LLVM 1.8 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 1.8. 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 ninth public release of the LLVM Compiler Infrastructure. This release incorporates a large number of enhancements and new features, including DWARF debugging support (C and C++ on Darwin/PPC), improved inline assembly support, a new nightly tester, llvm-config enhancements, many bugs fixed, and performance and compile time improvements.

New Features in LLVM 1.8
DWARF debugging support

The llvm-gcc4 C front-end now generates debugging info for C and C++. This information is propagated through the compiler and the code generator can currently produce DWARF debugging information from it. DWARF is a standard debugging format used on many platforms, but currently LLVM only includes target support for Mac OS X targets for the 1.8 release.

Inline Assembly Support

Inline assembly support is substantially improved in LLVM 1.8 over LLVM 1.7. Many unsupported features are now supported, and inline asm support in the X86 backend is far better. llvm-gcc4 now supports global register variables as well.

Loop Optimizer Improvements

The loop optimizer passes now uses "Loop-Closed SSA Form", which makes it easier to update SSA form as loop transformations change the code. An immediate benefit of this is that the loop unswitching pass can now unswitch loops in more cases.

Jump Table Support for Switches

The code generator now lowers switch statements to jump tables, providing significant performance boosts for applications (e.g. interpreters) whose performance is highly correlated to switch statement performance.

Deallocation of JIT'd Machine Code

The LLVM JIT now allows clients to deallocate machine code JIT'd to its code buffer. This is important for long living applications that depend on the JIT.

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:

- LLVM 1.8 includes an initial ARM backend. This backend is in early development stages.
- LLVM 1.8 now includes significantly better support for mingw and cygwin.
- The llvm-config tool is now built by default and has several new features.
- The X86 and PPC backends now use the correct platform ABI for passing vectors as arguments to functions.
- The X86 backend now includes support for the Microsoft ML assembler ("MASM").
- The PowerPC backend now pattern matches the 'rlwimi' instruction more aggressively.
- Most of LLVM is now built with "-pedantic", ensuring better portability to more C++ Compilers.
- The PowerPC backend now includes initial 64-bit support. The JIT is not complete, and the static compiler has a couple of known bugs, but support is mostly in place. LLVM 1.9 will include completed PPC-64 support.

Significant Changes in LLVM 1.8

- The LLVM "SparcV9" backend (deprecated in LLVM 1.7) has been removed in LLVM 1.8. The LLVM "Sparc" backend replaces it.
- The --version option now prints more useful information, including the build configuration for the 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.
- 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 Alpha JIT is experimental.
- "`-filetype=asm`" (the default) is the only supported value for the `-filetype` llc option.

Known problems with the Build System

- none yet

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-gcc3 has many significant problems that are fixed by llvm-gcc4. Two major ones include:

- With llvm-gcc3, 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);
          }

- With llvm-gcc3, Initialization of global union variables can only be done with the largest union member.

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.

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.
- 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`, `used`, `deprecated`, `warn_unused_result`, `weak`
     Ignored: `noinline`, `always_inline`, `pure`, `const`, `nothrow`, `malloc`, `no_instrument_function`, `cdecl`
     Unsupported: `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`, `used`, `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).

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.

Bugs

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

Notes

- 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 is very different from the model used in the Itanium ABI, so exceptions will not interact correctly.

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).

Known problems with the X86 back-end

- none yet.

Known problems with the PowerPC back-end

- PowerPC backend does not correctly implement ordered FP comparisons.

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 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 ARM back-end

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

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.