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ReleaseNotes.html
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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"
"http://www.w3.org/TR/html4/strict.dtd">
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
<link rel="stylesheet" href="llvm.css" type="text/css">
<title>LLVM 2.9 Release Notes</title>
</head>
<body>
<h1>LLVM 2.9 Release Notes</h1>
<img align=right src="http://llvm.org/img/DragonSmall.png"
width="136" height="136" alt="LLVM Dragon Logo">
<ol>
<li><a href="#intro">Introduction</a></li>
<li><a href="#subproj">Sub-project Status Update</a></li>
<li><a href="#externalproj">External Projects Using LLVM 2.9</a></li>
<li><a href="#whatsnew">What's New in LLVM 2.9?</a></li>
<li><a href="GettingStarted.html">Installation Instructions</a></li>
<li><a href="#knownproblems">Known Problems</a></li>
<li><a href="#additionalinfo">Additional Information</a></li>
</ol>
<div class="doc_author">
<p>Written by the <a href="http://llvm.org/">LLVM Team</a></p>
</div>
<!--
<h1 style="color:red">These are in-progress notes for the upcoming LLVM 2.9
release.<br>
You may prefer the
<a href="http://llvm.org/releases/2.8/docs/ReleaseNotes.html">LLVM 2.8
Release Notes</a>.</h1>
-->
<!-- *********************************************************************** -->
<h2>
<a name="intro">Introduction</a>
</h2>
<!-- *********************************************************************** -->
<div>
<p>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 <a
href="http://llvm.org/releases/">LLVM releases web site</a>.</p>
<p>For more information about LLVM, including information about the latest
release, please check out the <a href="http://llvm.org/">main LLVM
web site</a>. If you have questions or comments, the <a
href="http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev">LLVM Developer's
Mailing List</a> is a good place to send them.</p>
<p>Note that if you are reading this file from a Subversion checkout or the
main LLVM web page, this document applies to the <i>next</i> release, not the
current one. To see the release notes for a specific release, please see the
<a href="http://llvm.org/releases/">releases page</a>.</p>
</div>
<!-- Features that need text if they're finished for 3.1:
ARM EHABI
combiner-aa?
strong phi elim
loop dependence analysis
CorrelatedValuePropagation
lib/Transforms/IPO/MergeFunctions.cpp => consider for 3.1.
-->
<!-- *********************************************************************** -->
<h2>
<a name="subproj">Sub-project Status Update</a>
</h2>
<!-- *********************************************************************** -->
<div>
<p>
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.
</p>
<!--=========================================================================-->
<h3>
<a name="clang">Clang: C/C++/Objective-C Frontend Toolkit</a>
</h3>
<div>
<p><a href="http://clang.llvm.org/">Clang</a> 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.</p>
<p>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 <a
href="http://clang.llvm.org/cxx_status.html#cxx0x">C++'0x features</a>
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.</p>
<p>If Clang rejects your code but another compiler accepts it, please take a
look at the <a href="http://clang.llvm.org/compatibility.html">language
compatibility</a> guide to make sure this is not intentional or a known issue.
</p>
</div>
<!--=========================================================================-->
<h3>
<a name="dragonegg">DragonEgg: GCC front-ends, LLVM back-end</a>
</h3>
<div>
<p>
<a href="http://dragonegg.llvm.org/">DragonEgg</a> is a
<a href="http://gcc.gnu.org/wiki/plugins">gcc plugin</a> 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!
