LLVM 1.2 Release Notes

1. Introduction
2. What's New?
3. Portability and Supported Platforms
4. Installation Instructions
5. Known Problems
   - Known problems with the LLVM Core
   - Known problems with the C Front-end
   - Known problems with the C++ Front-end
   - Known problems with the X86 Back-end
   - Known problems with the Sparc Back-end
   - Known problems with the C back-end
6. Additional Information

Written by the LLVM team

Introduction

This document contains the release notes for the LLVM compiler infrastructure, release 1.2. Here we describe the status of LLVM, including any known problems and bug fixes from the previous release. The most up-to-date version of this document can be found on the LLVM 1.2 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 potentially more current releases, please check out the main 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, 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 third public release of the LLVM compiler infrastructure.

At this time, LLVM is known to correctly compile and run all non-unwinding C & C++ SPEC CPU2000 benchmarks, the Olden benchmarks, and the Ptrdist benchmarks. It has also been used to compile many other programs. LLVM now also works with a broad variety of C++ programs, though it has still received much less testing than the C front-end.

The LLVM native code generators are very stable but do not currently support unwinding (exception throwing or `longjmp`ing), which prevent them from working with programs like the `253.perlbmk` in SPEC CPU2000. The C backend and the rest of LLVM supports these programs, so you can still use LLVM with them. Support for unwinding will be added in a future release.

This release implements the following new features:

In this release, the following missing features were implemented:

In this release, the following Quality of Implementation issues were fixed:

1. JIT should lazily initialize global variables
2. [X86] X86 Backend never releases memory for machine code structures
3. [vmcore] OpaqueType objects memory leak
4. [llvmgcc] C front-end does not compile "extern inline" into linkonce

In this release, the following Code Quality issues were fixed:

1. [loopsimplify] Many pointless phi nodes are created

In this release, the following bugs in the previous release were fixed:

Bugs in the LLVM Core:

1. [licm] LICM promotes volatile memory locations to registers
2. [licm] Memory read after free causes infrequent crash
3. [indvars] Induction variable canonicalization always makes 32-bit indvars
4. [constantmerge] Merging globals can cause use of invalid pointers!

Bugs in the C/C++ front-end:

1. Need weak linkage on memory management functions in libc runtime to allow them to be overriden
2. [llvm-gcc] asserts when an extern inline function is redefined

Portability and Supported Platforms

LLVM has been extensively tested on Intel and AMD machines running Red Hat Linux and FreeBSD. It has also been tested on Sun UltraSPARC workstations running Solaris 8. Additionally, LLVM works on Mac OS X 10.3 and above, but only with the C backend or interpreter (no native backend for the PowerPC is available yet). The core LLVM infrastructure uses "autoconf" for portability, so hopefully we work on more platforms than that. However, it is likely that we missed something and that minor porting is required to get LLVM to work on new platforms. We welcome portability patches and 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.

Known problems with the LLVM Core

- In the JIT, `dlsym` on a symbol compiled by the JIT will not work.
- The JIT does not use mutexes to protect its internal data structures. As such, execution of a threaded program could cause these data structures to be corrupted.
- It is not possible to `dlopen` an LLVM bytecode file in the JIT.
- Linking in static archive files (.a files) is very slow (there is no symbol table in the archive).
- LLVM cannot handle structures with more than 256 elements.
- The gccld program does not link objects/archives in the order specified on the command line.
- Tail duplication does not update SSA form correctly.

Known problems with the C front-end
Bugs

- 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.
- Functions marked "extern inline" are not compiled into LLVM with linkonce linkage.
- The memory management functions in the libc runtime need weak linkage so that they can be overridden.

Notes

- 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. Labels as Values: Getting pointers to labels and computed gotos.
  3. Nested Functions: As in Algol and Pascal, lexical scoping of functions.
  4. Constructing Calls: Dispatching a call to another function.
  5. Extended Asm: Assembler instructions with C expressions as operands.
  6. Constraints: Constraints for asm operands.
  7. Asm Labels: Specifying the assembler name to use for a C symbol.
  8. Explicit Reg Vars: Defining variables residing in specified registers.
  9. Return Address: Getting the return or frame address of a function.
  10. Vector Extensions: Using vector instructions through built-in functions.
  11. Target Builtins: Built-in functions specific to particular targets.
  12. Thread-Local: Per-thread variables.
  13. 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`, `constructor`, `destructor`, `unused`, `deprecated`, `warn_unused_result`, `weak`
     Ignored: `noreturn`, `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` (ignored).

  The following extensions are known to be supported:

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

Bugs

- The C++ front-end inherits all problems afflicting the C front-end.
- Code is generated for empty classes.

Notes

- 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 `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 X86 back-end

- The X86 code generator does not currently support the `unwind` instruction, so code that throws a C++ exception or calls the C `longjmp` function will abort.

Known problems with the Sparc back-end

- The Sparc code generator does not currently support the `unwind` instruction, so code that throws a C++ exception or calls the C `longjmp` function will abort.
- The llc program can crash on legal code.

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.
- Initializers for global variables cannot include special floating point numbers like Not-A-Number or Infinity.
- Zero arg vararg functions are not supported. This should not affect LLVM produced by the C or C++ frontends.

Additional Information

A wide variety of additional information is available on the LLVM web page, including mailing lists 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.