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.
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 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 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 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 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++ 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
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.
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.
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. 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,
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);
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.
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.
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)
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.
LLVM IR has several new features for better support of new targets and that expose new optimization opportunities:
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:
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.
For more information, please see the Intro to the LLVM MC Project Blog Post.
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:
New features and major changes in the X86 target include:
New features of the ARM target include:
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:
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.
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.
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 C backend has numerous problems and is not being actively maintained. Depending on it for anything serious is not advised.
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.
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.