</p>
<p>
The 2.9 release has the following notable changes:
<ul>
<li>The plugin is much more stable when compiling Fortran.</li>
<li>Inline assembly where an asm output is tied to an input of a different size
is now supported in many more cases.</li>
<li>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.</li>
<li>Compiling Java programs no longer systematically crashes the plugin.</li>
</ul>
</div>
<!--=========================================================================-->
<h3>
<a name="compiler-rt">compiler-rt: Compiler Runtime Library</a>
</h3>
<div>
<p>
The new LLVM <a href="http://compiler-rt.llvm.org/">compiler-rt project</a>
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).</p>
<p>In the LLVM 2.9 timeframe, compiler_rt has had several minor changes for
better ARM support, and a fairly major license change. All of the code in the
compiler-rt project is now <a href="DeveloperPolicy.html#license">dual
licensed</a> under MIT and UIUC license, which allows you to use compiler-rt
in applications without the binary copyright reproduction clause. If you
prefer the LLVM/UIUC license, you are free to continue using it under that
license as well.</p>
</div>
<!--=========================================================================-->
<h3>
<a name="lldb">LLDB: Low Level Debugger</a>
</h3>
<div>
<p>
<a href="http://lldb.llvm.org/">LLDB</a> 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.</p>
<p>
LLDB is has advanced by leaps and bounds in the 2.9 timeframe. It is
dramatically more stable and useful, and includes both a new <a
href="http://lldb.llvm.org/tutorial.html">tutorial</a> and a <a
href="http://lldb.llvm.org/lldb-gdb.html">side-by-side comparison with
GDB</a>.</p>
</div>
<!--=========================================================================-->
<h3>
<a name="libc++">libc++: C++ Standard Library</a>
</h3>
<div>
<p>
<a href="http://libcxx.llvm.org/">libc++</a> 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.</p>
<p>
In the LLVM 2.9 timeframe, libc++ has had numerous bugs fixed, and is now being
co-developed with Clang's C++'0x mode.</p>
<p>
Like compiler_rt, libc++ is now <a href="DeveloperPolicy.html#license">dual
licensed</a> under the MIT and UIUC license, allowing it to be used more
permissively.
</p>
</div>
<!--=========================================================================-->
<h3>
<a name="LLBrowse">LLBrowse: IR Browser</a>
</h3>
<div>
<p>
<a href="http://llvm.org/svn/llvm-project/llbrowse/trunk/doc/LLBrowse.html">
LLBrowse</a> is an interactive viewer for LLVM modules. It can load any LLVM
module and displays its contents as an expandable tree view, facilitating an
easy way to inspect types, functions, global variables, or metadata nodes. It
is fully cross-platform, being based on the popular wxWidgets GUI toolkit.
</p>
</div>
<!--=========================================================================-->
<h3>
<a name="vmkit">VMKit</a>
</h3>
<div>
<p>The <a href="http://vmkit.llvm.org/">VMKit project</a> is an implementation
of a Java Virtual Machine (Java VM or JVM) that uses LLVM for static and
just-in-time compilation. As of LLVM 2.9, VMKit now supports generational
garbage collectors. The garbage collectors are provided by the MMTk framework,
and VMKit can be configured to use one of the numerous implemented collectors
of MMTk.
</p>
</div>
<!--=========================================================================-->
<!--
<h3>
<a name="klee">KLEE: A Symbolic Execution Virtual Machine</a>
</h3>
<div>
<p>
<a href="http://klee.llvm.org/">KLEE</a> is a symbolic execution framework for
programs in LLVM bitcode form. KLEE tries to symbolically evaluate "all" paths
through the application and records state transitions that lead to fault
states. This allows it to construct testcases that lead to faults and can even
be used to verify some algorithms.
