This document contains the release notes for the LLVM compiler infrastructure, release 1.6. Here we describe the status of LLVM, including any known problems and major improvements from the previous release. The most up-to-date version of this document can be found on the LLVM 1.6 web site. If you are not reading this on the LLVM web pages, you should probably go there because this document may be updated after the release.

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.

This is the seventh public release of the LLVM Compiler Infrastructure. This release incorporates a large number of enhancements and additions (primarily in the code generator), which combine to improve the quality of the code generated by LLVM by up to 30% in some cases. This release is also the first release to have first-class support for Mac OS X: all of the major bugs have been shaken out and it is now as well supported as Linux on X86.

LLVM now includes support for auto-generating large portions of the instruction selectors from target descriptions. This allows us to write patterns in the target .td file, instead of writing lots of nasty C++ code. Most of the PowerPC instruction selector is now generated from the PowerPC target description files and other targets are adding support that will be live for LLVM 1.7.

For example, here are some patterns used by the PowerPC backend. A floating-point multiply then subtract instruction (FMSUBS):

Exclusive-or by 16-bit immediate (XORI):

Exclusive-or by 16-bit immediate shifted right 16-bits (XORIS):

With these definitions, we teach the code generator how to combine these two instructions to xor an abitrary 32-bit immediate with the following definition. The first line specifies what to match (a xor with an arbitrary immediate) the second line specifies what to produce:

def : Pat<(xor GPRC:$in, imm:$imm),
          (XORIS (XORI GPRC:$in, (LO16 imm:$imm)), (HI16 imm:$imm))>;

Instruction selectors using the refined instruction selection framework can now use a simple pre-pass scheduler included with LLVM 1.6. This scheduler is currently simple (cannot be configured much by the targets), but will be extended in the future.

It is now straight-forward to parameterize a target implementation, and provide a mapping from CPU names to sets of target parameters. LLC now supports a -mcpu=cpu option that lets you choose a subtarget by CPU name: use "llvm-as < /dev/null | llc -march=XXX -mcpu=help" to get a list of supported CPUs for target "XXX". It also provides a -mattr=+attr1,-attr2 option that can be used to control individual features of a target (the previous command will list available features as well).

This functionality is nice when you want tell LLC something like "compile to code that is specialized for the PowerPC G5, but doesn't use altivec code. In this case, using "llc -march=ppc32 -mcpu=g5 -mattr=-altivec".

1. The JIT now uses mutexes to protect its internal data structures. This allows multi-threaded programs to be run from the JIT or interpreter without corruption of the internal data structures. See PR418 and PR540 for the details.
2. LLVM on Win32 no longer requires sed, flex, or bison when compiling with Visual C++.
3. The llvm-test suite can now use the NAG Fortran to C compiler to compile SPEC FP programs if available (allowing us to test all of SPEC'95 & 2000).
4. When bugpoint is grinding away and the user hits ctrl-C, it now gracefully stops and gives what it has reduced so far, instead of giving up completely. In addition, the JIT debugging mode of bugpoint is much faster.
5. LLVM now includes Xcode project files in the llvm/Xcode directory.
6. LLVM now supports Mac OS X on Intel.
7. LLVM now builds cleanly with GCC 4.1.

1. The -globalopt pass can now statically evaluate C++ static constructors when they are simple enough. For example, it can now statically initialize "struct X { int a; X() : a(4) {} } g;".
2. The Loop Strength Reduction pass has been completely rewritten, is far more aggressive, and is turned on by default in the RISC targets. On PPC, we find that it often speeds up programs from 10-40% depending on the program.
3. The code produced when exception handling is enabled is far more efficient in some cases, particularly on Mac OS X.

1. The Alpha backend is substantially more stable and robust than in LLVM 1.5. For example, it now fully supports varargs functions. The Alpha backend also now features beta JIT support.
2. The code generator contains a new component, the DAG Combiner. This allows us to optimize lowered code (e.g. after 64-bit operations have been lowered to use 32-bit registers on 32-bit targets) and do fine-grained bit-twiddling optimizations for the backend.
3. The SelectionDAG infrastructure is far more capable and mature, able to handle many new target peculiarities in a target-independent way.
4. The default register allocator is now far faster on some testcases, particularly on targets with a large number of registers (e.g. IA64 and PPC).

