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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> + + +<!-- +Almost dead code. + include/llvm/Analysis/LiveValues.h => Dan + lib/Transforms/IPO/MergeFunctions.cpp => consider for 2.8. + llvm/Analysis/PointerTracking.h => Edwin wants this, consider for 2.8. + ABCD, GEPSplitterPass + MSIL backend? + lib/Transforms/Utils/SSI.cpp -> ABCD depends on it. +--> + + +<!-- Features that need text if they're finished for 2.8: + combiner-aa? + strong phi elim + llvm.dbg.value: variable debug info for optimized code + loop dependence analysis + --> + + <!-- for announcement email: + Logo web page. + Many new papers added to /pubs/ + --> + +<!-- *********************************************************************** --> +<div class="doc_section"> + <a name="subproj">Sub-project Status Update</a> +</div> +<!-- *********************************************************************** --> + +<div class="doc_text"> +<p> +The LLVM 2.8 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> + +</div> + + +<!--=========================================================================--> +<div class="doc_subsection"> +<a name="clang">Clang: C/C++/Objective-C Frontend Toolkit</a> +</div> + +<div class="doc_text"> + +<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 and Objective-C on x86 (32- and 64-bit).</p> + +<p>In the LLVM 2.7 time-frame, the Clang team has made many improvements:</p> + +<ul> + +<li>C++ Support: Clang is now capable of self-hosting! While still +alpha-quality, Clang's C++ support has matured enough to build LLVM and Clang, +and C++ is now enabled by default. See the <a +href="http://clang.llvm.org/cxx_compatibility.html">Clang C++ compatibility +page</a> for common C++ migration issues.</li> + +<li>Objective-C: Clang now includes experimental support for an updated +Objective-C ABI on non-Darwin platforms. This includes support for non-fragile +instance variables and accelerated proxies, as well as greater potential for +future optimisations. The new ABI is used when compiling with the +-fobjc-nonfragile-abi and -fgnu-runtime options. Code compiled with these +options may be mixed with code compiled with GCC or clang using the old GNU ABI, +but requires the libobjc2 runtime from the GNUstep project.</li> + +<li>New warnings: Clang contains a number of new warnings, including +control-flow warnings (unreachable code, missing return statements in a +non-<code>void</code> function, etc.), sign-comparison warnings, and improved +format-string warnings.</li> + +<li>CIndex API and Python bindings: Clang now includes a C API as part of the +CIndex library. Although we may make some changes to the API in the future, it +is intended to be stable and has been designed for use by external projects. See +the Clang +doxygen <a href="http://clang.llvm.org/doxygen/group__CINDEX.html">CIndex</a> +documentation for more details. The CIndex API also includes a preliminary +set of Python bindings.</li> + +<li>ARM Support: Clang now has ABI support for both the Darwin and Linux ARM +ABIs. Coupled with many improvements to the LLVM ARM backend, Clang is now +suitable for use as a beta quality ARM compiler.</li> + +</ul> +</div> + +<!--=========================================================================--> +<div class="doc_subsection"> +<a name="clangsa">Clang Static Analyzer</a> +</div> + +<div class="doc_text"> + +<p>The <a href="http://clang-analyzer.llvm.org/">Clang Static Analyzer</a> + project is an effort to use static source code analysis techniques to + automatically find bugs in C and Objective-C programs (and hopefully <a + href="http://clang-analyzer.llvm.org/dev_cxx.html">C++ in the + future</a>!). The tool is very good at finding bugs that occur on specific + paths through code, such as on error conditions.</p> + +<p>In the LLVM 2.7 time-frame, the analyzer core has made several major and + minor improvements, including better support for tracking the fields of + structures, initial support (not enabled by default yet) for doing + interprocedural (cross-function) analysis, and new checks have been added. +</p> + +</div> + +<!--=========================================================================--> +<div class="doc_subsection"> +<a name="vmkit">VMKit: JVM/CLI Virtual Machine Implementation</a> +</div> + +<div class="doc_text"> +<p> +The <a href="http://vmkit.llvm.org/">VMKit project</a> is an implementation of +a JVM and a CLI Virtual Machine (Microsoft .NET is an +implementation of the CLI) using LLVM for static and just-in-time +compilation.</p> + +<p> +With the release of LLVM 2.7, VMKit has shifted to a great framework for writing +virtual machines. VMKit now offers precise and efficient garbage collection with +multi-threading support, thanks to the MMTk memory management toolkit, as well +as just in time and ahead of time compilation with LLVM. The major changes in +VMKit 0.27 are:</p> + +<ul> + +<li>Garbage collection: VMKit now uses the MMTk toolkit for garbage collectors. + The first collector to be ported is the MarkSweep collector, which is precise, + and drastically improves the performance of VMKit.