</p>
<p>UPDATE!</p>
</div>-->
</div>
<!-- *********************************************************************** -->
<h2>
<a name="externalproj">External Open Source Projects Using LLVM 2.9</a>
</h2>
<!-- *********************************************************************** -->
<div>
<p>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.</p>
<!--=========================================================================-->
<h3>Crack Programming Language</h3>
<div>
<p>
<a href="http://code.google.com/p/crack-language/">Crack</a> aims to provide the
ease of development of a scripting language with the performance of a compiled
language. The language derives concepts from C++, Java and Python, incorporating
object-oriented programming, operator overloading and strong typing.</p>
</div>
<!--=========================================================================-->
<h3>TTA-based Codesign Environment (TCE)</h3>
<div>
<p>TCE is a toolset for designing application-specific processors (ASP) based on
the Transport triggered architecture (TTA). The toolset provides a complete
co-design flow from C/C++ programs down to synthesizable VHDL and parallel
program binaries. Processor customization points include the register files,
function units, supported operations, and the interconnection network.</p>
<p>TCE uses Clang and LLVM for C/C++ language support, target independent
optimizations and also for parts of code generation. It generates new LLVM-based
code generators "on the fly" for the designed TTA processors and loads them in
to the compiler backend as runtime libraries to avoid per-target recompilation
of larger parts of the compiler chain.</p>
</div>
<!--=========================================================================-->
<h3>PinaVM</h3>
<div>
<p><a href="http://gitorious.org/pinavm/pages/Home">PinaVM</a> is an open
source, <a href="http://www.systemc.org/">SystemC</a> front-end. Unlike many
other front-ends, PinaVM actually executes the elaboration of the
program analyzed using LLVM's JIT infrastructure. It later enriches the
bitcode with SystemC-specific information.</p>
</div>
<!--=========================================================================-->
<h3>Pure</h3>
<div>
<p><a href="http://pure-lang.googlecode.com/">Pure</a> is an
algebraic/functional
programming language based on term rewriting. Programs are collections
of equations which are used to evaluate expressions in a symbolic
fashion. The interpreter uses LLVM as a backend to JIT-compile Pure
programs to fast native code. Pure offers dynamic typing, eager and lazy
evaluation, lexical closures, a hygienic macro system (also based on
term rewriting), built-in list and matrix support (including list and
matrix comprehensions) and an easy-to-use interface to C and other
programming languages (including the ability to load LLVM bitcode
modules, and inline C, C++, Fortran and Faust code in Pure programs if
the corresponding LLVM-enabled compilers are installed).</p>
<p>Pure version 0.47 has been tested and is known to work with LLVM 2.9
(and continues to work with older LLVM releases >= 2.5).</p>
</div>
<!--=========================================================================-->
<h3 id="icedtea">IcedTea Java Virtual Machine Implementation</h3>
<div>
<p>
<a href="http://icedtea.classpath.org/wiki/Main_Page">IcedTea</a> provides a
harness to build OpenJDK using only free software build tools and to provide
replacements for the not-yet free parts of OpenJDK. One of the extensions that
IcedTea provides is a new JIT compiler named <a
href="http://icedtea.classpath.org/wiki/ZeroSharkFaq">Shark</a> which uses LLVM
to provide native code generation without introducing processor-dependent
code.
</p>
<p> OpenJDK 7 b112, IcedTea6 1.9 and IcedTea7 1.13 and later have been tested
and are known to work with LLVM 2.9 (and continue to work with older LLVM
releases >= 2.6 as well).</p>
</div>
<!--=========================================================================-->
<h3>Glasgow Haskell Compiler (GHC)</h3>
<div>
<p>GHC is an open source, state-of-the-art programming suite for Haskell,
a standard lazy functional programming language. It includes an
optimizing static compiler generating good code for a variety of
platforms, together with an interactive system for convenient, quick
development.</p>
<p>In addition to the existing C and native code generators, GHC 7.0 now
supports an LLVM code generator. GHC supports LLVM 2.7 and later.</p>
</div>
<!--=========================================================================-->
<h3>Polly - Polyhedral optimizations for LLVM</h3>
<div>
<p>Polly is a project that aims to provide advanced memory access optimizations
to better take advantage of SIMD units, cache hierarchies, multiple cores or
even vector accelerators for LLVM. Built around an abstract mathematical
description based on Z-polyhedra, it provides the infrastructure to develop
advanced optimizations in LLVM and to connect complex external optimizers. In
its first year of existence Polly already provides an exact value-based
dependency analysis as well as basic SIMD and OpenMP code generation support.
Furthermore, Polly can use PoCC(Pluto) an advanced optimizer for data-locality
and parallelism.</p>
</div>
<!--=========================================================================-->
<h3>Rubinius</h3>
<div>
<p><a href="http://github.com/evanphx/rubinius">Rubinius</a> is an environment
for running Ruby code which strives to write as much of the implementation in
Ruby as possible. Combined with a bytecode interpreting VM, it uses LLVM to
optimize and compile ruby code down to machine code. Techniques such as type
feedback, method inlining, and deoptimization are all used to remove dynamism
from ruby execution and increase performance.</p>
</div>
<!--=========================================================================-->
<h3>
<a name="FAUST">FAUST Real-Time Audio Signal Processing Language</a>
</h3>
<div>
<p>
<a href="http://faust.grame.fr">FAUST</a> is a compiled language for real-time
audio signal processing. The name FAUST stands for Functional AUdio STream. Its
programming model combines two approaches: functional programming and block
diagram composition. In addition with the C, C++, JAVA output formats, the
Faust compiler can now generate LLVM bitcode, and works with LLVM 2.7-2.9.</p>
</div>
</div>
<!-- *********************************************************************** -->
<h2>
<a name="whatsnew">What's New in LLVM 2.9?</a>
</h2>
<!-- *********************************************************************** -->
<div>
<p>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.
