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======================
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LLVM 3.2 Release Notes
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======================
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.. contents::
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:local:
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Written by the `LLVM Team <http://llvm.org/>`_
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:red:`These are in-progress notes for the upcoming LLVM 3.2 release. You may
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prefer the` `LLVM 3.1 Release Notes <http://llvm.org/releases/3.1/docs
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/ReleaseNotes.html>`_.
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Introduction
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============
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This document contains the release notes for the LLVM Compiler Infrastructure,
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release 3.2. Here we describe the status of LLVM, including major improvements
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from the previous release, improvements in various subprojects of LLVM, and
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some of the current users of the code. All LLVM releases may be downloaded
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from the `LLVM releases web site <http://llvm.org/releases/>`_.
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For more information about LLVM, including information about the latest
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release, please check out the `main LLVM web site <http://llvm.org/>`_. If you
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have questions or comments, the `LLVM Developer's Mailing List
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<http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev>`_ is a good place to send
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them.
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Note that if you are reading this file from a Subversion checkout or the main
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LLVM web page, this document applies to the *next* release, not the current
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one. To see the release notes for a specific release, please see the `releases
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page <http://llvm.org/releases/>`_.
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Sub-project Status Update
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=========================
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The LLVM 3.2 distribution currently consists of code from the core LLVM
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repository, which roughly includes the LLVM optimizers, code generators and
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supporting tools, and the Clang repository. In addition to this code, the LLVM
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Project includes other sub-projects that are in development. Here we include
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updates on these subprojects.
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Clang: C/C++/Objective-C Frontend Toolkit
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-----------------------------------------
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`Clang <http://clang.llvm.org/>`_ is an LLVM front end for the C, C++, and
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Objective-C languages. Clang aims to provide a better user experience through
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expressive diagnostics, a high level of conformance to language standards, fast
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compilation, and low memory use. Like LLVM, Clang provides a modular,
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library-based architecture that makes it suitable for creating or integrating
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with other development tools. Clang is considered a production-quality
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compiler for C, Objective-C, C++ and Objective-C++ on x86 (32- and 64-bit), and
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for Darwin/ARM targets.
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In the LLVM 3.2 time-frame, the Clang team has made many improvements.
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Highlights include:
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#. More powerful warnings, especially `-Wuninitialized`
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#. Template type diffing in diagnostic messages
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#. Higher quality and more efficient debug info generation
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For more details about the changes to Clang since the 3.1 release, see the
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`Clang release notes. <http://clang.llvm.org/docs/ReleaseNotes.html>`_
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If Clang rejects your code but another compiler accepts it, please take a look
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at the `language compatibility <http://clang.llvm.org/compatibility.html>`_
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guide to make sure this is not intentional or a known issue.
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DragonEgg: GCC front-ends, LLVM back-end
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----------------------------------------
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`DragonEgg <http://dragonegg.llvm.org/>`_ is a `gcc plugin
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<http://gcc.gnu.org/wiki/plugins>`_ that replaces GCC's optimizers and code
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generators with LLVM's. It works with gcc-4.5 and gcc-4.6 (and partially with
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gcc-4.7), can target the x86-32/x86-64 and ARM processor families, and has been
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successfully used on the Darwin, FreeBSD, KFreeBSD, Linux and OpenBSD
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platforms. It fully supports Ada, C, C++ and Fortran. It has partial support
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for Go, Java, Obj-C and Obj-C++.
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The 3.2 release has the following notable changes:
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#. Able to load LLVM plugins such as Polly.
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#. Supports thread-local storage models.
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#. Passes knowledge of variable lifetimes to the LLVM optimizers.
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#. No longer requires GCC to be built with LTO support.
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compiler-rt: Compiler Runtime Library
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-------------------------------------
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The new LLVM `compiler-rt project <http://compiler-rt.llvm.org/>`_ is a simple
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library that provides an implementation of the low-level target-specific hooks
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required by code generation and other runtime components. For example, when
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compiling for a 32-bit target, converting a double to a 64-bit unsigned integer
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is compiled into a runtime call to the ``__fixunsdfdi`` function. The
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``compiler-rt`` library provides highly optimized implementations of this and
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other low-level routines (some are 3x faster than the equivalent libgcc
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routines).
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The 3.2 release has the following notable changes:
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#. ...
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LLDB: Low Level Debugger
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------------------------
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`LLDB <http://lldb.llvm.org>`_ is a ground-up implementation of a command line
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debugger, as well as a debugger API that can be used from other applications.
