LLVM 2.0 Release Notes

1. Introduction
2. What's New?
3. Installation Instructions
4. Portability and Supported Platforms
5. Known Problems
6. Additional Information

Written by the LLVM Team

Introduction

This document contains the release notes for the LLVM compiler infrastructure, release 2.0. Here we describe the status of LLVM, including major improvements from the previous release and any known problems. 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 CVS or the main LLVM web page, this document applies to the next release, not the current one. To see the release notes for the current or previous releases, see the releases page.

What's New?

This is the eleventh public release of the LLVM Compiler Infrastructure. Being the first major release since 1.0, this release is different in several ways from our previous releases:

1. We took this as an opportunity to break backwards compatibility with the LLVM 1.x bytecode and .ll file format. If you have LLVM 1.9 .ll files that you would like to upgrade to LLVM 2.x, we recommend the use of the stand alone llvm-upgrade tool (which is included with 2.0). We intend to keep compatibility with .ll and .bc formats within the 2.x release series, like we did within the 1.x series.
2. There are several significant change to the LLVM IR and internal APIs, such as a major overhaul of the type system, the completely new bitcode file format, etc (described below).
3. We designed the release around a 6 month release cycle instead of the usual 3-month cycle. This gave us extra time to develop and test some of the more invasive features in this release.
4. LLVM 2.0 no longer supports the llvm-gcc3 front-end. Users are required to upgrade to llvm-gcc4. llvm-gcc4 includes many features over llvm-gcc3, is faster, and is much easier to build from source.

Note that while this is a major version bump, this release has been extensively tested on a wide range of software. It is easy to say that this is our best release yet, in terms of both features and correctness. This is the first LLVM release to correctly compile and optimize major software like LLVM itself, Mozilla/Seamonkey, Qt 4.3rc1, kOffice, etc out of the box on linux/x86.

New Features in LLVM 2.0
Major Changes

Changes to the LLVM IR itself:

- Integer types are now completely signless. This means that we have types like i8/i16/i32 instead of ubyte/sbyte/short/ushort/int etc. LLVM operations that depend on sign have been split up into separate instructions (PR950). This eliminates cast instructions that just change the sign of the operands (e.g. int -> uint), which reduces the size of the IR and makes optimizers simpler to write.
- Integer types with arbitrary bitwidths (e.g. i13, i36, i42, i1057, etc) are now supported in the LLVM IR and optimizations (PR1043). However, neither llvm-gcc (PR1284) nor the native code generators (PR1270) support non-standard width integers yet.
- 'Type planes' have been removed (PR411). It is no longer possible to have two values with the same name in the same symbol table. This simplifies LLVM internals, allowing significant speedups.
- Global variables and functions in .ll files are now prefixed with @ instead of % (PR645).
- The LLVM 1.x "bytecode" format has been replaced with a completely new binary representation, named 'bitcode'. The Bitcode Format brings a number of advantages to the LLVM over the old bytecode format: it is denser (files are smaller), more extensible, requires less memory to read, is easier to keep backwards compatible (so LLVM 2.5 will read 2.0 .bc files), and has many other nice features.
- Load and store instructions now track the alignment of their pointer (PR400). This allows the IR to express loads that are not sufficiently aligned (e.g. due to '`#pragma packed`') or to capture extra alignment information.

Major new features:

- A number of ELF features are now supported by LLVM, including 'visibility', extern weak linkage, Thread Local Storage (TLS) with the `__thread` keyword, and symbol aliases. Among other things, this means that many of the special options needed to configure llvm-gcc on linux are no longer needed, and special hacks to build large C++ libraries like Qt are not needed.
- LLVM now has a new MSIL backend. `llc -march=msil` will now turn LLVM into MSIL (".net") bytecode. This is still fairly early development with a number of limitations.
- A new llvm-upgrade tool exists to migrates LLVM 1.9 .ll files to LLVM 2.0 syntax.

llvm-gcc Improvements

New features include:

- Precompiled Headers (PCH) are now supported.
- "`#pragma packed`" is now supported, as are the various features described above (visibility, extern weak linkage, __thread, aliases, etc).
- Tracking function parameter/result attributes is now possible.
- Many internal enhancements have been added, such as improvements to NON_LVALUE_EXPR, arrays with non-zero base, structs with variable sized fields, VIEW_CONVERT_EXPR, CEIL_DIV_EXPR, nested functions, and many other things. This is primarily to supports non-C GCC front-ends, like Ada.
- It is simpler to configure llvm-gcc for linux.

