LLVM 2.3 Release Notes
- Major Changes and Sub-project Status
- What's New?
- Installation Instructions
- Portability and Supported Platforms
- Known Problems
- Additional Information
This document contains the release notes for the LLVM compiler
infrastructure, release 2.3. Here we describe the status of LLVM, including
major improvements from the previous release and any 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 releases, please see the
This is the fourteenth public release of the LLVM Compiler Infrastructure.
It includes a large number of features and refinements from LLVM 2.2.
LLVM 2.3 no longer supports llvm-gcc 4.0, it has been replaced with
LLVM 2.3 no longer includes the llvm-upgrade tool. It was useful
for upgrading LLVM 1.9 files to LLVM 2.x syntax, but you can always use a
previous LLVM release to do this. One nice impact of this is that the LLVM
regression test suite no longer depends on llvm-upgrade, which makes it run
The llvm2cpp tool has been folded into llc, use
llc -march=cpp instead of llvm2cpp.
LLVM API Changes:
- Several core LLVM IR classes have migrated to use the
'FOOCLASS::Create(...)' pattern instead of 'new
FOOCLASS(...)' (e.g. where FOOCLASS=BasicBlock). We hope to
standardize on FOOCLASS::Create for all IR classes in the future,
but not all of them have been moved over yet.
- LLVM 2.3 renames the LLVMBuilder and LLVMFoldingBuilder classes to
- MRegisterInfo was renamed to
- The MappedFile class is gone, please use
- The '-enable-eh' flag to llc has been removed. Now code should
encode whether it is safe to omit unwind information for a function by
tagging the Function object with the 'nounwind' attribute.
- The ConstantFP::get method that uses APFloat now takes one argument
instead of two. The type argument has been removed, and the type is
now inferred from the size of the given APFloat value.
The core LLVM 2.3 distribution currently consists of code from the core LLVM
repository (which roughly contains the LLVM optimizer, code generators and
supporting tools) and the llvm-gcc repository. In addition to this code, the
LLVM Project includes other sub-projects that are in development. The two which
are the most actively developed are the new vmkit Project
and the Clang Project.
The "vmkit" project is a new addition to the LLVM family. It is an
implementation of a JVM and a CLI Virtual Machines (Microsoft .NET is an
implementation of the CLI) using the Just-In-Time compiler of LLVM.
The JVM, called JnJVM, executes real-world applications such as Apache
projects (e.g. Felix and Tomcat) and the SpecJVM98 benchmark. It uses the GNU
Classpath project for the base classes. The CLI implementation, called N3, is
its in early stages but can execute simple applications and the "pnetmark"
benchmark. It uses the pnetlib project as its core library.
The 'vmkit' VMs compare in performance with industrial and top open-source
VMs on scientific applications. Besides the JIT, the VMs use many features of
the LLVM framework, including the standard set of optimizations, atomic
operations, custom function provider and memory manager for JITed methods, and
specific virtual machine optimizations. vmkit is not an official part of LLVM
2.3 release. It is publicly available under the LLVM license and can be
svn co http://llvm.org/svn/llvm-project/vmkit/trunk vmkit
The Clang project is an effort to build
a set of new 'LLVM native' front-end technologies for the LLVM optimizer
and code generator. Clang is continuing to make major strides forward in all
areas. Its C and Objective-C parsing support is very solid, and the code
generation support is far enough along to build many C applications. While not
yet production quality, it is progressing very nicely. In addition, C++
front-end work has started to make significant progress.
At this point, Clang is most useful if you are interested in source-to-source
transformations (such as refactoring) and other source-level tools for C and
Objective-C. Clang now also includes tools for turning C code into pretty HTML,
and includes a new static
analysis tool in development. This tool focuses on automatically finding
bugs in C and Objective-C code.
LLVM 2.3 includes a huge number of bug fixes, performance tweaks and minor
improvements. Some of the major improvements and new features are listed in
LLVM 2.3 includes several major new capabilities:
The biggest change in LLVM 2.3 is Multiple Return Value (MRV) support.