</li> +<li>Line number information in the JVM: by using the debug metadata of LLVM, the + JVM now supports precise line number information, useful when printing a stack + trace.</li> +<li>Interface calls in the JVM: we implemented a variant of the Interface Method + Table technique for interface calls in the JVM. +</li> + +</ul> +</div> + + +<!--=========================================================================--> +<div class="doc_subsection"> +<a name="compiler-rt">compiler-rt: Compiler Runtime Library</a> +</div> + +<div class="doc_text"> +<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> +All of the code in the compiler-rt project is available under the standard LLVM +License, a "BSD-style" license. New in LLVM 2.7: compiler_rt now +supports ARM targets.</p> + +</div> + +<!--=========================================================================--> +<div class="doc_subsection"> +<a name="dragonegg">DragonEgg: llvm-gcc ported to gcc-4.5</a> +</div> + +<div class="doc_text"> +<p> +<a href="http://dragonegg.llvm.org/">DragonEgg</a> is a port of llvm-gcc to +gcc-4.5. Unlike llvm-gcc, which makes many intrusive changes to the underlying +gcc-4.2 code, dragonegg in theory does not require any gcc-4.5 modifications +whatsoever (currently one small patch is needed). This is thanks to the new +<a href="http://gcc.gnu.org/wiki/plugins">gcc plugin architecture</a>, which +makes it possible to modify the behaviour of gcc at runtime by loading a plugin, +which is nothing more than a dynamic library which conforms to the gcc plugin +interface. DragonEgg is a gcc plugin that causes the LLVM optimizers to be run +instead of the gcc optimizers, and the LLVM code generators instead of the gcc +code generators, just like llvm-gcc. To use it, you add +"-fplugin=path/dragonegg.so" to the gcc-4.5 command line, and gcc-4.5 magically +becomes llvm-gcc-4.5! +</p> + +<p> +DragonEgg is still a work in progress. Currently C works very well, while C++, +Ada and Fortran work fairly well. All other languages either don't work at all, +or only work poorly. For the moment only the x86-32 and x86-64 targets are +supported, and only on linux and darwin (darwin needs an additional gcc patch). +</p> + +<p> +DragonEgg is a new project which is seeing its first release with llvm-2.7. +</p> + +</div> + + +<!--=========================================================================--> +<div class="doc_subsection"> +<a name="mc">llvm-mc: Machine Code Toolkit</a> +</div> + +<div class="doc_text"> +<p> +The LLVM Machine Code (aka MC) sub-project of LLVM 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. It is a sub-project of LLVM which provides it with a number of advantages +over other compilers that do not have tightly integrated assembly-level tools. +For a gentle introduction, 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> + +<p>2.7 includes major parts of the work required by the new MC Project. A few + targets have been refactored to support it, and work is underway to support a + native assembler in LLVM. This work is not complete in LLVM 2.7, but it has + made substantially more progress on LLVM mainline.</p> + +<p>One minor example of what MC can do is to transcode an AT&T syntax + X86 .s file into intel syntax. You can do this with something like:</p> +<pre> + llvm-mc foo.s -output-asm-variant=1 -o foo-intel.s +</pre> + +</div> + + +<!-- *********************************************************************** --> +<div class="doc_section"> + <a name="externalproj">External Open Source Projects Using LLVM 2.7</a> +</div> +<!-- *********************************************************************** --> + +<div class="doc_text"> + +<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 2.7.</p> +</div> + +<!--=========================================================================--> +<div class="doc_subsection"> +<a name="pure">Pure</a> +</div> + +<div class="doc_text"> +<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. 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 C interface. The interpreter uses LLVM as a backend to + JIT-compile Pure programs to fast native code.</p> + +<p>Pure versions 0.43 and later have been tested and are known to work with +LLVM 2.7 (and continue to work with older LLVM releases >= 2.5).</p> + +</div> + +<!--=========================================================================--> +<div class="doc_subsection"> +<a name="RoadsendPHP">Roadsend PHP</a> +</div> + +<div class="doc_text"> +<p> +<a href="http://code.roadsend.com/rphp">Roadsend PHP</a> (rphp) is an open +source implementation of the PHP programming +language that uses LLVM for its optimizer, JIT and static compiler. This is a +reimplementation of an earlier project that is now based on LLVM. +</p> +</div> + +<!--=========================================================================--> +<div class="doc_subsection"> +<a name="UnladenSwallow">Unladen Swallow</a> +</div> + +<div class="doc_text"> +<p> +<a href="http://code.google.com/p/unladen-swallow/">Unladen Swallow</a> is a +branch of <a href="http://python.org/">Python</a> intended to be fully +compatible and significantly faster. It uses LLVM's optimization passes and JIT +compiler. +</p> +</div> + +<!