LLVM 2.2 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 2.2. 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 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 releases, please see the releases page.

What's New?

This is the thirteenth public release of the LLVM Compiler Infrastructure. It includes many features and refinements from LLVM 2.1.

Deprecated features in LLVM 2.2

This is the last LLVM release to support llvm-gcc 4.0, llvm-upgrade, and llvmc in its current form. llvm-gcc 4.0 has been replaced with llvm-gcc 4.2. llvm-upgrade is useful for upgrading llvm 1.9 files to llvm 2.x syntax, but you can always use an old release to do this. llvmc is currently mostly useless in llvm 2.2, and will be redesigned or removed in llvm 2.3.

llvm-gcc 4.0, llvm-gcc 4.2, and clang

LLVM 2.2 fully supports both the llvm-gcc 4.0 and llvm-gcc 4.2 front-ends (in LLVM 2.1, llvm-gcc 4.2 was beta). Since LLVM 2.1, the llvm-gcc 4.2 front-end has made leaps and bounds and is now at least as good as 4.0 in virtually every area, and is better in several areas (for example, exception handling correctness, support for Ada and FORTRAN). We strongly recommend that you migrate from llvm-gcc 4.0 to llvm-gcc 4.2 in this release cycle because LLVM 2.2 is the last release that will support llvm-gcc 4.0: LLVM 2.3 will only support the llvm-gcc 4.2 front-end.

The clang project is an effort to build a set of new 'llvm native' front-end technologies for the LLVM optimizer and code generator. Currently, its C and Objective-C support is maturing nicely, and it has advanced source-to-source analysis and transformation capabilities. If you are interested in building source-level tools for C and Objective-C (and eventually C++), you should take a look. However, note that clang is not an official part of the LLVM 2.2 release. If you are interested in this project, please see its web site.

Major New Features

LLVM 2.2 includes several major new capabilities:

- Scott Michel contributed an SPU backend, which generates code for the vector coprocessors on the Cell processor. (Status?)
- llvm-gcc 4.2 has significantly improved support for the GCC Ada (GNAT) and FORTRAN (gfortran) frontends. Duncan has the llvm-gcc 4.2 GNAT front-end supporting almost all of the ACATS testsuite (except 2 tests?). The llvm-gcc 4.2 gfortran front-end supports a broad range of FORTRAN code, but does not support EQUIVALENCE yet.
- Dale contributed full support for long double on x86/x86-64 (where it is 80 bits) and on Darwin PPC/PPC64 (where it is 128 bits). In previous LLVM releases, llvm-gcc silently mapped long double to double.
- Gordon Henriksen rewrote most of the Accurate Garbage Collection code in the code generator, making the generated code more efficient and adding support for the Ocaml garbage collector metadata format.
- Christopher Lamb contributed support for multiple address spaces in LLVM IR. This is useful for supporting targets that have 'near' vs 'far' pointers, 'RAM' vs 'ROM' pointers, or that have non-local memory that can be accessed with special instructions.
- LLVM now includes a new set of detailed tutorials, which explain how to implement a language with LLVM and shows how to use several important APIs.

LLVM Core Improvements

New features include:

- Gordon contributed support for C and Ocaml Bindings for the basic LLVM IR construction routines as well as several other auxiliary APIs.
- Anton added readnone/readonly attributes for modeling function side effects. Duncan hooked up GCC's pure/const attributes to use them and enhanced mod/ref analysis to use them.
- Devang added LLVMFoldingBuilder, a version of LLVMBuilder that implicitly simplifies the code as it is constructed.
- Ted Kremenek added a framework for generic object serialization to bitcode files. This support is only used by clang right now for ASTs but is extensible and could be used for serializing arbitrary other data into bitcode files.
- Duncan improved TargetData to distinguish between the size/alignment of a type in a register, in memory according to the platform ABI, and in memory when we have a choice.
- Duncan moved parameter attributes off of FunctionType and onto functions and calls. This makes it much easier to add attributes to a function in a transformation pass.
- Dan Gohman added support for vector sin, cos, and pow intrinsics.

