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.
This is the thirteenth public release of the LLVM Compiler Infrastructure. It includes many features and refinements from LLVM 2.1.
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 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.
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 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.
- 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.
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.
In addition to a huge array of bug fixes and minor performance tweaks, LLVM 2.2 supports a few major enhancements:
- Daniel Berlin and Curtis Dunham rewrote Andersen's alias analysis to be several orders of magnitude faster, implemented Offline Variable Substitution and Lazy Cycle Detection. Note that Andersen's is not enabled in llvm-gcc by default.
- 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.
New features include:
- Evan X86 now models EFLAGS in instructions.
- Evan: If conversion on by default for ARM.
- Bruno: MIPS PIC support.
- Arnold Schwaighofer: X86 tail call support.
- Dale darwin/x86-64 and darwin/ppc eh
- Evan: darwin/x86 debug info, improvements at -O0?
New features include:
- Devang added LLVMFoldingBuilder.
- Dan added support for vector sin, cos, and pow intrinsics.
- Ted added a framework for generic object serialization to bitcode files, only used by clang right now for ASTs but could be used for other stuff.
- Duncan fixed 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 function type and onto functions and calls, which makes it much easier to add attributes to a function in a transformation.
- Christopher Lamb: Multiple address spaces.
- Gordon: C and Ocaml Bindings
- Anton added readnone/readonly attributes for modeling function side effects and Duncan hooked up GCC's pure/const attributes to use them and enhanced alias analysis to use them.
New features include:
- Gordon Henriksen updated docs/Passes.html
- New lexer and parser for tblgen, new lexer for asmparser
- Dale GCC testsuite
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.
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.
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.
- 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 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.
- 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.
- The SPARC backend only supports the 32-bit SPARC ABI (-m32), it does not support the 64-bit SPARC ABI (-m64).
- On 21164s, some rare FP arithmetic sequences which may trap do not have the appropriate nops inserted to ensure restartability.
- 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.
- 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.
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.
llvm-gcc does not support __builtin_apply yet. See Constructing Calls: Dispatching a call to another function.
llvm-gcc partially supports these GCC extensions:
- 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.
- 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, pure, regparm section, stdcall, unused, used, visibility, warn_unused_result, weak
Ignored: nothrow, malloc, no_instrument_function
llvm-gcc supports the vast majority of GCC extensions, including:
- Pragmas: Pragmas accepted by GCC.
- Local Labels: Labels local to a block.
- Other Builtins: Other built-in functions.
- Variable Attributes: Specifying attributes of variables.
- Type Attributes: Specifying attributes of types.
- Thread-Local: Per-thread variables.
- Variable Length: Arrays whose length is computed at run time.
- Labels as Values: Getting pointers to labels and computed gotos.
- Statement Exprs: Putting statements and declarations inside expressions.
- Typeof:
typeof: referring to the type of an expression. - Lvalues: Using
?:, "," and casts in lvalues. - Conditionals: Omitting the middle operand of a
?:expression. - Long Long: Double-word integers.
- Complex: Data types for complex numbers.
- Hex Floats:Hexadecimal floating-point constants.
- Zero Length: Zero-length arrays.
- Empty Structures: Structures with no members.
- Variadic Macros: Macros with a variable number of arguments.
- Escaped Newlines: Slightly looser rules for escaped newlines.
- Extended Asm: Assembler instructions with C expressions as operands.
- Constraints: Constraints for asm operands.
- Asm Labels: Specifying the assembler name to use for a C symbol.
- Explicit Reg Vars: Defining variables residing in specified registers.
- Vector Extensions: Using vector instructions through built-in functions.
- Target Builtins: Built-in functions specific to particular targets.
- Subscripting: Any array can be subscripted, even if not an lvalue.
- Pointer Arith: Arithmetic on
void-pointers and function pointers. - Initializers: Non-constant initializers.
- Compound Literals: Compound literals give structures, unions, or arrays as values.
- Designated Inits: Labeling elements of initializers.
- Cast to Union: Casting to union type from any member of the union.
- Case Ranges: `case 1 ... 9' and such.
- Mixed Declarations: Mixing declarations and code.
- Function Prototypes: Prototype declarations and old-style definitions.
- C++ Comments: C++ comments are recognized.
- Dollar Signs: Dollar sign is allowed in identifiers.
- Character Escapes:
\estands for the character <ESC>. - Alignment: Inquiring about the alignment of a type or variable.
- Inline: Defining inline functions (as fast as macros).
- Alternate Keywords:
__const__,__asm__, etc., for header files. - Incomplete Enums:
enum foo;, with details to follow. - Function Names: Printable strings which are the name of the current function.
- Return Address: Getting the return or frame address of a function.
- Unnamed Fields: Unnamed struct/union fields within structs/unions.
- 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).
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.
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.
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