--=========================================================================--> +<div class="doc_subsection"> +<a name="tce">TTA-based Codesign Environment (TCE)</a> +</div> + +<div class="doc_text"> +<p> +<a href="http://tce.cs.tut.fi/">TCE</a> 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 llvm-gcc/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> + +<!--=========================================================================--> +<div class="doc_subsection"> +<a name="safecode">SAFECode Compiler</a> +</div> + +<div class="doc_text"> +<p> +<a href="http://safecode.cs.illinois.edu">SAFECode</a> is a memory safe C +compiler built using LLVM. It takes standard, unannotated C code, analyzes the +code to ensure that memory accesses and array indexing operations are safe, and +instruments the code with run-time checks when safety cannot be proven +statically. +</p> +</div> + +<!--=========================================================================--> +<div class="doc_subsection"> +<a name="icedtea">IcedTea Java Virtual Machine Implementation</a> +</div> + +<div class="doc_text"> +<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>Icedtea6 1.8 and later have been tested and are known to work with +LLVM 2.7 (and continue to work with older LLVM releases >= 2.6 as well). +</p> +</div> + +<!--=========================================================================--> +<div class="doc_subsection"> +<a name="llvm-lua">LLVM-Lua</a> +</div> + +<div class="doc_text"> +<p> +<a href="http://code.google.com/p/llvm-lua/">LLVM-Lua</a> uses LLVM + to add JIT and static compiling support to the Lua VM. Lua +bytecode is analyzed to remove type checks, then LLVM is used to compile the +bytecode down to machine code. +</p> +<p>LLVM-Lua 1.2.0 have been tested and is known to work with LLVM 2.7. +</p> +</div> + +<!--=========================================================================--> +<div class="doc_subsection"> +<a name="MacRuby">MacRuby</a> +</div> + +<div class="doc_text"> +<p> +<a href="http://macruby.org">MacRuby</a> is an implementation of Ruby based on +core Mac OS technologies, sponsored by Apple Inc. It uses LLVM at runtime for +optimization passes, JIT compilation and exception handling. It also allows +static (ahead-of-time) compilation of Ruby code straight to machine code. +</p> +<p>The upcoming MacRuby 0.6 release works with LLVM 2.7. +</p> +</div> + +<!--=========================================================================--> +<div class="doc_subsection"> +<a name="GHC">Glasgow Haskell Compiler (GHC)</a> +</div> + +<div class="doc_text"> +<p> +<a href="http://www.haskell.org/ghc/">GHC</a> 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 now +supports an <a +href="http://hackage.haskell.org/trac/ghc/wiki/Commentary/Compiler/Backends/LLVM">LLVM +code generator</a>. GHC supports LLVM 2.7.</p> + +</div> + + +<!-- *********************************************************************** --> +<div class="doc_section"> + <a name="whatsnew">What's New in LLVM 2.7?</a> +</div> +<!-- *********************************************************************** --> + +<div class="doc_text"> + +<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> + +</div> + +<!--=========================================================================--> +<div class="doc_subsection"> +<a name="orgchanges">LLVM Community Changes</a> +</div> + +<div class="doc_text"> + +<p>In addition to changes to the code, between LLVM 2.6 and 2.7, a number of +organization changes have happened: +</p> + +<ul> +<li>LLVM has a new <a href="http://llvm.org/Logo.html">official logo</a>!</li> + +<li>Ted Kremenek and Doug Gregor have stepped forward as <a + href="http://llvm.org/docs/DeveloperPolicy.html#owners">Code Owners</a> of the + Clang static analyzer and the Clang frontend, respectively.</li> + +<li>LLVM now has an <a href="http://blog.llvm.org">official Blog</a> at + <a href="http://blog.llvm.org">http://blog.llvm.org</a>. This is a great way + to learn about new LLVM-related features as they are implemented. Several + features in this release are already explained on the blog.</li> + +<li>The LLVM web pages are now checked into the SVN server, in the "www", + "www-pubs" and "www-releases" SVN modules. Previously they were hidden in a + largely inaccessible old CVS server.</li> + +<li><a href="http://llvm.org">llvm.org</a> is now hosted on a new (and much + faster) server. It is still graciously hosted at the University of Illinois + of Urbana Champaign.</li> +</ul> +</div> + +<!--=========================================================================--> +<div class="doc_subsection"> +<a name="majorfeatures">Major New Features</a> +</div> + +<div class="doc_text"> + +<p>LLVM 2.7 includes several major new capabilities:</p> + +<ul> +<li>2.7 includes initial support for the <a + href="http://en.wikipedia.org/wiki/MicroBlaze">MicroBlaze</a> target. + MicroBlaze is a soft processor core designed for Xilinx FPGAs.