Optimizer Improvements

New features include:

- The pass manager has been entirely rewritten, making it significantly smaller, simpler, and more extensible. Support has been added to run `FunctionPass`es interlaced with `CallGraphSCCPass`es, we now support loop transformations explicitly with `LoopPass`, and `ModulePass`es may now use the result of `FunctionPass`es.
- LLVM 2.0 includes a new loop rotation pass, which converts "for loops" into "do/while loops", where the condition is at the bottom of the loop.
- The Loop Strength Reduction pass has been improved, and we now support sinking expressions across blocks to reduce register pressure.
- The `-scalarrepl` pass can now promote unions containing FP values into a register, it can also handle unions of vectors of the same size.
- The [Post]DominatorSet classes have been removed from LLVM and clients switched to use the more-efficient ETForest class instead.
- The ImmediateDominator class has also been removed, and clients have been switched to use DominatorTree instead.
- The predicate simplifier pass has been improved, making it able to do simple value range propagation and eliminate more conditionals. However, note that predsimplify is not enabled by default in llvm-gcc.

Code Generator Enhancements

New features include:

- LLVM now supports software floating point, which allows LLVM to target chips that don't have hardware FPUs (e.g. ARM thumb mode).
- A new register scavenger has been implemented, which is useful for finding free registers after register allocation. This is useful when rewriting frame references on RISC targets, for example.
- Heuristics have been added to avoid coalescing vregs with very large live ranges to physregs. This was bad because it effectively pinned the physical register for the entire lifetime of the virtual register (PR711).
- Support now exists for very simple (but still very useful) rematerialization the register allocator, enough to move instructions like "load immediate" and constant pool loads.
- Switch statement lowering is significantly better, improving codegen for sparse switches that have dense subregions, and implemented support for the shift/and trick.
- LLVM now supports tracking physreg sub-registers and super-registers in the code generator, and includes extensive register allocator changes to track them.
- There is initial support for virtreg sub-registers (PR1350).

Other improvements include:

- Inline assembly support is much more solid that before. The two primary features still missing are support for 80-bit floating point stack registers on X86 (PR879), and support for inline asm in the C backend (PR802).
- DWARF debug information generation has been improved. LLVM now passes most of the GDB testsuite on MacOS and debug info is more dense.
- Codegen support for Zero-cost DWARF exception handling has been added (PR592). It is mostly complete and just in need of continued bug fixes and optimizations at this point. However, support in llvm-g++ is disabled with an #ifdef for the 2.0 release (PR870).
- The code generator now has more accurate and general hooks for describing addressing modes ("isLegalAddressingMode") to optimizations like loop strength reduction and code sinking.
- Progress has been made on a direct Mach-o .o file writer. Many small apps work, but it is still not quite complete.

In addition, the LLVM target description format has itself been extended in several ways:

- TargetData now supports better target parameterization in the .ll/.bc files, eliminating the 'pointersize/endianness' attributes in the files (PR761).
- TargetData was generalized for finer grained alignment handling, handling of vector alignment, and handling of preferred alignment
- LLVM now supports describing target calling conventions explicitly in .td files, reducing the amount of C++ code that needs to be written for a port.

Target-Specific Improvements

X86-specific Code Generator Enhancements:

- The MMX instruction set is now supported through intrinsics.
- The scheduler was improved to better reduce register pressure on X86 and other targets that are register pressure sensitive.
- Linux/x86-64 support is much better.
- PIC support for linux/x86 has been added.
- The X86 backend now supports the GCC regparm attribute.
- LLVM now supports inline asm with multiple constraint letters per operand (like "mri") which is common in X86 inline asms.

ARM-specific Code Generator Enhancements:

- The ARM code generator is now stable and fully supported.
- There are major new features, including support for ARM v4-v6 chips, vfp support, soft float point support, pre/postinc support, load/store multiple generation, constant pool entry motion (to support large functions), inline asm support, weak linkage support, static ctor/dtor support and many bug fixes.
- Added support for Thumb code generation (`llc -march=thumb`).
- The ARM backend now supports the ARM AAPCS/EABI ABI and PIC codegen on arm/linux.
- Several bugs were fixed for DWARF debug info generation on arm/linux.

PowerPC-specific Code Generator Enhancements:

- The PowerPC 64 JIT now supports addressing code loaded above the 2G boundary.
- Improved support for the Linux/ppc ABI and the linux/ppc JIT is fully functional now. llvm-gcc and static compilation are not fully supported yet though.
- Many PowerPC 64 bug fixes.

Other Improvements

More specific changes include:

- LLVM no longer relies on static destructors to shut itself down. Instead, it lazily initializes itself and shuts down when `llvm_shutdown()` is explicitly called.
- LLVM now has significantly fewer static constructors, reducing startup time.
- Several classes have been refactored to reduce the amount of code that gets linked into apps that use the JIT.
- Construction of intrinsic function declarations has been simplified.
- The gccas/gccld tools have been replaced with small shell scripts.
- Support has been added to llvm-test for running on low-memory or slow machines (make SMALL_PROBLEM_SIZE=1).

