# LLVM 2.9 Release Notes

1. Introduction
2. Sub-project Status Update
3. External Projects Using LLVM 2.9
4. What's New in LLVM 2.9?
5. Installation Instructions
6. Known Problems
7. Additional Information

Written by the LLVM Team

# These are in-progress notes for the upcoming LLVM 2.9 release.
You may prefer the LLVM 2.8 Release Notes.

# Introduction

This document contains the release notes for the LLVM Compiler Infrastructure, release 2.9. Here we describe the status of LLVM, including major improvements from the previous release and significant known problems. All LLVM releases may be downloaded from the LLVM releases web site.

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 a Subversion checkout or the main LLVM web page, this document applies to the next release, not the current one. To see the release notes for a specific release, please see the releases page.

# Sub-project Status Update

The LLVM 2.9 distribution currently consists of code from the core LLVM repository (which roughly includes the LLVM optimizers, code generators and supporting tools), the Clang repository and the llvm-gcc repository. In addition to this code, the LLVM Project includes other sub-projects that are in development. Here we include updates on these subprojects.

## Clang: C/C++/Objective-C Frontend Toolkit

Clang is an LLVM front end for the C, C++, and Objective-C languages. Clang aims to provide a better user experience through expressive diagnostics, a high level of conformance to language standards, fast compilation, and low memory use. Like LLVM, Clang provides a modular, library-based architecture that makes it suitable for creating or integrating with other development tools. Clang is considered a production-quality compiler for C, Objective-C, C++ and Objective-C++ on x86 (32- and 64-bit), and for darwin-arm targets.

In the LLVM 2.9 time-frame, the Clang team has made many improvements in C, C++ and Objective-C support. C++ support is now generally rock solid, has been exercised on a broad variety of code, and has several new C++'0x features implemented (such as rvalue references and variadic templates). LLVM 2.9 has also brought in a large range of bug fixes and minor features (e.g. __label__ support), and is much more compatible with the Linux Kernel.

If Clang rejects your code that is built with another compiler, please take a look at the language compatibility guide to make sure the issue isn't intentional or a known issue.

## DragonEgg: GCC front-ends, LLVM back-end

DragonEgg is a gcc plugin that replaces GCC's optimizers and code generators with LLVM's. Currently it requires a patched version of gcc-4.5. The plugin can target the x86-32 and x86-64 processor families and has been used successfully on the Darwin, FreeBSD and Linux platforms. The Ada, C, C++ and Fortran languages work well. The plugin is capable of compiling plenty of Obj-C, Obj-C++ and Java but it is not known whether the compiled code actually works or not!

The 2.9 release has the following notable changes:

- The plugin is much more stable when compiling Fortran.
- Inline asm where an asm output is tied to an input of a different size is now supported in many more cases.
- Basic support for the __float128 type was added. It is now possible to generate LLVM IR from programs using __float128 but code generation does not work yet.
- Compiling Java programs no longer systematically crashes the plugin.

## compiler-rt: Compiler Runtime Library

The new LLVM compiler-rt project is a simple library that provides an implementation of the low-level target-specific hooks required by code generation and other runtime components. For example, when compiling for a 32-bit target, converting a double to a 64-bit unsigned integer is compiled into a runtime call to the "__fixunsdfdi" function. The compiler-rt library provides highly optimized implementations of this and other low-level routines (some are 3x faster than the equivalent libgcc routines).

All of the code in the compiler-rt project is available under the standard LLVM License, a "BSD-style" license. compiler_rt is now dual licensed under MIT and UIUC license Several minor changes for better ARM support. New in LLVM 2.9, UPDATE

## LLDB: Low Level Debugger

LLDB is a brand new member of the LLVM umbrella of projects. LLDB is a next generation, high-performance debugger. It is built as a set of reusable components which highly leverage existing libraries in the larger LLVM Project, such as the Clang expression parser, the LLVM disassembler and the LLVM JIT.

LLDB is in early development and not included as part of the LLVM 2.9 release,

## libc++: C++ Standard Library

libc++ is another new member of the LLVM family. It is an implementation of the C++ standard library, written from the ground up to specifically target the forthcoming C++'0X standard and focus on delivering great performance.