</li> + +<li>2.7 includes a new LLVM IR "extensible metadata" feature. This feature + supports many different use cases, including allowing front-end authors to + encode source level information into LLVM IR, which is consumed by later + language-specific passes. This is a great way to do high-level optimizations + like devirtualization, type-based alias analysis, etc. See the <a + href="http://blog.llvm.org/2010/04/extensible-metadata-in-llvm-ir.html"> + Extensible Metadata Blog Post</a> for more information.</li> + +<li>2.7 encodes <a href="SourceLevelDebugging.html">debug information</a> +in a completely new way, built on extensible metadata. The new implementation +is much more memory efficient and paves the way for improvements to optimized +code debugging experience.</li> + +<li>2.7 now directly supports taking the address of a label and doing an + indirect branch through a pointer. This is particularly useful for + interpreter loops, and is used to implement the GCC "address of label" + extension. For more information, see the <a +href="http://blog.llvm.org/2010/01/address-of-label-and-indirect-branches.html"> +Address of Label and Indirect Branches in LLVM IR Blog Post</a>. + +<li>2.7 is the first release to start supporting APIs for assembling and + disassembling target machine code. These APIs are useful for a variety of + low level clients, and are surfaced in the new "enhanced disassembly" API. + For more information see the <a + href="http://blog.llvm.org/2010/01/x86-disassembler.html">The X86 + Disassembler Blog Post</a> for more information.</li> + +<li>2.7 includes major parts of the work required by the new MC Project, + see the <a href="#mc">MC update above</a> for more information.</li> + +</ul> + +</div> + +<!--=========================================================================--> +<div class="doc_subsection"> +<a name="coreimprovements">LLVM IR and Core Improvements</a> +</div> + +<div class="doc_text"> +<p>LLVM IR has several new features for better support of new targets and that +expose new optimization opportunities:</p> + +<ul> +<li>LLVM IR now supports a 16-bit "half float" data type through <a + href="LangRef.html#int_fp16">two new intrinsics</a> and APFloat support.</li> +<li>LLVM IR supports two new <a href="LangRef.html#fnattrs">function + attributes</a>: inlinehint and alignstack(n). The former is a hint to the + optimizer that a function was declared 'inline' and thus the inliner should + weight it higher when considering inlining it. The later + indicates to the code generator that the function diverges from the platform + ABI on stack alignment.</li> +<li>The new <a href="LangRef.html#int_objectsize">llvm.objectsize</a> intrinsic + allows the optimizer to infer the sizes of memory objects in some cases. + This intrinsic is used to implement the GCC <tt>__builtin_object_size</tt> + extension.</li> +<li>LLVM IR now supports marking load and store instructions with <a + href="LangRef.html#i_load">"non-temporal" hints</a> (building on the new + metadata feature). This hint encourages the code + generator to generate non-temporal accesses when possible, which are useful + for code that is carefully managing cache behavior. Currently, only the + X86 backend provides target support for this feature.</li> + +<li>LLVM 2.7 has pre-alpha support for <a + href="LangRef.html#t_union">unions in LLVM IR</a>. + Unfortunately, this support is not really usable in 2.7, so if you're + interested in pushing it forward, please help contribute to LLVM mainline.</li> + +</ul> + +</div> + +<!--=========================================================================--> +<div class="doc_subsection"> +<a name="optimizer">Optimizer Improvements</a> +</div> + +<div class="doc_text"> + +<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>The inliner now merges arrays stack objects in different callees when + inlining multiple call sites into one function. This reduces the stack size + of the resultant function.</li> +<li>The -basicaa alias analysis pass (which is the default) has been improved to + be less dependent on "type safe" pointers. It can now look through bitcasts + and other constructs more aggressively, allowing better load/store + optimization.</li> +<li>The load elimination optimization in the GVN Pass [<a +href="http://blog.llvm.org/2009/12/introduction-to-load-elimination-in-gvn.html">intro + blog post</a>] has been substantially improved to be more aggressive about + partial redundancy elimination and do more aggressive phi translation. Please + see the <a + href="http://blog.llvm.org/2009/12/advanced-topics-in-redundant-load.html"> + Advanced Topics in Redundant Load Elimination with a Focus on PHI Translation + Blog Post</a> for more details.</li> +<li>The module <a href="LangRef.html#datalayout">target data string</a> now + includes a notion of 'native' integer data types for the target. This + helps mid-level optimizations avoid promoting complex sequences of + operations to data types that are not natively supported (e.g. converting + i32 operations to i64 on 32-bit chips).</li> +<li>The mid-level optimizer is now conservative when operating on a module with + no target data. Previously, it would default to SparcV9 settings, which is + not what most people expected.</li> +<li>Jump threading is now much more aggressive at simplifying correlated + conditionals and threading blocks with otherwise complex logic. It has + subsumed the old "Conditional Propagation" pass, and -condprop has been + removed from LLVM 2.7.