</p>
<!--=========================================================================-->
<h3>
<a name="majorfeatures">Major New Features</a>
</h3>
<div>
<p>LLVM 2.9 includes several major new capabilities:</p>
<ul>
<li>Type Based Alias Analysis (TBAA) is now implemented and turned on by default
in Clang. This allows substantially better load/store optimization in some
cases. TBAA can be disabled by passing -fno-strict-aliasing.
</li>
<li>This release has seen a continued focus on quality of debug information.
LLVM now generates much higher fidelity debug information, particularly when
debugging optimized code.</li>
<li>Inline assembly now supports multiple alternative constraints.</li>
<li>A new backend for the NVIDIA PTX virtual ISA (used to target its GPUs) is
under rapid development. It is not generally useful in 2.9, but is making
rapid progress.</li>
</ul>
</div>
<!--=========================================================================-->
<h3>
<a name="coreimprovements">LLVM IR and Core Improvements</a>
</h3>
<div>
<p>LLVM IR has several new features for better support of new targets and that
expose new optimization opportunities:</p>
<ul>
<li>The <a href="LangRef.html#bitwiseops">udiv, ashr, lshr, and shl</a>
instructions now have support exact and nuw/nsw bits to indicate that they
don't overflow or shift out bits. This is useful for optimization of <a
href="http://llvm.org/PR8862">pointer differences</a> and other cases.</li>
<li>LLVM IR now supports the <a href="LangRef.html#globalvars">unnamed_addr</a>
attribute to indicate that constant global variables with identical
initializers can be merged. This fixed <a href="http://llvm.org/PR8927">an
issue</a> where LLVM would incorrectly merge two globals which were supposed
to have distinct addresses.</li>
<li>The new <a href="LangRef.html#fnattrs">hotpatch attribute</a> has been added
to allow runtime patching of functions.</li>
</ul>
</div>
<!--=========================================================================-->
<h3>
<a name="optimizer">Optimizer Improvements</a>
</h3>
<div>
<p>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:</p>
<ul>
<li>Link Time Optimization (LTO) has been improved to use MC for parsing inline
assembly and now can build large programs like Firefox 4 on both Mac OS X and
Linux.</li>
<li>The new -loop-idiom pass recognizes memset/memcpy loops (and memset_pattern
on darwin), turning them into library calls, which are typically better
optimized than inline code. If you are building a libc and notice that your
memcpy and memset functions are compiled into infinite recursion, please build
with -ffreestanding or -fno-builtin to disable this pass.</li>
<li>A new -early-cse pass does a fast pass over functions to fold constants,
simplify expressions, perform simple dead store elimination, and perform
common subexpression elimination. It does a good job at catching some of the
trivial redundancies that exist in unoptimized code, making later passes more
effective.</li>
<li>A new -loop-instsimplify pass is used to clean up loop bodies in the loop
optimizer.</li>
<li>The new TargetLibraryInfo interface allows mid-level optimizations to know
whether the current target's runtime library has certain functions. For
example, the optimizer can now transform integer-only printf calls to call
iprintf, allowing reduced code size for embedded C libraries (e.g. newlib).
</li>
<li>LLVM has a new <a href="WritingAnLLVMPass.html#RegionPass">RegionPass</a>
infrastructure for region-based optimizations.</li>
<li>Several optimizer passes have been substantially sped up:
GVN is much faster on functions with deep dominator trees and lots of basic
blocks. The dominator tree and dominance frontier passes are much faster to
compute, and preserved by more passes (so they are computed less often). The
-scalar-repl pass is also much faster and doesn't use DominanceFrontier.
</li>
<li>The Dead Store Elimination pass is more aggressive optimizing stores of
different types: e.g. a large store following a small one to the same address.