As of the LLVM 2.9 release, UPDATE! libc++ is now dual licensed under MIT and UIUC license

# External Open Source Projects Using LLVM 2.9

An exciting aspect of LLVM is that it is used as an enabling technology for a lot of other language and tools projects. This section lists some of the projects that have already been updated to work with LLVM 2.9.

# What's New in LLVM 2.9?

This release includes a huge number of bug fixes, performance tweaks and minor improvements. Some of the major improvements and new features are listed in this section.

## Major New Features

LLVM 2.9 includes several major new capabilities:

-       last release for llvm-gcc
      TBAA: On by default in clang.  Disable it with -fno-strict-aliasing.
        Could be more aggressive for structs.

      Triple::normalize is new, llvm triples are always stored in normalized form internally.

      Triple x86_64--mingw64 is obsoleted. Use x86_64--mingw32 instead.

      MC Assembler: X86 now generates much better diagnostics for common errors,
          is much faster at matching instructions, is much more bug-compatible with
          the GAS assembler, and is now generally useful for a broad range of X86
          assembly.

      New Nvidia PTX backend, not generally useful in 2.9 though.

      Much better debug info generated, particularly in optimized code situations.

      ARM Fast ISel

      ELF MC support: on by default in clang.  There are still known missing features
        for human written assembly.

      X86: Reimplemented all of MMX to introduce a new LLVM IR x86_mmx type.  Now
        random types like <2 x i32> are not iseld to mmx without emms.  The
        -disable-mmx flag is gone now.

      Some basic internals documentation for MC.

      MC Assembler support for .file and .loc.

      inline asm multiple alternative constraint support.

      LoopIdiom: memset/memcpy formation and memset_pattern on darwin.  Build with
        -ffreestanding or -fno-builtin if your memcpy is being compiled into infinite
        recursion.

      TargetLibraryInfo

      X86 support for FS/GS relative loads and stores using address space 256/257 are
        reliable now.

      ARM: New code placement pass.

      unnamed_addr + PR8927

      PointerTracking has been removed from mainline, moved to ClamAV.

      EarlyCSE pass.
      LoopInstSimplify pass.

      - DIBuilder provides simpler interface for front ends like Clang to encode debug info in LLVM IR.
        - This interface hides implementation details (e.g. DIDerivedType, existence of compile unit etc..) that any front end should not know about.
        For example, DIFactory DebugFactory;
        Ty = DebugFactory.CreateDerivedType(DW_TAG_volatile_type,
        findRegion(TYPE_CONTEXT(type)),
        StringRef(),
        getOrCreateFile(main_input_filename),
        0 /*line no*/,
        NodeSizeInBits(type),
        NodeAlignInBits(type),
        0 /*offset */,
        0 /* flags */,
        MainTy);
        can be replaced by
        DbgTy = DBuilder.createQualifiedType(DW_TAG_volatile_type, MainTy);
      DIFactory is gone now.

      PPC: Switched to MCInstPrinter, and MCCodeEmitter.  Ready to implement support
        for directly writing out mach-o object files, but noone seems interested.

      ARM: Improved code generation for Cortex-A8 and Cortex-A9 CPUs.

      Scheduler now models operand latency and pipeline forwarding.

      Can optimize printf to iprintf when no floating point is used, for embedded
        targets with smaller iprintf implementation.

      error_code + libsystem + PathV2 changes
        The system_error header from C++0x was added.
        * Use if (error_code ec = function()) to check for error conditions
        from functions which return it.
        * error_code::message returns a human readable description of the error.