</li> +<li>The -instcombine pass has been refactored from being one huge file to being + a library of its own. Internally, it uses a customized IRBuilder to clean + it up and simplify it.</li> + +<li>The optimal edge profiling pass is reliable and much more complete than in + 2.6. It can be used with the llvm-prof tool but isn't wired up to the + llvm-gcc and clang command line options yet.</li> + +<li>A new experimental alias analysis implementation, -scev-aa, has been added. + It uses LLVM's Scalar Evolution implementation to do symbolic analysis of + pointer offset expressions to disambiguate pointers. It can catch a few + cases that basicaa cannot, particularly in complex loop nests.</li> + +<li>The default pass ordering has been tweaked for improved optimization + effectiveness.</li> + +</ul> + +</div> + + +<!--=========================================================================--> +<div class="doc_subsection"> +<a name="executionengine">Interpreter and JIT Improvements</a> +</div> + +<div class="doc_text"> + +<ul> +<li>The JIT now supports generating debug information and is compatible with +the new GDB 7.0 (and later) interfaces for registering dynamically generated +debug info.</li> + +<li>The JIT now <a href="http://llvm.org/PR5184">defaults +to compiling eagerly</a> to avoid a race condition in the lazy JIT. +Clients that still want the lazy JIT can switch it on by calling +<tt>ExecutionEngine::DisableLazyCompilation(false)</tt>.</li> + +<li>It is now possible to create more than one JIT instance in the same process. +These JITs can generate machine code in parallel, +although <a href="http://llvm.org/docs/ProgrammersManual.html#jitthreading">you +still have to obey the other threading restrictions</a>.</li> + +</ul> + +</div> + +<!--=========================================================================--> +<div class="doc_subsection"> +<a name="codegen">Target Independent Code Generator Improvements</a> +</div> + +<div class="doc_text"> + +<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 'llc -asm-verbose' option (which is now the default) has been enhanced + to emit many useful comments to .s files indicating information about spill + slots and loop nest structure. This should make it much easier to read and + understand assembly files. This is wired up in llvm-gcc and clang to + the <tt>-fverbose-asm</tt> option.</li> + +<li>New LSR with "full strength reduction" mode, which can reduce address + register pressure in loops where address generation is important.</li> + +<li>A new codegen level Common Subexpression Elimination pass (MachineCSE) + is available and enabled by default. It catches redundancies exposed by + lowering.</li> +<li>A new pre-register-allocation tail duplication pass is available and enabled + by default, it can substantially improve branch prediction quality in some + cases.</li> +<li>A new sign and zero extension optimization pass (OptimizeExtsPass) + is available and enabled by default. This pass can takes advantage + architecture features like x86-64 implicit zero extension behavior and + sub-registers.</li> +<li>The code generator now supports a mode where it attempts to preserve the + order of instructions in the input code. This is important for source that + is hand scheduled and extremely sensitive to scheduling. It is compatible + with the GCC <tt>-fno-schedule-insns</tt> option.</li> +<li>The target-independent code generator now supports generating code with + arbitrary numbers of result values. Returning more values than was + previously supported is handled by returning through a hidden pointer. In + 2.7, only the X86 and XCore targets have adopted support for this + though.</li> +<li>The code generator now supports generating code that follows the + <a href="LangRef.html#callingconv">Glasgow Haskell Compiler Calling + Convention</a> and ABI.</li> +<li>The "<a href="CodeGenerator.html#selectiondag_select">DAG instruction + selection</a>" phase of the code generator has been largely rewritten for + 2.7. Previously, tblgen spit out tons of C++ code which was compiled and + linked into the target to do the pattern matching, now it emits a much + smaller table which is read by the target-independent code. The primary + advantages of this approach is that the size and compile time of various + targets is much improved. The X86 code generator shrunk by 1.5MB of code, + for example.</li> +<li>Almost the entire code generator has switched to emitting code through the + MC interfaces instead of printing textually to the .s file. This led to a + number of cleanups and speedups. In 2.7, debug an exception handling + information does not go through MC yet.</li> +</ul> +</div> + +<!--=========================================================================--> +<div class="doc_subsection"> +<a name="x86">X86-32 and X86-64 Target Improvements</a> +</div> + +<div class="doc_text"> +<p>New features of the X86 target include: +</p> + +<ul> +<li>The X86 backend now optimizes tails calls much more aggressively for + functions that use the standard C calling convention.