format for the actual compiled script code, as well as reflects a Java interface for developers to control the execution of the compiled bitcode. Executable machine code is then generated from this bitcode by an LLVM backend on the device. Renderscript is thus able to provide a mechanism by which Android developers can improve performance of their applications while retaining portability.

SAFECode is a memory safe C/C++ compiler built using LLVM. It takes standard, unannotated C/C++ code, analyzes the code to ensure that memory accesses and array indexing operations are safe, and instruments the code with run-time checks when safety cannot be proven statically. SAFECode can be used as a debugging aid (like Valgrind) to find and repair memory safety bugs. It can also be used to protect code from security attacks at run-time.

The Stupid D Compiler is a project seeking to write a self-hosting compiler for the D programming language without using the frontend of the reference compiler (DMD).

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.

Tart is a general-purpose, strongly typed programming language designed for application developers. Strongly inspired by Python and C#, Tart focuses on practical solutions for the professional software developer, while avoiding the clutter and boilerplate of legacy languages like Java and C++. Although Tart is still in development, the current implementation supports many features expected of a modern programming language, such as garbage collection, powerful bidirectional type inference, a greatly simplified syntax for template metaprogramming, closures and function literals, reflection, operator overloading, explicit mutability and immutability, and much more. Tart is flexible enough to accommodate a broad range of programming styles and philosophies, while maintaining a strong commitment to simplicity, minimalism and elegance in design.

ThreadSanitizer is a data race detector for (mostly) C and C++ code, available for Linux, Mac OS and Windows. On different systems, we use binary instrumentation frameworks (Valgrind and Pin) as frontends that generate the program events for the race detection algorithm. On Linux, there's an option of using LLVM-based compile-time instrumentation.

ZooLib is Open Source under the MIT License. It provides GUI, filesystem access, TCP networking, thread-safe memory management, threading and locking for Mac OS X, Classic Mac OS, Microsoft Windows, POSIX operating systems with X11, BeOS, Haiku, Apple's iOS and Research in Motion's BlackBerry.

My current work is to use CLang's static analyzer to improve ZooLib's code quality. I also plan to set up LLVM compiles of the demo programs and test programs using CLang and LLVM on all the platforms that CLang, LLVM and ZooLib all support.

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:

One of the biggest changes is that 3.0 has a new exception handling system. The old system used LLVM intrinsics to convey the exception handling information to the code generator. It worked in most cases, but not all. Inlining was especially difficult to get right. Also, the intrinsics could be moved away from the invoke instruction, making it hard to recover that information.

The new EH system makes exception handling a first-class member of the IR. It adds two new instructions:

• landingpad — this instruction defines a landing pad basic block. It contains all of the information that's needed by the code generator. It's also required to be the first non-PHI instruction in the landing pad. In addition, a landing pad may be jumped to only by the unwind edge of an invoke instruction.
• resume — this instruction causes the current exception to resume traveling up the stack. It replaces the @llvm.eh.resume intrinsic.

Converting from the old EH API to the new EH API is rather simple, because a lot of complexity has been removed. The two intrinsics, @llvm.eh.exception and @llvm.eh.selector have been superceded by the landingpad instruction. Instead of generating a call to @llvm.eh.exception and @llvm.eh.selector:

Function *ExcIntr = Intrinsic::getDeclaration(TheModule,
                                              Intrinsic::eh_exception);
Function *SlctrIntr = Intrinsic::getDeclaration(TheModule,
                                                Intrinsic::eh_selector);

// The exception pointer.
Value *ExnPtr = Builder.CreateCall(ExcIntr, "exc_ptr");

std::vector<Value*> Args;
Args.push_back(ExnPtr);
Args.push_back(Builder.CreateBitCast(Personality,
                                     Type::getInt8PtrTy(Context)));

// Add selector clauses to Args.