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LLDB makes use of the Clang parser to provide high-fidelity expression parsing
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(particularly for C++) and uses the LLVM JIT for target support.
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The 3.2 release has the following notable changes:
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#. ...
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libc++: C++ Standard Library
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----------------------------
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Like compiler_rt, libc++ is now :ref:`dual licensed
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<copyright-license-patents>` under the MIT and UIUC license, allowing it to be
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used more permissively.
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Within the LLVM 3.2 time-frame there were the following highlights:
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#. ...
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VMKit
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-----
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The `VMKit project <http://vmkit.llvm.org/>`_ is an implementation of a Java
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Virtual Machine (Java VM or JVM) that uses LLVM for static and just-in-time
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compilation.
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The 3.2 release has the following notable changes:
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#. ...
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Polly: Polyhedral Optimizer
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---------------------------
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`Polly <http://polly.llvm.org/>`_ is an *experimental* optimizer for data
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locality and parallelism. It provides high-level loop optimizations and
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automatic parallelisation.
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Within the LLVM 3.2 time-frame there were the following highlights:
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#. isl, the integer set library used by Polly, was relicensed to the MIT license
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#. isl based code generation
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#. MIT licensed replacement for CLooG (LGPLv2)
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#. Fine grained option handling (separation of core and border computations,
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control overhead vs. code size)
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#. Support for FORTRAN and dragonegg
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#. OpenMP code generation fixes
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External Open Source Projects Using LLVM 3.2
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============================================
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An exciting aspect of LLVM is that it is used as an enabling technology for a
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lot of other language and tools projects. This section lists some of the
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projects that have already been updated to work with LLVM 3.2.
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Crack
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-----
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`Crack <http://code.google.com/p/crack-language/>`_ aims to provide the ease of
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development of a scripting language with the performance of a compiled
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language. The language derives concepts from C++, Java and Python,
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incorporating object-oriented programming, operator overloading and strong
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typing.
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FAUST
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-----
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`FAUST <http://faust.grame.fr/>`_ is a compiled language for real-time audio
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signal processing. The name FAUST stands for Functional AUdio STream. Its
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programming model combines two approaches: functional programming and block
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diagram composition. In addition with the C, C++, Java, JavaScript output
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formats, the Faust compiler can generate LLVM bitcode, and works with LLVM
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2.7-3.1.
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Glasgow Haskell Compiler (GHC)
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------------------------------
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`GHC <http://www.haskell.org/ghc/>`_ is an open source compiler and programming
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suite for Haskell, a lazy functional programming language. It includes an
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optimizing static compiler generating good code for a variety of platforms,
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together with an interactive system for convenient, quick development.
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GHC 7.0 and onwards include an LLVM code generator, supporting LLVM 2.8 and
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later.
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Julia
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-----
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`Julia <https://github.com/JuliaLang/julia>`_ is a high-level, high-performance
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dynamic language for technical computing. It provides a sophisticated
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compiler, distributed parallel execution, numerical accuracy, and an extensive
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mathematical function library. The compiler uses type inference to generate
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fast code without any type declarations, and uses LLVM's optimization passes
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and JIT compiler. The `Julia Language <http://julialang.org/>`_ is designed
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around multiple dispatch, giving programs a large degree of flexibility. It is
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ready for use on many kinds of problems.
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LLVM D Compiler
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---------------
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`LLVM D Compiler <https://github.com/ldc-developers/ldc>`_ (LDC) is a compiler
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for the D programming Language. It is based on the DMD frontend and uses LLVM
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as backend.
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Open Shading Language
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---------------------
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`Open Shading Language (OSL)
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<https://github.com/imageworks/OpenShadingLanguage/>`_ is a small but rich
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language for programmable shading in advanced global illumination renderers and
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other applications, ideal for describing materials, lights, displacement, and
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pattern generation. It uses LLVM to JIT complex shader networks to x86 code at
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runtime.
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OSL was developed by Sony Pictures Imageworks for use in its in-house renderer
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used for feature film animation and visual effects, and is distributed as open
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source software with the "New BSD" license.
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Portable OpenCL (pocl)
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----------------------
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In addition to producing an easily portable open source OpenCL implementation,
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another major goal of `pocl <http://pocl.sourceforge.net/>`_ is improving
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performance portability of OpenCL programs with compiler optimizations,
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reducing the need for target-dependent manual optimizations. An important part
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of pocl is a set of LLVM passes used to statically parallelize multiple
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work-items with the kernel compiler, even in the presence of work-group
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barriers. This enables static parallelization of the fine-grained static
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concurrency in the work groups in multiple ways (SIMD, VLIW, superscalar, ...).
