LLVM 1.3 Release Notes

1. Introduction
2. What's New?
3. Installation Instructions
4. Portability and Supported Platforms
5. Known Problems
   - Experimental features included in this release
   - 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 1.3. 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 1.3 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 fourth public release of the LLVM compiler infrastructure. This release primarily improves the performance of the code produced by all aspects of the LLVM compiler and adds some new features, though it does fix a few bugs as well.

At this time, LLVM is known to correctly compile and run all 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 less testing than the C front-end.

This release implements the following new features:

1. The LLVM `select` instruction is now fully implemented and supported by all transformations, native code generators, and the interpreter.
2. Bugpoint can now narrow down code-generation bugs to a loop nest, where before it could only narrow them down to a function being miscompiled.
3. Bugpoint can now debug arbitrary modes of llc and lli, by passing them command line flags (e.g., `-regalloc=linearscan`, `-enable-correct-eh-support`, etc.)
4. The Control Flow Graph in the native code generators is no longer constrained to be the same as the CFG for the LLVM input code.
5. The LLVM induction variable analysis routines have been rewritten.
6. LLVM now has new loop unrolling and loop unswitching passes.
7. The induction variable substitution pass performs linear function test replacement and exit value replacement optimizations.

In this release, the following missing features were implemented:

1. LLVM cannot handle structures with more than 256 elements
2. [bugpoint] External functions used in non-instruction entities, such as global constant initializer
3. Stacker does not handle targets with 64-bit pointers.
4. Bugpoint doesn't support uses of external fns by immediate constant exprs

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

1. LLVM tools will happily spew bytecode onto your terminal
2. [llvmgcc] type names are not emitted for structure typedefs

In this release, the following build problems were fixed:

1. Minor configure bugs with -disable/enable-povray and -disable-spec
2. shell scripts output by gccld don't work if you change PATH

This release includes the following Code Quality improvements:

1. Fixed: [vmcore] Code quality problem due to long operand of getelementptr
2. The X86 backend now generates substantially better code for 64-bit integer and floating point operations.
3. The -inline pass no longer inlines mutually recursive functions until it hits the inlining threshold.
4. The -inline pass no longer misses obvious inlining opportunities just because the callee eventually calls into an external function.
5. The -simplifycfg pass can now "if convert" simple statements into the new `select` instruction.
6. The -loopsimplify pass can now break natural loops with multiple backedges into multiple nested loops. This enables a variety of subsequent optimizations.
7. The -adce pass can now eliminate calls to functions that do not not write to memory.
8. The link-time optimizer now runs the -prune-eh pass (to remove unused exception handlers).
9. The -simplifycfg pass can now eliminate simple correlated branches (such as "`if (A < B && A < B)`", and can turn short-circuiting operators into the strict versions when useful (such as "`if (A < B || A > C)`" into "`if (A < B | A > C)`"
10. LLVM now has infrastructure for (simple and sparse conditional) constant propagation of function calls. It currently supports a few math library functions like sqrt/sin/cos/etc.

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

Bugs in the LLVM Core:

1. [vmcore] Linker causes erroneous asssertion
2. [loopsimplify] Loop simplify incorrectly updates dominator information
3. [tailduplicate] DemoteRegToStack breaks SSA form
4. [X86] JIT miscompiles unsigned short to floating point cast
5. [adce] Crash handling unreachable code that unwinds
6. [sparc] LLC can't emit 2 functions of the same name, both having constant pools
7. [livevar] Live variables missed physical register use of aliased definition

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

1. [llvmgcc] Crash on use of undeclared enum type
2. [llvmgcc] Variable length array indexing miscompiled

Portability and Supported Platforms

LLVM has been extensively tested on Intel and AMD machines running Red Hat Linux. It has also been tested under FreeBSD, and on Sun UltraSPARC workstations running Solaris 8. Additionally, LLVM works on Mac OS X 10.3 and above, but only with the C backend and interpreter; no native machine-code generator for the PowerPC is available yet.

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.

In this release, the following portability problems were fixed:

1. warnings compiling Stacker compiler on Mac OS X
2. Archive file reader doesn't understand abbreviated names in headers

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 following passes are incomplete or buggy: `-pgmdep, -memdep, -ipmodref, -sortstructs, -swapstructs, -cee`
- The `-pre` pass is incomplete (there are cases it doesn't handle that it should) and not thoroughly tested.
- The `llvm-ar` tool is incomplete and probably buggy.
- The `llvm-db` tool is in a very early stage of development.

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).
- The gccld program does not link objects/archives in the order specified on the command line.
- The lower-invoke pass does not mark values live across a setjmp as volatile. This missing feature only affects targets whose setjmp/longjmp libraries do not save and restore the entire register file.

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.
- [llvm-gcc] Error when an implicitly external function is re-declared as static

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. 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`, `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. 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 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.

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

- None so far.

Known problems with the Sparc back-end

- There are several programs in the LLVM testsuite that the Sparc code generator is known to miscompile.

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.