</li> +<li>The X86 backend now models scalar SSE registers as subregs of the SSE vector + registers, making the code generator more aggressive in cases where scalars + and vector types are mixed.</li> + +</ul> + +</div> + +<!--=========================================================================--> +<div class="doc_subsection"> +<a name="ARM">ARM Target Improvements</a> +</div> + +<div class="doc_text"> +<p>New features of the ARM target include: +</p> + +<ul> + +<li>The ARM backend now generates instructions in unified assembly syntax.</li> + +<li>llvm-gcc now has complete support for the ARM v7 NEON instruction set. This + support differs slightly from the GCC implementation. Please see the + <a +href="http://blog.llvm.org/2010/04/arm-advanced-simd-neon-intrinsics-and.html"> + ARM Advanced SIMD (NEON) Intrinsics and Types in LLVM Blog Post</a> for + helpful information if migrating code from GCC to LLVM-GCC.</li> + +<li>The ARM and Thumb code generators now use register scavenging for stack + object address materialization. This allows the use of R3 as a general + purpose register in Thumb1 code, as it was previous reserved for use in + stack address materialization. Secondly, sequential uses of the same + value will now re-use the materialized constant.</li> + +<li>The ARM backend now has good support for ARMv4 targets and has been tested + on StrongARM hardware. Previously, LLVM only supported ARMv4T and + newer chips.</li> + +<li>Atomic builtins are now supported for ARMv6 and ARMv7 (__sync_synchronize, + __sync_fetch_and_add, etc.).</li> + +</ul> + + +</div> + +<!--=========================================================================--> +<div class="doc_subsection"> +<a name="newapis">New Useful APIs</a> +</div> + +<div class="doc_text"> + +<p>This release includes a number of new APIs that are used internally, which + may also be useful for external clients. +</p> + +<ul> +<li>The optimizer uses the new CodeMetrics class to measure the size of code. + Various passes (like the inliner, loop unswitcher, etc) all use this to make + more accurate estimates of the code size impact of various + optimizations.</li> +<li>A new <a href="http://llvm.org/doxygen/InstructionSimplify_8h-source.html"> + llvm/Analysis/InstructionSimplify.h</a> interface is available for doing + symbolic simplification of instructions (e.g. <tt>a+0</tt> -> <tt>a</tt>) + without requiring the instruction to exist. This centralizes a lot of + ad-hoc symbolic manipulation code scattered in various passes.</li> +<li>The optimizer now uses a new <a + href="http://llvm.org/doxygen/SSAUpdater_8h-source.html">SSAUpdater</a> + class which efficiently supports + doing unstructured SSA update operations. This centralized a bunch of code + scattered throughout various passes (e.g. jump threading, lcssa, + loop rotate, etc) for doing this sort of thing. The code generator has a + similar <a href="http://llvm.org/doxygen/MachineSSAUpdater_8h-source.html"> + MachineSSAUpdater</a> class.</li> +<li>The <a href="http://llvm.org/doxygen/Regex_8h-source.html"> + llvm/Support/Regex.h</a> header exposes a platform independent regular + expression API. Building on this, the <a + href="TestingGuide.html#FileCheck">FileCheck</a> utility now supports + regular exressions.</li> +<li>raw_ostream now supports a circular "debug stream" accessed with "dbgs()". + By default, this stream works the same way as "errs()", but if you pass + <tt>-debug-buffer-size=1000</tt> to opt, the debug stream is capped to a + fixed sized circular buffer and the output is printed at the end of the + program's execution. This is helpful if you have a long lived compiler + process and you're interested in seeing snapshots in time.</li> +</ul> + + +</div> + +<!--=========================================================================--> +<div class="doc_subsection"> +<a name="otherimprovements">Other Improvements and New Features</a> +</div> + +<div class="doc_text"> +<p>Other miscellaneous features include:</p> + +<ul> +<li>You can now build LLVM as a big dynamic library (e.g. "libllvm2.7.so"). To + get this, configure LLVM with the --enable-shared option.</li> + +<li>LLVM command line tools now overwrite their output by default. Previously, + they would only do this with -f. This makes them more convenient to use, and + behave more like standard unix tools.</li> + +<li>The opt and llc tools now autodetect whether their input is a .ll or .bc + file, and automatically do the right thing. This means you don't need to + explicitly use the llvm-as tool for most things.</li> +</ul> + +</div> + + +<!--=========================================================================--> +<div class="doc_subsection"> +<a name="changes">Major Changes and Removed Features</a> +</div> + +<div class="doc_text"> + +<p>If you're already an LLVM user or developer with out-of-tree changes based +on LLVM 2.6, this section lists some "gotchas" that you may run into upgrading +from the previous release.