API Changes

LLVM 2.0 contains a revamp of the type system and several other significant internal changes. If you are programming to the C++ API, be aware of the following major changes:

- Pass registration is slightly different in LLVM 2.0 (you now need an `intptr_t` in your constructor), as explained in the Writing an LLVM Pass document.
- `ConstantBool`, `ConstantIntegral` and `ConstantInt` classes have been merged together, we now just have `ConstantInt`.
- `Type::IntTy`, `Type::UIntTy`, `Type::SByteTy`, ... are replaced by `Type::Int8Ty`, `Type::Int16Ty`, etc. LLVM types have always corresponded to fixed size types (e.g. long was always 64-bits), but the type system no longer includes information about the sign of the type. Also, the `Type::isPrimitiveType()` method now returns false for integers.
- Several classes (`CallInst`, `GetElementPtrInst`, `ConstantArray`, etc), that once took `std::vector` as arguments now take ranges instead. For example, you can create a `GetElementPtrInst` with code like:

            Value *Ops[] = { Op1, Op2, Op3 };
            GEP = new GetElementPtrInst(BasePtr, Ops, 3);

  This avoids creation of a temporary vector (and a call to malloc/free). If you have an `std::vector`, use code like this:

            std::vector<Value*> Ops = ...;
            GEP = new GetElementPtrInst(BasePtr, &Ops[0], Ops.size());

- `CastInst` is now abstract and its functionality is split into several parts, one for each of the new cast instructions.
- `Instruction::getNext()/getPrev()` are now private (along with `BasicBlock::getNext`, etc), for efficiency reasons (they are now no longer just simple pointers). Please use `BasicBlock::iterator`, etc instead.
- `Module::getNamedFunction()` is now called `Module::getFunction()`.
- `SymbolTable.h` has been split into `ValueSymbolTable.h` and `TypeSymbolTable.h`.

Portability and Supported Platforms

LLVM is known to work on the following platforms:

- Intel and AMD machines running Red Hat Linux, Fedora Core and FreeBSD (and probably other unix-like systems).
- PowerPC and X86-based Mac OS X systems, running 10.2 and above in 32-bit and 64-bit modes.
- Intel and AMD machines running on Win32 using MinGW libraries (native)
- Intel and AMD machines running on Win32 with the Cygwin libraries (limited support is available for native builds with Visual C++).
- Sun UltraSPARC workstations running Solaris 8.
- Alpha-based machines running Debian GNU/Linux.
- Itanium-based machines running Linux and HP-UX.

The core LLVM infrastructure uses GNU autoconf to adapt itself to the machine and operating system on which it is built. However, minor porting may be required to get LLVM to work on new platforms. We welcome your portability patches and reports of successful builds or error messages.

Known Problems

This section contains all known problems with the LLVM system, listed by component. As new problems are discovered, they will be added to these sections. If you run into a problem, please check the LLVM bug database and submit a bug if there isn't already one.

Experimental features included with this release

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 `-cee` pass is known to be buggy, and may be removed in in a future release.
- C++ EH support is disabled for this release.
- The MSIL backend is experimental.
- The IA64 code generator is experimental.
- The Alpha JIT is experimental.
- "`-filetype=asm`" (the default) is the only supported value for the `-filetype` llc option.

Known problems with the X86 back-end

- The X86 backend does not yet support inline assembly that uses the X86 floating point stack.

Known problems with the PowerPC back-end

- PowerPC backend does not correctly implement ordered FP comparisons.
- The Linux PPC32/ABI support needs testing for the interpreter and static compilation, and lacks support for debug information.

Known problems with the ARM back-end

- Thumb mode works only on ARMv6 or higher processors. On sub-ARMv6 processors, thumb program can crash or produces wrong results (PR1388).
- Compilation for ARM Linux OABI (old ABI) is supported, but not fully tested.
- There is a bug in QEMU-ARM (<= 0.9.0) which causes it to incorrectly execute programs compiled with LLVM. Please use more recent versions of QEMU.

Known problems with the SPARC back-end

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

Known problems with the Alpha back-end

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

Known problems with the IA64 back-end

- C++ programs are likely to fail on IA64, as calls to `setjmp` are made where the argument is not 16-byte aligned, as required on IA64. (Strictly speaking this is not a bug in the IA64 back-end; it will also be encountered when building C++ programs using the C back-end.)
- The C++ front-end does not use IA64 ABI compliant layout of v-tables. In particular, it just stores function pointers instead of function descriptors in the vtable. This bug prevents mixing C++ code compiled with LLVM with C++ objects compiled by other C++ compilers.
- There are a few ABI violations which will lead to problems when mixing LLVM output with code built with other compilers, particularly for floating-point programs.
- Defining vararg functions is not supported (but calling them is ok).
- The Itanium backend has bitrotted somewhat.

