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 Chris Lattner

This document contains the release notes for the LLVM compiler infrastructure, release 1.0. Here we describe how to install LLVM, as well as any known problems. The most up-to-date version of this document can be found on the LLVM 1.0 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.

This is the first public release of the LLVM compiler infrastructure. As such, it is all new! In particular, we are providing a stable C compiler, beta C++ compiler, a C back-end, stable X86 and Sparc V9 static and JIT code generators, as well as a large suite of scalar and interprocedural optimizations.

The default optimizer sequence used by the C/C++ front-ends is:

1. Interprocedural dead code elimination (-globaldce)
2. Exception handling pruning (-prune-eh)
3. Function inlining (-inline)
4. Instruction combining (-instcombine)
5. Cast elimination (-raise)
6. Tail duplication (-tailduplicate)
7. CFG simplification (-simplifycfg)
8. Scalar replacement of aggregates (-scalarrepl)
9. Tail call elimination (-tailcallelim)
10. Instruction combining (-instcombine)
11. Reassociation (-reassociate)
12. Instruction combining (-instcombine)
13. CFG simplification (-simplifycfg)
14. Loop canonicalization (-loopsimplify)
15. Loop invariant code motion, with scalar promotion (-licm)
16. Global common subexpression elimination, with load elimination (-gcse)
17. Sparse conditional constant propagation (-sccp)
18. Instruction combining (-instcombine)
19. Induction variable canonicalization (-indvars)
20. Aggressive dead code elimination (-adce)
21. CFG simplication (-simplifycfg)
22. Dead type elimination (-deadtypeelim)
23. Global constant merging (-constmerge)

At link-time, the following optimizations are run:

1. Global constant merging (-constmerge)
2. [optional] Internalization [which marks most functions and global variables static] (-internalize)
3. Interprocedural dead argument elimination (-deadargelim)
4. Instruction combining (-instcombine)
5. CFG simplification (-simplifycfg)
6. Interprocedural dead code elimination (-globaldce)

TODO: Works on: SPEC CPU 2000

TODO: Works on: Olden/Ptrdist benchmarks

The What's New section should give a bulletted list of what is included, perhaps a couple of lists (Stable, Beta).

It would be useful to give a pointer to a list of the available passes (at least for the standard compiler techniques).

LLVM has only been extensively tested on ia32-linux and sparc-solaris machines. The core LLVM infrastructure uses "autoconf" for portability, so hopefully we work on more platforms than that. However, it is extremely likely that we missed something. We welcome portability patches and error messages.

FIXME

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.

---

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.

---

Known problems with the C front-end

• 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.

• 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);
      }
  

• 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.
  2. getcontext, setcontext, makecontext - These functions have not been tested.

• Bugs:
  Oversized integer bitfields cause crash.
  LLVM needs explicit support for weak variables.
  

• 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, which 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).

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Known problems with the C++ front-end

For this release, the C++ front-end is considered to be of beta quality. It works for a large number of simple programs, but has not been extensively tested. We welcome bug reports though!
• 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 .
• Code for executing destructors when unwinding is not shared.

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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.

• Some executables produced by LLC seem to intermittently crash (extremely infrequently). The cause of the problem has not been diagnosed, and does not affect the JIT.

---

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.

---

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 that include floating point numbers may not be initialized with exactly the right floating point number, if the number is not accurately representable in decimal. This prevents the Olden "power" benchmark from producing exactly the right results with the C back-end.

• The code produces by the C back-end has only been tested with the Sun CC and GCC compilers. It is possible that it will have to be adjusted to support other C compilers.

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