LLVM 1.9 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 1.9. 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 (corresponding to LLVM CVS) can be found on the LLVM releases 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.

What's New?

This is the tenth public release of the LLVM Compiler Infrastructure. This release incorporates a large number of enhancements, new features, and bug fixes. We recommend that all users of previous LLVM versions upgrade.

New Features in LLVM 1.9
New X86-64 Backend

LLVM 1.9 now fully supports the x86-64 instruction set on Mac OS/X, and supports it on Linux (and other operating systems) when compiling in -static mode. LLVM includes JIT support for X86-64, and supports both Intel EMT-64T and AMD-64 architectures. The X86-64 instruction set permits addressing a 64-bit addressing space and provides the compiler with twice the number of integer registers to use.

Link-Time Optimization integration with native linkers

LLVM now includes liblto which can be used to integrate LLVM Link-Time Optimization support into a native linker. This allows LLVM .bc to transparently participate with linking an application, even when some .o files are in LLVM form and some are not.

DWARF debugging support for Linux, Cygwin and MinGW on X86

llvm-gcc4 now supports generating debugging info for Linux, Cygwin and MinGW. This extends the PPC/Darwin and X86/Darwin debugging support available in the 1.8 release. DWARF is a standard debugging format used on many platforms.

Optimizer Improvements

The mid-level optimizer is now faster and produces better code in many cases. Significant changes include:

- LLVM includes a new 'predicate simplifier' pass, which currently performs dominator tree-based optimizations.
- The complete loop unroll pass now supports unrolling of multiple basic block loops.
- The 'globalopt' pass can now perform the scalar replacement of aggregates transformation on some heap allocations.
- The globalsmodref-aa alias analysis can now track 'indirect pointer globals' more accurately.
- The instruction combiner can now perform element propagation analysis of vector expressions, eliminating computation of vector elements that are not used.

Code Generator Enhancements

The LLVM Target-Independent code generator now supports more target features and optimizes many cases more aggressively. New features include:

- LLVM now includes a late branch folding pass which optimizes code layout, performs several branch optzns, and deletes unreachable code.
- The code generator now support targets that have pre/post-increment addressing modes.
- LLVM now supports dynamically-loadable register allocators and schedulers.
- LLVM 1.9 includes several improvements to inline asm support, including support for new constraints and modifiers.
- The register coalescer is now more aggressive than before, allowing it to eliminate more copies.

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

- tblgen now allows definition of 'multiclasses' which can be used to factor instruction patterns more aggressively in .td files.
- LLVM has a new TargetAsmInfo class which captures a variety of information about the target assembly language format.
- .td files now support "`${:foo}`" syntax for encoding subtarget-specific assembler syntax into instruction descriptions.

Further, several significant target-specific enhancements are included in LLVM 1.9:

- The LLVM ARM backend now supports more instructions and the use of a frame pointer. It is now possible to build libgcc and a simple cross compiler, but it is not considered "complete" yet.
- LLVM supports the Win32 dllimport/dllexport linkage and stdcall/fastcall calling conventions.

Other Improvements

This release includes many other improvements, including improvements to the optimizers and code generators (improving the generated code) changes to speed up the compiler in many ways (improving algorithms and fine tuning code), and changes to reduce the code size of the compiler itself.

More specific changes include:

- The llvm-test framework now supports SPEC2006.
- LLVM now includes a FAQ about the `getelementptr` instruction.
- Bugpoint now supports a new "`-find-bugs`" mode. This mode makes bugpoint permute pass sequences to try to expose bugs due to pass sequencing.
- The JIT now supports lazily streaming code from multiple modules at a time, implicitly linking the code as it goes.

Significant API Changes in LLVM 1.9

Several significant API changes have been made. If you are maintaining out-of-tree code, please be aware that:

- The ConstantSInt and ConstantUInt classes have been merged into the ConstantInt class.

- As a step towards making LLVM's integer types signless, several new instructions have been added to LLVM. The `Div` instruction is now `UDiv`, `SDiv`, and `FDiv`. The `Rem` instruction is now `URem`, `SRem` and `FRem`. See the Language Reference for details on these new instructions.

