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

At this time, LLVM is known to work properly with SPEC CPU 2000, the Olden benchmarks, and the Ptrdist benchmarks among many other programs. Note however that the Sparc and X86 backends do not currently support exception throwing or long jumping (including 253.perlbmk in SPEC). For these programs you must use the C backend.

LLVM has only been extensively tested on Intel and AMD machines running Red Hat Linux, and Sun UltraSPARC workstations running Solaris 8. 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.

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 (there is no symbol table in the archive).

• Interpreter does not support invoke or unwind.

• LLVM cannot handle structures with more than 256 elements.

• The interpreter does not support invoke or unwind
• Interpreter does not support the vaarg instruction
• llvm-nm cannot read archive files

• [inliner] Inlining invoke with PHI in unwind target is broken
• [linker] linkonce globals should link successfully to external globals
• [constmerge] Constant merging pass merges constants with external linkage
• [scalarrepl] Scalar Replacement of aggregates is decimating structures it shouldn't be
• [instcombine] Resolving invoke inserts cast after terminator
• llvm-as crashes when labels are used in phi nodes
• [build problem] Callgraph.cpp not pulled in from libipa.a
• [gccld] The -r (relinking) option does not work correctly
• [bcreader] Cannot read shift constant expressions from bytecode file
• [lowersetjmp] Lowersetjmp pass breaks dominance properties!
• SymbolTable::getUniqueName is very inefficient
• Bad path to the C/C++ frontend causes build problems
• [buildscripts] Building into objdir with .o in it fails
• [setjmp/longjmp] Linking C programs which use setjmp/longjmp sometimes fail with references to the C++ runtime library!
• AsmParser Misses Symbol Redefinition Error
• gccld -Lfoo -lfoo fails to find ./foo/libfoo.a
• [bcreader] Incorrect cast causes misread forward constant references
• LLVM header files should be -Wold-style-cast clean
• [adce] ADCE considers blocks without postdominators to be unreachable
• [vmcore] Symbol table doesn't rename colliding variables during type resolution
• bugpoint must not pass -R to Mach-O linker

---

Known problems with the C front-end

• #ident is not recognized by C frontend
• C front-end miscompiles the builtin_expect intrinsic!
• llvmgcc asserts when compiling functions renamed with asm's
• C frontend crashes on some programs with lots of types
• llvm-gcc crashes compiling global union initializer
• C front-end crash on empty structure
• CFrontend crashes when compiling C99 compound expressions
• llvm-gcc infinite loops on "case MAXINT:"
• llvm-gcc tries to add bools
• Casting a string constant to void crashes llvm-gcc
• C front-end miscompiles unsigned enums whose LLVM types are signed
• [llvm-gcc] ?: operator as lvalue not implemented
• [C/C++] Bogus warning about taking the address of 'register' variable
• crash assigning into an array in a struct which contains a bitfield.
• Oversized integer bitfields cause crash.

• [llvm-gcc] Bitfields & large array don't mix well
.
• [llvm-gcc] Complex division is not supported
• [llvm-gcc] Illegal union field reference
• [llvm-gcc] crash on union initialization
• [llvm-gcc] Pointer & constant results in invalid shift

• 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, 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).

---

Known problems with the C++ front-end

For this release, the C++ front-end is considered to be fully functional but of beta quality. It has been tested and works for a number of simple programs that collectively exercise most of the language. Nevertheless, it has not been in use as long as the C front-end. Please report any bugs or problems.

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

• C++ frontend can crash when compiling virtual base classes
• 1.0 precompiled libstdc++ does not include wchar_t support
• [C++] Catch blocks make unparsable labels
• [C++] Initializing array with constructable objects fail
• [c++] C++ Frontend lays out superclasses like anonymous bitfields!
• [llvmg++] Enum types are incorrectly shrunk to smaller than 'int' size
• [llvmg++] Cannot use pointer to member to initialize global
• [llvmg++] Front-end attempts to return structure by value
• [llvmg++] Pointer to member initializers not supported in constructors
• [llvmg++] ?: expressions do not run correct number of destructors!
• Code for executing destructors when unwinding is not shared (this is a quality of implementation problem, which does not effect functionality).

• [llvmg++] call through array of pointers to member functions causes assertion

• 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.

---

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.

• [X86] Emission of global bool initializers broken
• [X86] div and rem constant exprs invalidate iterators!

---

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 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.
• 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.

• C backend fails on constant cast expr to ptr-to-anonymous struct
• Variables in scope of output setjmp calls should be volatile. Note that this does not effect correctness on many platforms, such as X86.

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/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.