Code Generator Improvements

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

- Owen refactored the existing LLVM dominator and loop information code to allow it work on the machine code representation. He contributed support for dominator and loop information on machine code and merged the code for forward and backward dominator computation.
- Dan added support for emitting debug information with .file and .loc directives on that support it, instead of emitting large tables in the .s file.
- Evan extended the DAG scheduler to model physical register dependencies explicitly and have the BURR scheduler pick a correct schedule based on the dependencies. This reduces our use of the 'flag' operand hack.
- Evan added initial support for register coalescing of subregister references.
- Rafael Espindola implemented initial support for a new 'byval' attribute, which allows more efficient by-value argument passing in the LLVM IR. Evan finished support for it and enabled it in the X86 (32- and 64-bit) and C backends.
- The LLVM TargetInstrInfo class can now answer queries about the mod/ref and side-effect behavior of MachineInstr's. This information is inferred automatically by TableGen from .td files for all instructions with patterns.
- Evan implemented simple live interval splitting on basic block boundaries. This allows the register allocator to be more successful at keeping values in registers in some parts of a value's live range, even if they need to be spilled in some other block.
- The new MachineRegisterInfo.h class provides support for efficiently iterating over all defs/uses of a register, and this information is automatically kept up-to-date. This support is similar to the use_iterator in the LLVM IR level.
- The MachineInstr, MachineOperand and TargetInstrDesc classes are simpler, more consistent, and better documented.

Optimizer Improvements

In addition to a huge array of bug fixes and minor performance tweaks, the LLVM 2.2 optimizers support a few major enhancements:

- Daniel Berlin and Curtis Dunham rewrote Andersen's alias analysis to be several orders of magnitude faster, and implemented Offline Variable Substitution and Lazy Cycle Detection. Note that Andersen's is not enabled in llvm-gcc by default, but can be accessed through 'opt'.
- Dan Gohman contributed several enhancements to Loop Strength Reduction (LSR) to make it more aggressive with SSE intrinsics.
- Evan added support for simple exit value substitution to LSR.
- Evan enhanced LSR to support induction variable reuse when the induction variables have different widths.

Target Specific Improvements

New target-specific features include:

- Evan contributed support to the X86 backend to model the mod/ref behavior of the EFLAGS register explicitly in all instructions. This gives more freedom to the scheduler, and is a more explicit way to model the instructions.
- Dale contributed support for exception handling on Darwin/x86-64 and Darwin/ppc.
- Evan turned on if-conversion by default for ARM, allowing LLVM to take advantage of its predication features.
- Bruno added PIC support to the MIPS backend, fixed many bugs and improved support for architecture variants.
- Arnold Schwaighofer added support for X86 tail calls (limitations? details?).
- Evan contributed several enhancements to Darwin/x86 debug information, and improvements at -O0 (details?).

Other Improvements

New features include:

- Gordon expanded and updated the LLVM Analysis and Transformation Passes reference to include descriptions for each pass.
- We rewrote the lexer and parser used by TableGen to make them simpler and cleaner. This gives tblgen support for 'caret diagnostics'. The .ll file lexer was also rewritten to support caret diagnostics but doesn't use this support yet.
- Dale has been grinding through the GCC testsuite, and marked many LLVM-incompatible tests as not-to-be-run (for example, if they are grepping through some GCC dump file that LLVM doesn't produce), he also found and fixed many LLVM bugs exposed by the testsuite.

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 10.3 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 will be removed in LLVM 2.3.
- The MSIL, IA64, Alpha, and MIPS backends are experimental.
- The LLC "`-filetype=asm`" (the default) is the only supported value for this option.
- The llvmc tool is not supported.

Known problems with the X86 back-end

- The X86 backend does not yet support inline assembly that uses the X86 floating point stack.
- The X86 backend occasionally has alignment problems on operating systems that don't require 16-byte stack alignment (including most non-darwin OS's like linux).
- The X86 backend generates inefficient floating point code when configured to generate code for systems that don't have SSE2.

Known problems with the PowerPC back-end

- 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 programs can crash or produce 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.
- The C backend does not support vectors yet.
- 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.

Known problems with the C front-end
Bugs

llvm-gcc 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

- 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 taking the address of a nested function (except on the X86-32 target) or non-local gotos.
  2. Function Attributes: Declaring that functions have no side effects or that they can never return.
     Supported: `alias`, `always_inline`, `cdecl`, `const`, `constructor`, `destructor`, `deprecated`, `fastcall`, `format`, `format_arg`, `non_null`, `noinline`, `noreturn`, `nothrow`, `pure`, `regparm` `section`, `stdcall`, `unused`, `used`, `visibility`, `warn_unused_result`, `weak`
     Ignored: `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.

- Exception handling only works well on the X86 and PowerPC targets.

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.