The MemCpyOptimizer pass handles several new forms of memcpy elimination.</li>
<li>LLVM now optimizes various idioms for overflow detection into check of the
flag register on various CPUs. For example, we now compile:
<pre>
unsigned long t = a+b;
if (t < a) ...
</pre>
into:
<pre>
addq %rdi, %rbx
jno LBB0_2
</pre>
</li>
</ul>
</div>
<!--=========================================================================-->
<h3>
<a name="mc">MC Level Improvements</a>
</h3>
<div>
<p>
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.</p>
<ul>
<li>ELF MC support has matured enough for the integrated assembler to be turned
on by default in Clang on X86-32 and X86-64 ELF systems.</li>
<li>MC supports and CodeGen uses the <tt>.file</tt> and <tt>.loc</tt> directives
for producing line number debug info. This produces more compact line
tables and easier to read .s files.</li>
<li>MC supports the <tt>.cfi_*</tt> directives for producing DWARF
frame information, but it is still not used by CodeGen by default.</li>
<li>The MC assembler 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.</li>
<li>We now have some basic <a href="CodeGenerator.html#mc">internals
documentation</a> for MC.</li>
<li>.td files can now specify assembler aliases directly with the <a
href="CodeGenerator.html#na_instparsing">MnemonicAlias and InstAlias</a>
tblgen classes.</li>
<li>LLVM now has an experimental format-independent object file manipulation
library (lib/Object). It supports both PE/COFF and ELF. The llvm-nm tool has
been extended to work with native object files, and the new llvm-objdump tool
supports disassembly of object files (but no relocations are displayed yet).
</li>
<li>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.</li>
</ul>
<p>For more information, please see the <a
href="http://blog.llvm.org/2010/04/intro-to-llvm-mc-project.html">Intro to the
LLVM MC Project Blog Post</a>.
</p>
</div>
<!--=========================================================================-->
<h3>
<a name="codegen">Target Independent Code Generator Improvements</a>
</h3>
<div>
<p>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:</p>
<ul>
<li>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 without causing spills to be generated.
</li>
<li>LiveDebugVariables is a new pass that keeps track of debugging information
for user variables that are promoted to registers in optimized builds.</li>
<li>The scheduler now models operand latency and pipeline forwarding.</li>
<li>A major register allocator infrastructure rewrite is underway. It is not on
by default for 2.9 and you are not advised to use it, but it has made
substantial progress in the 2.9 timeframe:
<ul>
<li>A new -regalloc=basic "basic" register allocator can be used as a simple
fallback when debugging. It uses the new infrastructure.</li>
<li>New infrastructure is in place 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.</li>
<li>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.</li>
<li>Rematerialization works with live range splitting.</li>
<li>The 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.</li>
</ul>
</li>
</ul>
</div>
<!--=========================================================================-->
<h3>
<a name="x86">X86-32 and X86-64 Target Improvements</a>
</h3>
<div>
<p>New features and major changes in the X86 target include:
</p>
<ul>
<li>LLVM 2.9 includes a complete reimplementation of the MMX instruction set.
The reimplementation uses a new LLVM IR <a
href="LangRef.html#t_x86mmx">x86_mmx</a> type to ensure that MMX operations
are <em>only</em> generated from source that uses MMX builtin operations. With
this, random types like <2 x i32> are not turned into MMX operations
(which can be catastrophic without proper "emms" insertion). Because the X86
code generator always generates reliable code, the -disable-mmx flag is now
removed.
</li>
<li>X86 support for FS/GS relative loads and stores using <a
href="CodeGenerator.html#x86_memory">address space 256/257</a> works reliably
now.</li>
<li>LLVM 2.9 generates much better code in several cases by using adc/sbb to
avoid generation of conditional move instructions for conditional increment
and other idioms.</li>
<li>The X86 backend has adopted a new preRA scheduling mode, "list-ilp", to
shorten the height of instruction schedules without inducing register spills.