1. A vast number of bugs have been fixed in the PowerPC backend and in llvm-gcc when configured for Mac OS X (particularly relating to ABI issues). For example: PR449, PR594, PR603, PR609, PR630, PR643, and several others without bugzilla bugs.
2. Several bugs in tail call support have been fixed.
3. configure does not correctly detect gcc version on cygwin.
4. Many many other random bugs have been fixed. Query our bugzilla with a target of 1.6 for more information.

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).
• Sun UltraSPARC workstations running Solaris 8.
• Intel and AMD machines running on Win32 with the Cygwin libraries (limited support is available for native builds with Visual C++).
• PowerPC and X86-based Mac OS X systems, running 10.2 and above.
• 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 following passes are incomplete or buggy, and may be removed in future releases: -cee, -pre
• The llvm-db tool is in a very early stage of development, but can be used to step through programs and inspect the stack.
• The SparcV8 and IA64 code generators are experimental.
• The Alpha JIT is experimental.

• In the JIT, dlsym() on a symbol compiled by the JIT will not work.

• C99 Variable sized arrays do not release stack memory when they go out of scope. Thus, the following program may run out of stack space:

      for (i = 0; i != 1000000; ++i) {
        int X[n];
        foo(X);
      }
  

• Initialization of global union variables can only be done with the largest union member.

• Inline assembly is not yet supported.
• "long double" is 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.
• The following Unix system functionality has not been tested and may not work:
  1. sigsetjmp, siglongjmp - These are not turned into the appropriate invoke/unwind instructions. Note that setjmp and longjmp are compiled correctly.
  2. getcontext, setcontext, makecontext - These functions have not been tested.
• Although many GCC extensions are supported, some are not. In particular, the following extensions are known to not be supported:
  1. Local Labels: Labels local to a block.
  2. Nested Functions: As in Algol and Pascal, lexical scoping of functions.
  3. Constructing Calls: Dispatching a call to another function.
  4. Extended Asm: Assembler instructions with C expressions as operands.
  5. Constraints: Constraints for asm operands.
  6. Asm Labels: Specifying the assembler name to use for a C symbol.
  7. Explicit Reg Vars: Defining variables residing in specified registers.
  8. Vector Extensions: Using vector instructions through built-in functions.
  9. Target Builtins: Built-in functions specific to particular targets.
  10. Thread-Local: Per-thread variables.
  11. Pragmas: Pragmas accepted by GCC.

  The following GCC extensions are partially supported. An ignored attribute means that the LLVM compiler ignores the presence of the attribute, but the code should still work. An unsupported attribute is one which is ignored by the LLVM compiler and will cause a different interpretation of the program.

  1. Variable Length: Arrays whose length is computed at run time.
     Supported, but allocated stack space is not freed until the function returns (noted above).
  2. Function Attributes: Declaring that functions have no side effects or that they can never return.
     Supported: format, format_arg, non_null, noreturn, constructor, destructor, unused, deprecated, warn_unused_result, weak
     Ignored: noinline, always_inline, pure, const, nothrow, malloc, no_instrument_function, cdecl
     Unsupported: used, section, alias, visibility, regparm, stdcall, fastcall, all other target specific attributes
  3. Variable Attributes: Specifying attributes of variables.
     Supported: cleanup, common, nocommon, deprecated, transparent_union, unused, weak
     Unsupported: aligned, mode, packed, section, shared, tls_model, vector_size, dllimport, dllexport, all target specific attributes.
  4. Type Attributes: Specifying attributes of types.
     Supported: transparent_union, unused, deprecated, may_alias
     Unsupported: aligned, packed, all target specific attributes.
  5. Other Builtins: Other built-in functions.
     We support all builtins which have a C language equivalent (e.g., __builtin_cos), __builtin_alloca, __builtin_types_compatible_p, __builtin_choose_expr, __builtin_constant_p, and __builtin_expect (currently ignored). We also support builtins for ISO C99 floating point comparison macros (e.g., __builtin_islessequal), __builtin_prefetch, __builtin_popcount[ll], __builtin_clz[ll], and __builtin_ctz[ll].