MRVs allow LLVM IR to directly represent functions that return multiple
values without having to pass them "by reference" in the LLVM IR. This
allows a front-end to generate more efficient code, as MRVs are generally
returned in registers if a target supports them. See the LLVM IR Reference for more details.
MRVs are fully supported in the LLVM IR, but are not yet fully supported in
on all targets. However, it is generally safe to return up to 2 values from
a function: most targets should be able to handle at least that. MRV
support is a critical requirement for X86-64 ABI support, as X86-64 requires
the ability to return multiple registers from functions, and we use MRVs to
accomplish this in a direct way.
LLVM 2.3 includes a complete reimplementation of the "llvmc"
tool. It is designed to overcome several problems with the original
llvmc and to provide a superset of the features of the
The main features of llvmc2 are:
- Extended handling of command line options and smart rules for
dispatching them to different tools.
- Flexible (and extensible) rules for defining different tools.
- The different intermediate steps performed by tools are represented
as edges in the abstract graph.
- The 'language' for driver behavior definition is tablegen and thus
it's relatively easy to add new features.
- The definition of driver is transformed into set of C++ classes, thus
no runtime interpretation is needed.
LLVM 2.3 includes a completely rewritten interface for Link Time Optimization. This interface
is written in C, which allows for easier integration with C code bases, and
incorporates improvements we learned about from the first incarnation of the
The Kaleidoscope tutorial now
includes a "port" of the tutorial that uses the Ocaml bindings to implement
the Kaleidoscope language.
LLVM 2.3 fully supports the llvm-gcc 4.2 front-end, and includes support
for the C, C++, Objective-C, Ada, and Fortran front-ends.
- llvm-gcc 4.2 includes numerous fixes to better support the Objective-C
front-end. Objective-C now works very well on Mac OS/X.
- Fortran EQUIVALENCEs are now supported by the gfortran front-end.
- llvm-gcc 4.2 includes many other fixes which improve conformance with the
relevant parts of the GCC testsuite.
New features include:
- LLVM IR now directly represents "common" linkage, instead of representing it
as a form of weak linkage.
- LLVM IR now has support for atomic operations, and this functionality can
be accessed through the llvm-gcc "__sync_synchronize",
"__sync_val_compare_and_swap", and related builtins. Support for atomics are
available in the Alpha, X86, X86-64, and PowerPC backends.
- The C and Ocaml bindings have extended to cover pass managers, several
transformation passes, iteration over the LLVM IR, target data, and parameter
In addition to a huge array of bug fixes and minor performance tweaks, the
LLVM 2.3 optimizers support a few major enhancements:
Loop index set splitting on by default.
This transformation hoists conditions from loop bodies and reduces a loop's
iteration space to improve performance. For example,
for (i = LB; i < UB; ++i)
if (i <= NV)
is transformed into:
NUB = min(NV+1, UB)
for (i = LB; i < NUB; ++i)
- LLVM now includes a new memcpy optimization pass which removes
dead memcpy calls, unneeded copies of aggregates, and performs
return slot optimization. The LLVM optimizer now notices long sequences of
consecutive stores and merges them into memcpy's where profitable.
- Alignment detection for vector memory references and for memcpy and
memset is now more aggressive.
- The Aggressive Dead Code Elimination (ADCE) optimization has been rewritten
to make it both faster and safer in the presence of code containing infinite
loops. Some of its prior functionality has been factored out into the loop
deletion pass, which is safe for infinite loops. The new ADCE pass is
no longer based on control dependence, making it run faster.
- The 'SimplifyLibCalls' pass, which optimizes calls to libc and libm
functions for C-based languages, has been rewritten to be a FunctionPass
instead a ModulePass. This allows it to be run more often and to be
included at -O1 in llvm-gcc. It was also extended to include more
optimizations and several corner case bugs were fixed.