</li>
<li>The MC assembler supports 3dNow! and 3DNowA instructions.</li>
<li>Several bugs have been fixed for Windows x64 code generator.</li>
</ul>
</div>
<!--=========================================================================-->
<h3>
<a name="ARM">ARM Target Improvements</a>
</h3>
<div>
<p>New features of the ARM target include:
</p>
<ul>
<li>The ARM backend now has a fast instruction selector, which dramatically
improves -O0 compile times.</li>
<li>The ARM backend has new tuning for Cortex-A8 and Cortex-A9 CPUs.</li>
<li>The __builtin_prefetch builtin (and llvm.prefetch intrinsic) is compiled
into prefetch instructions instead of being discarded.</li>
<li> 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.</li>
<li>Countless ARM microoptimizations have landed in LLVM 2.9.</li>
</ul>
</div>
<!--=========================================================================-->
<h3>
<a name="OtherTS">Other Target Specific Improvements</a>
</h3>
<div>
<ul>
<li>MicroBlaze: major updates for aggressive delay slot filler, MC-based
assembly printing, assembly instruction parsing, ELF .o file emission, and MC
instruction disassembler have landed.</li>
<li>SPARC: Many improvements, including using the Y registers for
multiplications and addition of a simple delay slot filler.</li>
<li>PowerPC: The backend has been largely MC'ized and is ready to support
directly writing out mach-o object files. No one seems interested in finishing
this final step though.</li>
<li>Mips: Improved o32 ABI support, including better varags handling.
More instructions supported in codegen: madd, msub, rotr, rotrv and clo.
It also now supports lowering block addresses.</li>
</ul>
</div>
<!--=========================================================================-->
<h3>
<a name="changes">Major Changes and Removed Features</a>
</h3>
<div>
<p>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.</p>
<ul>
<li><b>This is the last release to support the llvm-gcc frontend.</b></li>
<li>LLVM has a new <a href="CodingStandards.html#ll_naming">naming
convention standard</a>, though the codebase hasn't fully adopted it yet.</li>
<li>The new DIBuilder class provides a simpler interface for front ends to
encode debug info in LLVM IR, and has replaced DIFactory.</li>
<li>LLVM IR and other tools always work on normalized target triples (which have
been run through <tt>Triple::normalize</tt>).</li>
<li>The target triple x86_64--mingw64 is obsoleted. Use x86_64--mingw32
instead.</li>
<li>The PointerTracking pass has been removed from mainline, and moved to The
ClamAV project (its only client).</li>
<li>The LoopIndexSplit, LiveValues, SimplifyHalfPowrLibCalls, GEPSplitter, and
PartialSpecialization passes were removed. They were unmaintained,
buggy, or deemed to be a bad idea.</li>
</ul>
</div>
<!--=========================================================================-->
<h3>
<a name="api_changes">Internal API Changes</a>
</h3>
<div>
<p>In addition, many APIs have changed in this release. Some of the major
LLVM API changes are:</p>
<ul>
<li>include/llvm/System merged into include/llvm/Support.</li>
<li>The <a href="http://llvm.org/PR5207">llvm::APInt API</a> was significantly
cleaned up.</li>
<li>In the code generator, MVT::Flag was renamed to MVT::Glue to more accurately
describe its behavior.</li>
<li>The system_error header from C++0x was added, and is now pervasively used to
capture and handle i/o and other errors in LLVM.</li>
<li>The old sys::Path API has been deprecated in favor of the new PathV2 API,
which is more efficient and flexible.</li>
</ul>
</div>
</div>
<!-- *********************************************************************** -->
<h2>
<a name="knownproblems">Known Problems</a>
</h2>
<!-- *********************************************************************** -->
<div>
<p>This section contains significant known problems with the LLVM system,
listed by component. If you run into a problem, please check the <a
href="http://llvm.org/bugs/">LLVM bug database</a> and submit a bug if
there isn't already one.</p>
<!-- ======================================================================= -->
<h3>
<a name="experimental">Experimental features included with this release</a>
</h3>
<div>
<p>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 <a
href="http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev">LLVMdev list</a>.</p>
<ul>
<li>The Alpha, Blackfin, CellSPU, MicroBlaze, MSP430, MIPS, PTX, SystemZ
and XCore backends are experimental.</li>
<li><tt>llc</tt> "<tt>-filetype=obj</tt>" is experimental on all targets
other than darwin and ELF X86 systems.</li>
</ul>
</div>
<!-- ======================================================================= -->
<h3>
<a name="x86-be">Known problems with the X86 back-end</a>
</h3>
<div>
<ul>
<li>The X86 backend does not yet support
all <a href="http://llvm.org/PR879">inline assembly that uses the X86
floating point stack</a>. It supports the 'f' and 't' constraints, but not
'u'.</li>
<li>The X86-64 backend does not yet support the LLVM IR instruction
<tt>va_arg</tt>. Currently, front-ends support variadic
argument constructs on X86-64 by lowering them manually.</li>
<li>Windows x64 (aka Win64) code generator has a few issues.