        PathV1 has been deprecated in favor of PathV2 (sorry I didn't finish
        this before the release).
        * No Path class, use a r-value convertible to a twine instead.
        * Assumes all paths are UTF-8.

      new macho-dump tool

      Major regalloc rewrite, not on by default for 2.9 and not advised to use it.
       * New basic register allocator that can be used as a safe fallback when
         debugging. Enable with -regalloc=basic.
       * New infrastructure for live range splitting. SplitKit can break a live
         interval into smaller pieces while preserving SSA form, and SpillPlacement
         can help find the best split points. This is a work in progress so the API
         is changing quickly.
       * The inline spiller has learned to clean up after live range splitting. It
         can hoist spills out of loops, and it can eliminate redundant spills.
         Rematerialization works with live range splitting.
       * New greedy register allocator using live range splitting. This will be the
         default register allocator in the next LLVM release, but it is not turned on
         by default in 2.9.

      ARM: __builtin_prefetch turns into prefetch instructions.

      MC assembler support for 3dNow! and 3DNowA instructions.

      tblgen support for assembler aliases: MnemonicAlias and InstAlias

      LoopIndexSplit pass was removed, unmaintained.
      LiveValues, SimplifyHalfPowrLibCalls, and GEPSplitter were removed.

      include/llvm/System merged into include/llvm/Support.

      Win32 PE-COFF support in the MC assembler has made a lot of progress in the 2.9
        timeframe, but is still not generally useful.  Please see
        "http://llvm.org/bugs/showdependencytree.cgi?id=9100&hide_resolved=1" for open bugs?

      New RegionPass infrastructure
        for region-based optimizations.

      MicroBlaze: major updates for aggressive delay slot filler, MC-based assembly
         printing, assembly instruction parsing, ELF .o file emission, and MC
         instruction disassembler.

      Countless ARM microoptimizations.

      Speedups to various mid-level passes:
        GVN is much faster on functions with deep dominator trees / lots of BBs.
        DomTree and DominatorFrontier are much faster to compute, and preserved by
          more passes (so they are computed less often)
        SRoA is also much faster and doesn't use DominanceFrontier.

      new 'hotpatch' attribute: LangRef.html#fnattrs

      APInt API changes, see PR5207.

      DSE is more aggressive with stores of different types: e.g. a large store
        following a small one to the same address.

      New naming rules in coding standards: CodingStandards.html#ll_naming

      LiveDebugVariables is a new pass that keeps track of debugging information for
        user variables that are kept in registers in optimized builds.

      We now optimize various idioms for overflow detection into check of the flag
        register on various CPUs, e.g.:
         unsigned long t = a+b;
         if (t < a) ...
        into:
        	addq	%rdi, %rbx
        	jno	LBB0_2

      X86: Much better codegen for several cases using adc/sbb instead of cmovs for
        conditional increment and other idioms.

      MVT::Flag renamed to MVT::Glue

      Removed the PartialSpecialization pass, it was unmaintained and buggy.

      SPARC: Many improvements, including using the Y registers for multiplications
        and addition of a simple delay slot filler.

      udiv, ashr, lshr, shl now have exact and nuw/nsw bits: PR8862 / LangRef.html

      lib/Object and llvm-objdump

        Target Independent Code Gen:
        The pre-register-allocation (preRA) instruction scheduler models register pressure
        much more accurately in some cases. This allows the adoption of more
        aggressive scheduling heuristics.

        The X86 backend has adopted a new preRA scheduling
        mode, "list-ilp", to shorten the height of instruction schedules
        without inducing register spills.

        The ARM backend preRA scheduler now models machine resources at cycle
        granularity. This allows the scheduler to both accurately model
        instruction latency and avoid overcommitting functional units.

## LLVM IR and Core Improvements

LLVM IR has several new features for better support of new targets and that expose new optimization opportunities:

## Optimizer Improvements

In addition to a large array of minor performance tweaks and bug fixes, this release includes a few major enhancements and additions to the optimizers:

- TBAA.
- LTO has been improved to use MC for parsing inline asm and now can build large programs like Firefox 4 on both OS X and Linux.

## MC Level Improvements

The LLVM Machine Code (aka MC) subsystem was created to solve a number of problems in the realm of assembly, disassembly, object file format handling, and a number of other related areas that CPU instruction-set level tools work in.