- LLVM now includes a simple 'Jump Threading' pass, which attempts to simplify
conditional branches using information about predecessor blocks, simplifying
the control flow graph. This pass is pretty basic at this point, but
catches some important cases and provides a foundation to build on.
- Several corner case bugs which could lead to deleting volatile memory
accesses have been fixed.
- Several optimizations have been sped up, leading to faster code generation
with the same code quality.
We put a significant amount of work into the code generator infrastructure,
which allows us to implement more aggressive algorithms and make it run
- The code generator now has support for carrying information about memory
references throughout the entire code generation process, via the
MachineMemOperand class. In the future this will be used to improve
both pre-pass and post-pass scheduling, and to improve compiler-debugging
- The target-independent code generator infrastructure now uses LLVM's
class to handle integer values, which allows it to support integer types
larger than 64 bits (for example i128). Note that support for such types is
also dependent on target-specific support. Use of APInt is also a step
toward support for non-power-of-2 integer sizes.
- LLVM 2.3 includes several compile time speedups for code with large basic
blocks, particularly in the instruction selection phase, register
allocation, scheduling, and tail merging/jump threading.
- LLVM 2.3 includes several improvements which make llc's
--view-sunit-dags visualization of scheduling dependency graphs
easier to understand.
- The code generator allows targets to write patterns that generate subreg
references directly in .td files now.
- memcpy lowering in the backend is more aggressive, particularly for
memcpy calls introduced by the code generator when handling
pass-by-value structure argument copies.
- Inline assembly with multiple register results now returns those results
directly in the appropriate registers, rather than going through memory.
Inline assembly that uses constraints like "ir" with immediates now use the
'i' form when possible instead of always loading the value in a register.
This saves an instruction and reduces register use.
- Added support for PIC/GOT style tail calls on X86/32 and initial
support for tail calls on PowerPC 32 (it may also work on PowerPC 64 but is
not thoroughly tested).
New target-specific features include:
- llvm-gcc's X86-64 ABI conformance is far improved, particularly in the
area of passing and returning structures by value. llvm-gcc compiled code
now interoperates very well on X86-64 systems with other compilers.
- Support for Win64 was added. This includes code generation itself, JIT
support, and necessary changes to llvm-gcc.
- The LLVM X86 backend now supports the support SSE 4.1 instruction set, and
the llvm-gcc 4.2 front-end supports the SSE 4.1 compiler builtins. Various
generic vector operations (insert/extract/shuffle) are much more efficient
when SSE 4.1 is enabled. The JIT automatically takes advantage of these
instructions, but llvm-gcc must be explicitly told to use them, e.g. with
- The X86 backend now does a number of optimizations that aim to avoid
converting numbers back and forth from SSE registers to the X87 floating
point stack. This is important because most X86 ABIs require return values
to be on the X87 Floating Point stack, but most CPUs prefer computation in
the SSE units.
- The X86 backend supports stack realignment, which is particularly useful for
vector code on OS's without 16-byte aligned stacks, such as Linux and
- The X86 backend now supports the "sseregparm" options in GCC, which allow
functions to be tagged as passing floating point values in SSE
- Trampolines (taking the address of a nested function) now work on
- __builtin_prefetch is now compiled into the appropriate prefetch
instructions instead of being ignored.
- 128-bit integers are now supported on X86-64 targets. This can be used
through __attribute__((TImode)) in llvm-gcc.
- The register allocator can now rematerialize PIC-base computations, which is
an important optimization for register use.
- The "t" and "f" inline assembly constraints for the X87 floating point stack
now work. However, the "u" constraint is still not fully supported.
New target-specific features include:
- The LLVM C backend now supports vector code.
- The Cell SPU backend includes a number of improvements. It generates better
code and its stability/completeness is improving.
New features include:
- LLVM now builds with GCC 4.3.
- Bugpoint now supports running custom scripts (with the -run-custom
option) to determine how to execute the command and whether it is making
LLVM is known to work on the following platforms:
- Intel and AMD machines (IA32) running Red Hat Linux, Fedora Core and FreeBSD
(and probably other unix-like systems).