<ul>
<li>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.</li>
<li>On mingw-w64, you will see unresolved symbol <tt>__chkstk</tt>
due to <a href="http://llvm.org/bugs/show_bug.cgi?id=8919">Bug 8919</a>.
It is fixed in <a href="http://lists.cs.uiuc.edu/pipermail/llvm-commits/Week-of-Mon-20110321/118499.html">r128206</a>.</li>
<li>Miss-aligned MOVDQA might crash your program. It is due to
<a href="http://llvm.org/bugs/show_bug.cgi?id=9483">Bug 9483</a>,
lack of handling aligned internal globals.</li>
</ul>
</li>
</ul>
</div>
<!-- ======================================================================= -->
<h3>
<a name="ppc-be">Known problems with the PowerPC back-end</a>
</h3>
<div>
<ul>
<li>The Linux PPC32/ABI support needs testing for the interpreter and static
compilation, and lacks support for debug information.</li>
</ul>
</div>
<!-- ======================================================================= -->
<h3>
<a name="arm-be">Known problems with the ARM back-end</a>
</h3>
<div>
<ul>
<li>Thumb mode works only on ARMv6 or higher processors. On sub-ARMv6
processors, thumb programs can crash or produce wrong
results (<a href="http://llvm.org/PR1388">PR1388</a>).</li>
<li>Compilation for ARM Linux OABI (old ABI) is supported but not fully tested.
</li>
</ul>
</div>
<!-- ======================================================================= -->
<h3>
<a name="sparc-be">Known problems with the SPARC back-end</a>
</h3>
<div>
<ul>
<li>The SPARC backend only supports the 32-bit SPARC ABI (-m32); it does not
support the 64-bit SPARC ABI (-m64).</li>
</ul>
</div>
<!-- ======================================================================= -->
<h3>
<a name="mips-be">Known problems with the MIPS back-end</a>
</h3>
<div>
<ul>
<li>64-bit MIPS targets are not supported yet.</li>
</ul>
</div>
<!-- ======================================================================= -->
<h3>
<a name="alpha-be">Known problems with the Alpha back-end</a>
</h3>
<div>
<ul>
<li>On 21164s, some rare FP arithmetic sequences which may trap do not have the
appropriate nops inserted to ensure restartability.</li>
</ul>
</div>
<!-- ======================================================================= -->
<h3>
<a name="c-be">Known problems with the C back-end</a>
</h3>
<div>
<p>The C backend has numerous problems and is not being actively maintained.
Depending on it for anything serious is not advised.</p>
<ul>
<li><a href="http://llvm.org/PR802">The C backend has only basic support for
inline assembly code</a>.</li>
<li><a href="http://llvm.org/PR1658">The C backend violates the ABI of common
C++ programs</a>, preventing intermixing between C++ compiled by the CBE and
C++ code compiled with <tt>llc</tt> or native compilers.</li>
<li>The C backend does not support all exception handling constructs.</li>
<li>The C backend does not support arbitrary precision integers.</li>
</ul>
</div>
<!-- ======================================================================= -->
<h3>
<a name="llvm-gcc">Known problems with the llvm-gcc front-end</a>
</h3>
<div>
<p><b>LLVM 2.9 will be the last release of llvm-gcc.</b></p>
<p>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
<tt>__builtin_apply</tt> 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).</p>
<p>Fortran support generally works, but there are still several unresolved bugs
in <a href="http://llvm.org/bugs/">Bugzilla</a>. Please see the
tools/gfortran component for details. Note that llvm-gcc is missing major