- MC is now used by default for ELF systems on x86 and x86-64.
- MC supports and CodeGen uses the `.loc` directives for producing line number debug info. This produces more compact line tables.
- MC supports the `.cfi_*` directives for producing DWARF frame information, but it is still not used by CodeGen by default.
- COFF support?

For more information, please see the Intro to the LLVM MC Project Blog Post.

## Target Independent Code Generator Improvements

We have put a significant amount of work into the code generator infrastructure, which allows us to implement more aggressive algorithms and make it run faster:

FastISel for ARM.

## X86-32 and X86-64 Target Improvements

New features and major changes in the X86 target include:

- Several bugs have been fixed for Windows x64 code generator.

## ARM Target Improvements

New features of the ARM target include:

## Major Changes and Removed Features

If you're already an LLVM user or developer with out-of-tree changes based on LLVM 2.8, this section lists some "gotchas" that you may run into upgrading from the previous release.

In addition, many APIs have changed in this release. Some of the major LLVM API changes are:

## Development Infrastructure Changes

This section lists changes to the LLVM development infrastructure. This mostly impacts users who actively work on LLVM or follow development on mainline, but may also impact users who leverage the LLVM build infrastructure or are interested in LLVM qualification.

# Known Problems

This section contains significant known problems with the LLVM system, listed by component. 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 Alpha, Blackfin, CellSPU, MicroBlaze, MSP430, MIPS, PTX, SystemZ and XCore backends are experimental.
- `llc` "`-filetype=obj`" is experimental on all targets other than darwin-i386 and darwin-x86_64. FIXME: Not true on ELF anymore?

## Known problems with the X86 back-end

- The X86 backend does not yet support all inline assembly that uses the X86 floating point stack. It supports the 'f' and 't' constraints, but not 'u'.
- The X86-64 backend does not yet support the LLVM IR instruction `va_arg`. Currently, front-ends support variadic argument constructs on X86-64 by lowering them manually.
- Windows x64 (aka Win64) code generator has a few issues.
  - llvm-gcc cannot build the mingw-w64 runtime currently due to lack of support for the 'u' inline assembly constraint and for X87 floating point inline assembly.
  - On mingw-w64, you will see unresolved symbol `__chkstk` due to Bug 8919. It is fixed in r128206.
  - Miss-aligned MOVDQA might crash your program. It is due to Bug 9483, lack of handling aligned internal globals.

## Known problems with the PowerPC back-end

- The Linux PPC32/ABI support needs testing for the interpreter and static compilation, and lacks support for debug information.

## Known problems with the ARM back-end

- Thumb mode works only on ARMv6 or higher processors. On sub-ARMv6 processors, thumb programs can crash or produce wrong results (PR1388).
- Compilation for ARM Linux OABI (old ABI) is supported but not fully tested.

## 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 MIPS back-end

- 64-bit MIPS targets are not supported yet.

## 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 C back-end

The C backend has numerous problems and is not being actively maintained. Depending on it for anything serious is not advised.

- The C backend has only basic support for inline assembly code.
- The C backend violates the ABI of common C++ programs, preventing intermixing between C++ compiled by the CBE and C++ code compiled with `llc` or native compilers.
- The C backend does not support all exception handling constructs.
- The C backend does not support arbitrary precision integers.

## Known problems with the llvm-gcc front-end

llvm-gcc is generally very stable for the C family of languages. The only major language feature of GCC not supported by llvm-gcc is the `__builtin_apply` family of builtins. However, some extensions are only supported on some targets. For example, trampolines are only supported on some targets (these are used when you take the address of a nested function).

Fortran support generally works, but there are still several unresolved bugs in Bugzilla. Please see the tools/gfortran component for details. Note that llvm-gcc is missing major Fortran performance work in the frontend and library that went into GCC after 4.2. If you are interested in Fortran, we recommend that you consider using dragonegg instead.

The llvm-gcc 4.2 Ada compiler has basic functionality, but is no longer being actively maintained. If you are interested in Ada, we recommend that you consider using dragonegg instead.

# Additional Information

A wide variety of additional information is available on the LLVM web page, in particular in the documentation section. The web page also contains versions of the API documentation which is up-to-date with the Subversion 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.