</p> + +<ul> + +<li> +The Andersen's alias analysis ("anders-aa") pass, the Predicate Simplifier +("predsimplify") pass, the LoopVR pass, the GVNPRE pass, and the random sampling +profiling ("rsprofiling") passes have all been removed. They were not being +actively maintained and had substantial problems. If you are interested in +these components, you are welcome to ressurect them from SVN, fix the +correctness problems, and resubmit them to mainline.</li> + +<li>LLVM now defaults to building most libraries with RTTI turned off, providing +a code size reduction. Packagers who are interested in building LLVM to support +plugins that require RTTI information should build with "make REQUIRE_RTTI=1" +and should read the new <a href="Packaging.html">Advice on Packaging LLVM</a> +document.</li> + +<li>The LLVM interpreter now defaults to <em>not</em> using <tt>libffi</tt> even +if you have it installed. This makes it more likely that an LLVM built on one +system will work when copied to a similar system. To use <tt>libffi</tt>, +configure with <tt>--enable-libffi</tt>.</li> + +<li>Debug information uses a completely different representation, an LLVM 2.6 +.bc file should work with LLVM 2.7, but debug info won't come forward.</li> + +<li>The LLVM 2.6 (and earlier) "malloc" and "free" instructions got removed, + along with LowerAllocations pass. Now you should just use a call to the + malloc and free functions in libc. These calls are optimized as well as + the old instructions were.</li> +</ul> + +<p>In addition, many APIs have changed in this release. Some of the major LLVM +API changes are:</p> + +<ul> + +<li>The <tt>add</tt>, <tt>sub</tt>, and <tt>mul</tt> instructions no longer +support floating-point operands. The <tt>fadd</tt>, <tt>fsub</tt>, and +<tt>fmul</tt> instructions should be used for this purpose instead.</li> + +</ul> + +</div> + + + +<!-- *********************************************************************** --> +<div class="doc_section"> + <a name="portability">Portability and Supported Platforms</a> +</div> +<!-- *********************************************************************** --> + +<div class="doc_text"> + +<p>LLVM is known to work on the following platforms:</p> + +<ul> +<li>Intel and AMD machines (IA32, X86-64, AMD64, EMT-64) running Red Hat + Linux, Fedora Core, FreeBSD and AuroraUX (and probably other unix-like + systems).</li> +<li>PowerPC and X86-based Mac OS X systems, running 10.4 and above in 32-bit + and 64-bit modes.</li> +<li>Intel and AMD machines running on Win32 using MinGW libraries (native).</li> +<li>Intel and AMD machines running on Win32 with the Cygwin libraries (limited + support is available for native builds with Visual C++).</li> +<li>Sun x86 and AMD64 machines running Solaris 10, OpenSolaris 0906.</li> +<li>Alpha-based machines running Debian GNU/Linux.</li> +</ul> + +<p>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.</p> + +</div> + +<!-- *********************************************************************** --> +<div class="doc_section"> + <a name="knownproblems">Known Problems</a> +</div> +<!-- *********************************************************************** --> + +<div class="doc_text"> + +<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> + +<ul> +<li>LLVM will not correctly compile on Solaris and/or OpenSolaris +using the stock GCC 3.x.x series 'out the box', +See: <a href="GettingStarted.html#brokengcc">Broken versions of GCC and other tools</a>. +However, A <a href="http://pkg.auroraux.org/GCC">Modern GCC Build</a> +for x86/x86-64 has been made available from the third party AuroraUX Project +that has been meticulously tested for bootstrapping LLVM & Clang.</li> +</ul> + +</div> + +<!-- ======================================================================= --> +<div class="doc_subsection"> + <a name="experimental">Experimental features included with this release</a> +</div> + +<div class="doc_text"> + +<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 MSIL, Alpha, SPU, MIPS, PIC16, Blackfin, MSP430, SystemZ and MicroBlaze + backends are experimental.</li> +<li><tt>llc</tt> "<tt>-filetype=asm</tt>" (the default) is the only + supported value for this option. The MachO writer is experimental, and + works much better in mainline SVN.</li> +</ul> + +</div> + +<!-- ======================================================================= --> +<div class="doc_subsection"> + <a name="x86-be">Known problems with the X86 back-end</a> +</div> + +<div class="doc_text"> + +<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 backend generates inefficient floating point code when configured + to generate code for systems that don't have SSE2.</li> + <li>Win64 code generation wasn't widely tested. Everything should work, but we + expect small issues to happen. Also, llvm-gcc cannot build the mingw64 + runtime currently due to lack of support for the 'u' inline assembly + constraint and for X87 floating point inline assembly.</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> +</ul> + +</div> + +<!-- ======================================================================= --> +<div class="doc_subsection"> + <a name="ppc-be">Known problems with the PowerPC back-end</a> +</div> + +<div class="doc_text"> + +<ul> +<li>The Linux PPC32/ABI support needs testing for the interpreter and static +compilation, and lacks support for debug information.