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Pure
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----
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`Pure <http://pure-lang.googlecode.com/>`_ is an algebraic/functional
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programming language based on term rewriting. Programs are collections of
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equations which are used to evaluate expressions in a symbolic fashion. The
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interpreter uses LLVM as a backend to JIT-compile Pure programs to fast native
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code. Pure offers dynamic typing, eager and lazy evaluation, lexical closures,
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a hygienic macro system (also based on term rewriting), built-in list and
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matrix support (including list and matrix comprehensions) and an easy-to-use
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interface to C and other programming languages (including the ability to load
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LLVM bitcode modules, and inline C, C++, Fortran and Faust code in Pure
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programs if the corresponding LLVM-enabled compilers are installed).
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Pure version 0.54 has been tested and is known to work with LLVM 3.1 (and
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continues to work with older LLVM releases >= 2.5).
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TTA-based Co-design Environment (TCE)
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-------------------------------------
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`TCE <http://tce.cs.tut.fi/>`_ is a toolset for designing application-specific
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processors (ASP) based on the Transport triggered architecture (TTA). The
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toolset provides a complete co-design flow from C/C++ programs down to
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synthesizable VHDL/Verilog and parallel program binaries. Processor
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customization points include the register files, function units, supported
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operations, and the interconnection network.
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TCE uses Clang and LLVM for C/C++ language support, target independent
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optimizations and also for parts of code generation. It generates new
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LLVM-based code generators "on the fly" for the designed TTA processors and
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loads them in to the compiler backend as runtime libraries to avoid per-target
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recompilation of larger parts of the compiler chain.
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Installation Instructions
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=========================
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See :doc:`GettingStarted`.
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What's New in LLVM 3.2?
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=======================
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This release includes a huge number of bug fixes, performance tweaks and minor
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improvements. Some of the major improvements and new features are listed in
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this section.
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Major New Features
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------------------
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..
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Features that need text if they're finished for 3.2:
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ARM EHABI
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combiner-aa?
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strong phi elim
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loop dependence analysis
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CorrelatedValuePropagation
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Integrated assembler on by default for arm/thumb?
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Near dead:
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Analysis/RegionInfo.h + Dom Frontiers
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SparseBitVector: used in LiveVar.
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llvm/lib/Archive - replace with lib object?
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LLVM 3.2 includes several major changes and big features:
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#. New NVPTX back-end (replacing existing PTX back-end) based on NVIDIA sources
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#. ...
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LLVM IR and Core Improvements
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-----------------------------
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LLVM IR has several new features for better support of new targets and that
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expose new optimization opportunities:
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#. Thread local variables may have a specified TLS model. See the :ref:`Language
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Reference Manual <globalvars>`.
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#. ...
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Optimizer Improvements
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----------------------
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In addition to many minor performance tweaks and bug fixes, this release
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includes a few major enhancements and additions to the optimizers:
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Loop Vectorizer - We've added a loop vectorizer and we are now able to
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vectorize small loops. The loop vectorizer is disabled by default and can be
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enabled using the ``-mllvm -vectorize-loops`` flag. The SIMD vector width can
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be specified using the flag ``-mllvm -force-vector-width=4``. The default
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value is ``0`` which means auto-select.
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We can now vectorize this function:
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.. code-block:: c++
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unsigned sum_arrays(int *A, int *B, int start, int end) {
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unsigned sum = 0;
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for (int i = start; i < end; ++i)
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sum += A[i] + B[i] + i;
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return sum;
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}
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We vectorize under the following loops:
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#. The inner most loops must have a single basic block.
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#. The number of iterations are known before the loop starts to execute.
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#. The loop counter needs to be incremented by one.
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#. The loop trip count **can** be a variable.
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#. Loops do **not** need to start at zero.
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#. The induction variable can be used inside the loop.
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#. Loop reductions are supported.
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#. Arrays with affine access pattern do **not** need to be marked as
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'``noalias``' and are checked at runtime.
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#. ...
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SROA - We've re-written SROA to be significantly more powerful and generate
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code which is much more friendly to the rest of the optimization pipeline.