Known problems with the C back-end

- The C backend does not support inline assembly code.

Known problems with the C front-end
Bugs

llvm-gcc4 does not currently support Link-Time Optimization on most platforms "out-of-the-box". Please inquire on the llvmdev mailing list if you are interested.

Notes

- "long double" is silently transformed by the front-end into "double". There is no support for floating point data types of any size other than 32 and 64 bits.

- llvm-gcc does not support `__builtin_apply` yet. See Constructing Calls: Dispatching a call to another function.

- llvm-gcc partially supports these GCC extensions:

  1. Nested Functions: As in Algol and Pascal, lexical scoping of functions.
     Nested functions are supported, but llvm-gcc does not support non-local gotos or taking the address of a nested function.
  2. Function Attributes: Declaring that functions have no side effects or that they can never return.
     Supported: `alias`, `always_inline`, `cdecl`, `constructor`, `destructor`, `deprecated`, `fastcall`, `format`, `format_arg`, `non_null`, `noreturn`, `regparm` `section`, `stdcall`, `unused`, `used`, `visibility`, `warn_unused_result`, `weak`
     Ignored: `noinline`, `pure`, `const`, `nothrow`, `malloc`, `no_instrument_function`

- llvm-gcc supports the vast majority of GCC extensions, including:

  1. Pragmas: Pragmas accepted by GCC.
  2. Local Labels: Labels local to a block.
  3. Other Builtins: Other built-in functions.
  4. Variable Attributes: Specifying attributes of variables.
  5. Type Attributes: Specifying attributes of types.
  6. Thread-Local: Per-thread variables.
  7. Variable Length: Arrays whose length is computed at run time.
  8. Labels as Values: Getting pointers to labels and computed gotos.
  9. Statement Exprs: Putting statements and declarations inside expressions.
  10. Typeof: `typeof`: referring to the type of an expression.
  11. Lvalues: Using `?:`, "`,`" and casts in lvalues.
  12. Conditionals: Omitting the middle operand of a `?:` expression.
  13. Long Long: Double-word integers.
  14. Complex: Data types for complex numbers.
  15. Hex Floats:Hexadecimal floating-point constants.
  16. Zero Length: Zero-length arrays.
  17. Empty Structures: Structures with no members.
  18. Variadic Macros: Macros with a variable number of arguments.
  19. Escaped Newlines: Slightly looser rules for escaped newlines.
  20. Extended Asm: Assembler instructions with C expressions as operands.
  21. Constraints: Constraints for asm operands.
  22. Asm Labels: Specifying the assembler name to use for a C symbol.
  23. Explicit Reg Vars: Defining variables residing in specified registers.
  24. Vector Extensions: Using vector instructions through built-in functions.
  25. Target Builtins: Built-in functions specific to particular targets.
  26. Subscripting: Any array can be subscripted, even if not an lvalue.
  27. Pointer Arith: Arithmetic on `void`-pointers and function pointers.
  28. Initializers: Non-constant initializers.
  29. Compound Literals: Compound literals give structures, unions, or arrays as values.
  30. Designated Inits: Labeling elements of initializers.
  31. Cast to Union: Casting to union type from any member of the union.
  32. Case Ranges: `case 1 ... 9' and such.
  33. Mixed Declarations: Mixing declarations and code.
  34. Function Prototypes: Prototype declarations and old-style definitions.
  35. C++ Comments: C++ comments are recognized.
  36. Dollar Signs: Dollar sign is allowed in identifiers.
  37. Character Escapes: `\e` stands for the character <ESC>.
  38. Alignment: Inquiring about the alignment of a type or variable.
  39. Inline: Defining inline functions (as fast as macros).
  40. Alternate Keywords:`__const__`, `__asm__`, etc., for header files.
  41. Incomplete Enums: `enum foo;`, with details to follow.
  42. Function Names: Printable strings which are the name of the current function.
  43. Return Address: Getting the return or frame address of a function.
  44. Unnamed Fields: Unnamed struct/union fields within structs/unions.
  45. Attribute Syntax: Formal syntax for attributes.

If you run into GCC extensions which have not been included in any of these lists, please let us know (also including whether or not they work).

Known problems with the C++ front-end

The C++ front-end is considered to be fully tested and works for a number of non-trivial programs, including LLVM itself, Qt, Mozilla, etc.

- llvm-gcc4 only has partial support for C++ Exception Handling, and it is not enabled by default.

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 CVS 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.