- `ConstantBool::True` and `ConstantBool::False` have been renamed to `ConstantBool::getTrue()` and `ConstantBool::getFalse()`.

- The 'analyze' tool has been merged into the 'opt' tool.

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).
- Intel and AMD machines running on Win32 using MinGW libraries (native)
- 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 in 32-bit and 64-bit modes.
- 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.
- The IA64 code generator is experimental.
- The ARM 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. See the bug for details on workarounds on Linux.

Known problems with the PowerPC back-end

- PowerPC backend does not correctly implement ordered FP comparisons.
- The 64-bit PowerPC backend is not fully stable. If you desire PPC64 support, please use mainline CVS LLVM, which has several important bug fixes.

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 C back-end

- 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.
- The C backend does not correctly implement the `llvm.stacksave` or `llvm.stackrestore` intrinsics. This means that some code compiled by it can run out of stack space if they depend on these (e.g. C99 varargs).
- The C backend does not support inline assembly code.

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).

Known problems with the ARM back-end

- The ARM backend is currently in early development stages, it is not ready for production use.

Known problems with the LLVM Core

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

Known problems with the C front-end
Bugs

llvm-gcc4 is far more stable and produces better code than llvm-gcc3, but does not currently support Link-Time Optimization or C++ Exception Handling, which llvm-gcc3 does.

llvm-gcc4 does not support the GCC indirect goto extension, but llvm-gcc3 does.

Notes

- "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.
- 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. Thread-Local: Per-thread variables.
  5. 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: `constructor`, `destructor`, `deprecated`, `fastcall`, `format`, `format_arg`, `non_null`, `noreturn`, `section`, `stdcall`, `unused`, `used`, `visibility`, `warn_unused_result`, `weak`
     Ignored: `noinline`, `always_inline`, `pure`, `const`, `nothrow`, `malloc`, `no_instrument_function`, `cdecl`
     Unsupported: `alias`, `regparm`, all other target specific attributes
  3. Variable Attributes: Specifying attributes of variables.
     Supported: `cleanup`, `common`, `nocommon`, `deprecated`, `dllimport`, `dllexport`, `section`, `transparent_union`, `unused`, `used`, `weak`
     Unsupported: `aligned`, `mode`, `packed`, `shared`, `tls_model`, `vector_size`, 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. Extended Asm: Assembler instructions with C expressions as operands.
  14. Constraints: Constraints for asm operands.
  15. Asm Labels: Specifying the assembler name to use for a C symbol.
  16. Explicit Reg Vars: Defining variables residing in specified registers.
  17. Vector Extensions: Using vector instructions through built-in functions.
  18. Target Builtins: Built-in functions specific to particular targets.
  19. Subscripting: Any array can be subscripted, even if not an lvalue.
  20. Pointer Arith: Arithmetic on `void`-pointers and function pointers.
  21. Initializers: Non-constant initializers.
  22. Compound Literals: Compound literals give structures, unions, or arrays as values.
  23. Designated Inits: Labeling elements of initializers.
  24. Cast to Union: Casting to union type from any member of the union.
  25. Case Ranges: `case 1 ... 9' and such.
  26. Mixed Declarations: Mixing declarations and code.
  27. Function Prototypes: Prototype declarations and old-style definitions.
  28. C++ Comments: C++ comments are recognized.
  29. Dollar Signs: Dollar sign is allowed in identifiers.
  30. Character Escapes: `\e` stands for the character <ESC>.
  31. Alignment: Inquiring about the alignment of a type or variable.
  32. Inline: Defining inline functions (as fast as macros).
  33. Alternate Keywords:`__const__`, `__asm__`, etc., for header files.
  34. Incomplete Enums: `enum foo;`, with details to follow.
  35. Function Names: Printable strings which are the name of the current function.
  36. Return Address: Getting the return or frame address of a function.
  37. Unnamed Fields: Unnamed struct/union fields within structs/unions.
  38. 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

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.

Notes

- llvm-gcc4 does not support C++ exception handling at all yet.
- Destructors for local objects are not always run when a `longjmp` is performed. In particular, destructors for objects in the `longjmp`ing 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-gcc3 is very different from the model used in the Itanium ABI, so exceptions will not interact correctly.

Additional Information

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.