  The following extensions are known to be supported:

  1. Labels as Values: Getting pointers to labels and computed gotos.
  2. Statement Exprs: Putting statements and declarations inside expressions.
  3. Typeof: typeof: referring to the type of an expression.
  4. Lvalues: Using ?:, "," and casts in lvalues.
  5. Conditionals: Omitting the middle operand of a ?: expression.
  6. Long Long: Double-word integers.
  7. Complex: Data types for complex numbers.
  8. Hex Floats:Hexadecimal floating-point constants.
  9. Zero Length: Zero-length arrays.
  10. Empty Structures: Structures with no members.
  11. Variadic Macros: Macros with a variable number of arguments.
  12. Escaped Newlines: Slightly looser rules for escaped newlines.
  13. Subscripting: Any array can be subscripted, even if not an lvalue.
  14. Pointer Arith: Arithmetic on void-pointers and function pointers.
  15. Initializers: Non-constant initializers.
  16. Compound Literals: Compound literals give structures, unions, or arrays as values.
  17. Designated Inits: Labeling elements of initializers.
  18. Cast to Union: Casting to union type from any member of the union.
  19. Case Ranges: `case 1 ... 9' and such.
  20. Mixed Declarations: Mixing declarations and code.
  21. Function Prototypes: Prototype declarations and old-style definitions.
  22. C++ Comments: C++ comments are recognized.
  23. Dollar Signs: Dollar sign is allowed in identifiers.
  24. Character Escapes: \e stands for the character <ESC>.
  25. Alignment: Inquiring about the alignment of a type or variable.
  26. Inline: Defining inline functions (as fast as macros).
  27. Alternate Keywords:__const__, __asm__, etc., for header files.
  28. Incomplete Enums: enum foo;, with details to follow.
  29. Function Names: Printable strings which are the name of the current function.
  30. Return Address: Getting the return or frame address of a function.
  31. Unnamed Fields: Unnamed struct/union fields within structs/unions.
  32. 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).

For this release, the C++ front-end is considered to be fully tested and works for a number of non-trivial programs, including LLVM itself.

• The C++ front-end inherits all problems afflicting the C front-end.

• The C++ front-end is based on a pre-release of the GCC 3.4 C++ parser. This parser is significantly more standards compliant (and picky) than prior GCC versions. For more information, see the C++ section of the GCC 3.4 release notes.
• Destructors for local objects are not always run when a longjmp is performed. In particular, destructors for objects in the longjmping function and in the setjmp receiver function may not be run. Objects in intervening stack frames will be destroyed, however (which is better than most compilers).
• The LLVM C++ front-end follows the Itanium C++ ABI. This document, which is not Itanium specific, specifies a standard for name mangling, class layout, v-table layout, RTTI formats, and other C++ representation issues. Because we use this API, code generated by the LLVM compilers should be binary compatible with machine code generated by other Itanium ABI C++ compilers (such as G++, the Intel and HP compilers, etc). However, the exception handling mechanism used by LLVM is very different from the model used in the Itanium ABI, so exceptions will not interact correctly.

• The C back-end produces code that violates the ANSI C Type-Based Alias Analysis rules. As such, special options may be necessary to compile the code (for example, GCC requires the -fno-strict-aliasing option). This problem probably cannot be fixed.
• Zero arg vararg functions are not supported. This should not affect LLVM produced by the C or C++ frontends.

• Memory Mapped I/O Intrinsics do not fence memory

• None yet

• [sparcv9] SparcV9 backend miscompiles several programs in the LLVM test suite

• 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).

• Many features are still missing (e.g. support for 64-bit integer arithmetic). This back-end is in pre-beta state.

A wide variety of additional information is available on the LLVM web page, including documentation and publications describing algorithms and components implemented in LLVM. 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.