LLVM 1.7 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 1.7. 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 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 eighth public release of the LLVM Compiler Infrastructure. This release incorporates a large number of enhancements and new features, including vector support (Intel SSE and Altivec), a new GCC4.0-based C/C++ front-end, Objective C/C++ support, inline assembly support, and many other big features.

New Features in LLVM 1.7
GCC4.0-based llvm-gcc front-end

LLVM 1.7 includes a brand new llvm-gcc, based on GCC 4.0.1. This version of llvm-gcc solves many serious long-standing problems with llvm-gcc, including all of those blocked by the llvm-gcc 4 meta bug. In addition, llvm-gcc4 implements support for many new features, including GCC inline assembly, generic vector support, SSE and Altivec intrinsics, and several new GCC attributes. Finally, llvm-gcc4 is significantly faster than llvm-gcc3, respects -O options, its -c/-S options correspond to GCC's (they emit native code), supports Objective C/C++, and it has debugging support well underway.

If you can use it, llvm-gcc4 offers significant new functionality, and we hope that it will replace llvm-gcc3 completely in a future release. Unfortunately, it does not currently support C++ exception handling at all, and it only works on Apple Mac OS/X machines with X86 or PowerPC processors.

Inline Assembly Support

The LLVM IR and llvm-gcc4 front-end now fully support arbitrary GCC inline assembly. The LLVM X86 and PowerPC code generators have initial support for it, being able to compile basic statements, but are missing some features. Please report any inline asm statements that crash the compiler or that are miscompiled as bugs.

New SPARC backend

LLVM 1.7 includes a new, fully functional, SPARC backend built in the target-independent code generator. This SPARC backend includes support for SPARC V8 and SPARC V9 subtargets (controlling whether V9 features can be used), and targets the 32-bit SPARC ABI.

The LLVM 1.7 release is the last release that will include the LLVM "SparcV9" backend, which was the very first LLVM native code generator. It will be removed in LLVM 1.8, being replaced with the new SPARC backend.

Generic Vector Support

LLVM now includes significantly extended support for SIMD vectors in its core instruction set. It now includes three new instructions for manipulating vectors: `extractelement`, `insertelement`, and `shufflevector`. Further, many bugs in vector handling have been fixed, and vectors are now supported by the target-independent code generator. For example, if a vector operation is not supported by a particular target, it will be correctly broken down and executed as scalar operations.

Because llvm-gcc3 does not support GCC generic vectors or vector intrinsics, llvm-gcc4 must be used.

Intel SSE and PowerPC Altivec support

The LLVM X86 backend now supports Intel SSE 1, 2, and 3, and now uses scalar SSE operations to implement scalar floating point math when the target supports SSE1 (for floats) or SSE2 (for doubles). Vector SSE instructions are generated by llvm-gcc4 when the generic vector mechanism or specific SSE intrinsics are used.

The LLVM PowerPC backend now supports the Altivec instruction set, including both GCC -maltivec and -faltivec modes. Altivec instructions are generated by llvm-gcc4 when the generic vector mechanism or specific Altivec intrinsics are used.

Optimizer Improvements

- The Loop Unswitching pass (`-loop-unswitch`) has had several bugs fixed, has several new features, and is enabled by default in llvmgcc3 now.
- The Loop Strength Reduction pass (`-loop-reduce`) is now enabled for the X86 and Alpha backends.
- The Instruction Combining pass (`-instcombine`) now includes a framework and implementation for simplifying code based on whether computed bits are demanded or not.
- The Scalar Replacement of Aggregates pass (`-scalarrepl`) can now promote simple unions to registers.
- The Reassociation pass (`-reassociate`) can now factor expressions, e.g. turning "A*A+A*B" into "A*(A+B)".
- Several LLVM passes are significantly faster.

Code Generator Improvements

- LLVM has a new prepass (before register allocation) list scheduler, which supports bottom-up and top-down scheduling, pluggable priority functions and pluggable hazard recognizers. The X86 backend uses this to reduce register pressure and RISC targets schedule based on operation latency.
- The tblgen-based target description framework introduced in LLVM 1.6 has several new features, useful for targets that can fold loads and stores into operations, and features that make the .td files more expressive.
- The instruction selector is significantly faster in 1.7 than in 1.6.
- The X86, Alpha and Itanium backends use new DAG-DAG instruction selectors, making them easier to maintain and generate slightly better code.
- The X86 backend now supports generation of Scalar SSE code for scalar FP expressions. LLVM provides significantly better performance with Scalar SSE instructions than it does with the Intel floating point stack instructions.
- The Itanium backend now has a bundling pass, which improves performance by ~10% and reduces code size (previously it unconditionally inserted a stop bit after every instruction).

Other New Features

- The Mac OS/X PowerPC and X86 backends now have initial support for Darwin DWARF debugging information, however, debug info generation has been disabled for the 1.7 release in llvmgcc4.
- LLVM includes the new llvm-config utility, which makes it easier to build and link programs against the LLVM libraries when not using the LLVM makefiles.
- LLVM now supports first class global ctor/dtor initialization lists, no longer forcing targets to use "__main".
- LLVM supports assigning globals and functions to a particular section in the result executable using the GCC section attribute.
- Adding intrinsics to LLVM is now significantly easier.
- llvmgcc4 now fully supports C99 Variable Length Arrays, including dynamic stack deallocation.

Significant Changes in LLVM 1.7

- The official LLVM URL is now http://llvm.org/.
- The LLVM intrinsics used to be overloaded based on type: for example, `llvm.ctpop` could work with any integer datatype. They are now separated into different intrinsics with suffixes to denote their argument type (e.g. `llvm.ctpop.i32`)). Old LLVM .ll and .bc files that use these intrinsics will continue to work with new LLVM versions (they are transparently upgraded by the parsers), but will cause a warning to be emitted.
- The `llvm.readport`, `llvm.writeport`, `llvm.readio`, and `llvm.writeio` intrinsics have been removed. The first two were ever only supported by the X86 backend, the last two were never correctly supported by any target, and none were accessible through the C front-end. Inline assembly support can now be used to implement these operations.
- The `llvm-db` tool had basic support for stepping through code, which used the JIT. This code has been removed, and DWARF emission support added instead. `llvm-db` still exists in CVS if someone wanted to write a `ptrace` backend for it.

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).
- 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. See those blocked on the llvm-gcc4 meta bug. 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.

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

- Indirect calls crash JIT on Darwin/x86.

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 SparcV9 back-end

- [sparcv9] SparcV9 backend miscompiles several programs in the LLVM test suite
- The SparcV9 backend is slated to be removed before the LLVM 1.8 release.

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.