// The selector call.
Builder.CreateCall(SlctrIntr, Args, "exc_sel");

You should instead generate a landingpad instruction, that returns an exception object and selector value:

LandingPadInst *LPadInst =
  Builder.CreateLandingPad(StructType::get(Int8PtrTy, Int32Ty, NULL),
                           Personality, 0);

Value *LPadExn = Builder.CreateExtractValue(LPadInst, 0);
Builder.CreateStore(LPadExn, getExceptionSlot());

Value *LPadSel = Builder.CreateExtractValue(LPadInst, 1);
Builder.CreateStore(LPadSel, getEHSelectorSlot());

It's now trivial to add the individual clauses to the landingpad instruction.

// Adding a catch clause
Constant *TypeInfo = getTypeInfo();
LPadInst->addClause(TypeInfo);

// Adding a C++ catch-all
LPadInst->addClause(Constant::getNullValue(Builder.getInt8PtrTy()));

// Adding a cleanup
LPadInst->setCleanup(true);

// Adding a filter clause
std::vector<Constant*> TypeInfos;
Constant *TypeInfo = getFilterTypeInfo();
TypeInfos.push_back(Builder.CreateBitCast(TypeInfo, Builder.getInt8PtrTy()));

ArrayType *FilterTy = ArrayType::get(Int8PtrTy, TypeInfos.size());
LPadInst->addClause(ConstantArray::get(FilterTy, TypeInfos));

Converting from using the @llvm.eh.resume intrinsic to the resume instruction is trivial. It takes the exception pointer and exception selector values returned by the landingpad instruction:

Type *UnwindDataTy = StructType::get(Builder.getInt8PtrTy(),
                                     Builder.getInt32Ty(), NULL);
Value *UnwindData = UndefValue::get(UnwindDataTy);
Value *ExcPtr = Builder.CreateLoad(getExceptionObjSlot());
Value *ExcSel = Builder.CreateLoad(getExceptionSelSlot());
UnwindData = Builder.CreateInsertValue(UnwindData, ExcPtr, 0, "exc_ptr");
UnwindData = Builder.CreateInsertValue(UnwindData, ExcSel, 1, "exc_sel");
Builder.CreateResume(UnwindData);

The induction variable simplification pass in 3.0 only modifies induction variables when profitable. Sign and zero extension elimination, linear function test replacement, loop unrolling, and other simplifications that require induction variable analysis have been generalized so they no longer require loops to be rewritten in a typically suboptimal form prior to optimization. This new design preserves more IR level information, avoids undoing earlier loop optimizations (particularly hand-optimized loops), and no longer strongly depends on the code generator rewriting loops a second time in a now optimal form--an intractable problem.

The original behavior can be restored with -mllvm -enable-iv-rewrite; however, support for this mode will be short lived. As such, bug reports should be filed for any significant performance regressions when moving from -mllvm -enable-iv-rewrite to the 3.0 default mode.

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 LLVMC front end code was removed while separating out language independence.
• The LowerSetJmp pass wasn't used effectively by any target and has been removed.
• The old TailDup pass was not used in the standard pipeline and was unable to update ssa form, so it 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.
• The old atomic intrinscs (llvm.memory.barrier and llvm.atomic.*) are now gone. Please use the new atomic instructions, described in the atomics guide.

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 way the type system works has been rewritten: PATypeHolder and OpaqueType are gone, and all APIs deal with Type* instead of const Type*. If you need to create recursive structures, then create a named structure, and use setBody() when all its elements are built. Type merging and refining is gone too: named structures are not merged with other structures, even if their layout is identical. (of course anonymous structures are still uniqued by layout).
• TargetSelect.h moved to Support/ from Target/
• UpgradeIntrinsicCall no longer upgrades pre-2.9 intrinsic calls (for example llvm.memset.i32).
• It is mandatory to initialize all out-of-tree passes too and their dependencies now with INITIALIZE_PASS{BEGIN,END,} and INITIALIZE_{PASS,AG}_DEPENDENCY.
• The interface for MemDepResult in MemoryDependenceAnalysis has been enhanced with new return types Unknown and NonFuncLocal, in addition to the existing types Clobber, Def, and NonLocal.

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 PPC32/ELF support lacks PIC support.

• 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 2.9 was 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.