- PowerPC and X86-based Mac OS X systems, running 10.3 and above in 32-bit and
- Intel and AMD machines running on Win32 using MinGW libraries (native).
- Intel and AMD machines running on Win32 with the Cygwin libraries (limited
support is available for native builds with Visual C++).
- Sun UltraSPARC workstations running Solaris 10.
- Alpha-based machines running Debian GNU/Linux.
- Itanium-based (IA64) 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.
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.
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 MSIL, IA64, Alpha, SPU, and MIPS backends are experimental.
- The llc "-filetype=asm" (the default) is the only supported
value for this option.
- 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
- The X86 backend generates inefficient floating point code when configured
to generate code for systems that don't have SSE2.
- Win64 code generation wasn't widely tested. Everything should work, but we
expect small issues to happen. Also, llvm-gcc cannot build mingw64 runtime
bugs due to lack of support for the
'u' inline assembly constraint and X87 floating point inline assembly.
- The X86-64 backend does not yet support position-independent code (PIC)
generation on Linux targets.
- The X86-64 backend does not yet support the LLVM IR instruction
va_arg. Currently, the llvm-gcc front-end supports variadic
argument constructs on X86-64 by lowering them manually.
- The Linux PPC32/ABI support needs testing for the interpreter and static
compilation, and lacks support for debug information.
- Thumb mode works only on ARMv6 or higher processors. On sub-ARMv6
processors, thumb programs can crash or produce wrong
- Compilation for ARM Linux OABI (old ABI) is supported, but not fully tested.
- There is a bug in QEMU-ARM (<= 0.9.0) which causes it to incorrectly
programs compiled with LLVM. Please use more recent versions of QEMU.
- The SPARC backend only supports the 32-bit SPARC ABI (-m32), it does not
support the 64-bit SPARC ABI (-m64).
- On 21164s, some rare FP arithmetic sequences which may trap do not have the
appropriate nops inserted to ensure restartability.
- The Itanium backend is highly experimental, and has a number of known
issues. We are looking for a maintainer for the Itanium backend. If you
are interested, please contact the llvmdev mailing list.
llvm-gcc does not currently support Link-Time
Optimization on most platforms "out-of-the-box". Please inquire on the
llvmdev mailing list if you are interested.
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
If you run into GCC extensions which are not supported, please let us know.
The C++ front-end is considered to be fully
tested and works for a number of non-trivial programs, including LLVM
itself, Qt, Mozilla, etc.
- Exception handling works well on the X86 and PowerPC targets, including
X86-64 darwin. This works when linking to a libstdc++ compiled by GCC. It is
supported on X86-64 linux, but that is disabled by default in this release.
The llvm-gcc 4.2 Ada compiler works fairly well, however this is not a mature
technology and problems should be expected.
- The Ada front-end currently only builds on X86-32. This is mainly due
to lack of trampoline support (pointers to nested functions) on other platforms,
however it also fails to build on X86-64
which does support trampolines.
- The Ada front-end fails to bootstrap.
Workaround: configure with --disable-bootstrap.
- The c380004 and c393010 ACATS tests
fail (c380004 also fails with gcc-4.2 mainline). When built at -O3, the
cxg2021 ACATS test also fails.
- Some gcc specific Ada tests continue to crash the compiler. The testsuite
reports most tests as having failed even though they pass.
- The -E binder option (exception backtraces)
does not work and will result in programs
crashing if an exception is raised. Workaround: do not use -E.
- Only discrete types are allowed to start
or finish at a non-byte offset in a record. Workaround: do not pack records
or use representation clauses that result in a field of a non-discrete type
starting or finishing in the middle of a byte.
- The lli interpreter considers
'main' as generated by the Ada binder to be invalid.
Workaround: hand edit the file to use pointers for argv and
envp rather than integers.
- The -fstack-check option is
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
LLVM Compiler Infrastructure
Last modified: $Date: 2008/06/09 08:20:32 $