</li> +</ul> + +</div> + +<!-- ======================================================================= --> +<div class="doc_subsection"> + <a name="arm-be">Known problems with the ARM back-end</a> +</div> + +<div class="doc_text"> + +<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> + +<!-- ======================================================================= --> +<div class="doc_subsection"> + <a name="sparc-be">Known problems with the SPARC back-end</a> +</div> + +<div class="doc_text"> + +<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> + +<!-- ======================================================================= --> +<div class="doc_subsection"> + <a name="mips-be">Known problems with the MIPS back-end</a> +</div> + +<div class="doc_text"> + +<ul> +<li>64-bit MIPS targets are not supported yet.</li> +</ul> + +</div> + +<!-- ======================================================================= --> +<div class="doc_subsection"> + <a name="alpha-be">Known problems with the Alpha back-end</a> +</div> + +<div class="doc_text"> + +<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> + +<!-- ======================================================================= --> +<div class="doc_subsection"> + <a name="c-be">Known problems with the C back-end</a> +</div> + +<div class="doc_text"> + +<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> + + +<!-- ======================================================================= --> +<div class="doc_subsection"> + <a name="c-fe">Known problems with the llvm-gcc C and C++ front-end</a> +</div> + +<div class="doc_text"> + +<p>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> + +</div> + +<!-- ======================================================================= --> +<div class="doc_subsection"> + <a name="fortran-fe">Known problems with the llvm-gcc Fortran front-end</a> +</div> + +<div class="doc_text"> +<ul> +<li>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.</li> +</ul> +</div> + +<!-- ======================================================================= --> +<div class="doc_subsection"> + <a name="ada-fe">Known problems with the llvm-gcc Ada front-end</a> +</div> + +<div class="doc_text"> +The llvm-gcc 4.2 Ada compiler works fairly well; however, this is not a mature +technology, and problems should be expected. +<ul> +<li>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 <a href="http://llvm.org/PR2006">also fails to build on X86-64</a> +which does support trampolines.</li> +<li>The Ada front-end <a href="http://llvm.org/PR2007">fails to bootstrap</a>. +This is due to lack of LLVM support for <tt>setjmp</tt>/<tt>longjmp</tt> style +exception handling, which is used internally by the compiler. +Workaround: configure with <tt>--disable-bootstrap</tt>.</li> +<li>The c380004, <a href="http://llvm.org/PR2010">c393010</a> +and <a href="http://llvm.org/PR2421">cxg2021</a> ACATS tests fail +(c380004 also fails with gcc-4.2 mainline). +If the compiler is built with checks disabled then <a href="http://llvm.org/PR2010">c393010</a> +causes the compiler to go into an infinite loop, using up all system memory.</li> +<li>Some GCC specific Ada tests continue to crash the compiler.</li> +<li>The <tt>-E</tt> binder option (exception backtraces) +<a href="http://llvm.org/PR1982">does not work</a> and will result in programs +crashing if an exception is raised. Workaround: do not use <tt>-E</tt>.</li> +<li>Only discrete types <a href="http://llvm.org/PR1981">are allowed to start +or finish at a non-byte offset</a> 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.</li> +<li>The <tt>lli</tt> interpreter <a href="http://llvm.org/PR2009">considers +'main' as generated by the Ada binder to be invalid</a>. +Workaround: hand edit the file to use pointers for <tt>argv</tt> and +<tt>envp</tt> rather than integers.</li> +<li>The <tt>-fstack-check</tt> option <a href="http://llvm.org/PR2008">is +ignored</a>.</li> +</ul> +</div> + +<!-- *********************************************************************** --> +<div class="doc_section"> + <a name="additionalinfo">Additional Information</a> +</div> +<!-- *********************************************************************** --> + +<div class="doc_text"> + +<p>A wide variety of additional information is available on the <a +href="http://llvm.org">LLVM web page</a>, in particular in the <a +href="http://llvm.org/docs/">documentation</a> 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 "<tt>llvm/doc/</tt>" directory in the LLVM tree.</p> + +<p>If you have any questions or comments about LLVM, please feel free to contact +us via the <a href="http://llvm.org/docs/#maillist"> mailing +lists</a>.</p> + +</div> + +<!-- *********************************************************************** --> + +<hr> +<address> + <a href="http://jigsaw.w3.org/css-validator/check/referer"><img + src="http://jigsaw.w3.org/css-validator/images/vcss-blue" alt="Valid CSS"></a> + <a href="http://validator.w3.org/check/referer"><img + src="http://www.w3.org/Icons/valid-html401-blue" alt="Valid HTML 4.01"></a> + + <a href="http://llvm.org/">LLVM Compiler Infrastructure</a><br> + Last modified: $Date$ +</address> + +</body> +</html> |