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Previously this pass had scaling problems that required it to only operate on
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relatively small aggregates, and at times it would mistakenly replace a large
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aggregate with a single very large integer in order to make it a scalar SSA
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value. The result was a large number of i1024 and i2048 values representing any
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small stack buffer. These in turn slowed down many subsequent optimization
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paths.
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The new SROA pass uses a different algorithm that allows it to only promote to
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scalars the pieces of the aggregate actively in use. Because of this it doesn't
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require any thresholds. It also always deduces the scalar values from the uses
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of the aggregate rather than the specific LLVM type of the aggregate. These
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features combine to both optimize more code with the pass but to improve the
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compile time of many functions dramatically.
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#. Branch weight metadata is preseved through more of the optimizer.
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#. ...
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MC Level Improvements
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---------------------
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The LLVM Machine Code (aka MC) subsystem was created to solve a number of
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problems in the realm of assembly, disassembly, object file format handling,
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and a number of other related areas that CPU instruction-set level tools work
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in. For more information, please see the `Intro to the LLVM MC Project Blog
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Post <http://blog.llvm.org/2010/04/intro-to-llvm-mc-project.html>`_.
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#. ...
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.. _codegen:
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Target Independent Code Generator Improvements
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----------------------------------------------
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We have put a significant amount of work into the code generator
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infrastructure, which allows us to implement more aggressive algorithms and
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make it run faster:
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#. ...
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Stack Coloring - We have implemented a new optimization pass to merge stack
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objects which are used in disjoin areas of the code. This optimization reduces
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the required stack space significantly, in cases where it is clear to the
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optimizer that the stack slot is not shared. We use the lifetime markers to
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tell the codegen that a certain alloca is used within a region.
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We now merge consecutive loads and stores.
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X86-32 and X86-64 Target Improvements
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-------------------------------------
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New features and major changes in the X86 target include:
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#. ...
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.. _ARM:
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ARM Target Improvements
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-----------------------
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New features of the ARM target include:
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#. ...
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.. _armintegratedassembler:
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MIPS Target Improvements
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------------------------
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New features and major changes in the MIPS target include:
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#. ...
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PowerPC Target Improvements
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---------------------------
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Many fixes and changes across LLVM (and Clang) for better compliance with the
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64-bit PowerPC ELF Application Binary Interface, interoperability with GCC, and
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overall 64-bit PowerPC support. Some highlights include:
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#. MCJIT support added.
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#. PPC64 relocation support and (small code model) TOC handling added.
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#. Parameter passing and return value fixes (alignment issues, padding, varargs
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support, proper register usage, odd-sized structure support, float support,
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extension of return values for i32 return values).
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#. Fixes in spill and reload code for vector registers.
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#. C++ exception handling enabled.
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#. Changes to remediate double-rounding compatibility issues with respect to
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GCC behavior.
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#. Refactoring to disentangle ``ppc64-elf-linux`` ABI from Darwin ppc64 ABI
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support.
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#. Assorted new test cases and test case fixes (endian and word size issues).
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#. Fixes for big-endian codegen bugs, instruction encodings, and instruction
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constraints.
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#. Implemented ``-integrated-as`` support.
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#. Additional support for Altivec compare operations.
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#. IBM long double support.
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There have also been code generation improvements for both 32- and 64-bit code.
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Instruction scheduling support for the Freescale e500mc and e5500 cores has
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been added.
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PTX/NVPTX Target Improvements
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-----------------------------
|
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The PTX back-end has been replaced by the NVPTX back-end, which is based on the
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LLVM back-end used by NVIDIA in their CUDA (nvcc) and OpenCL compiler. Some
|
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highlights include:
|
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#. Compatibility with PTX 3.1 and SM 3.5.
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#. Support for NVVM intrinsics as defined in the NVIDIA Compiler SDK.
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#. Full compatibility with old PTX back-end, with much greater coverage of LLVM
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SIR.
|
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Please submit any back-end bugs to the LLVM Bugzilla site.
|
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|
Other Target Specific Improvements
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----------------------------------
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#. ...
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Major Changes and Removed Features
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----------------------------------
|
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|
|
If you're already an LLVM user or developer with out-of-tree changes based on
|
|
LLVM 3.2, this section lists some "gotchas" that you may run into upgrading
|
|
from the previous release.
|
|
|
|
#. The CellSPU port has been removed. It can still be found in older versions.
|
|
#. ...
|
|
|
|
Internal API Changes
|
|
--------------------
|
|
|
|
In addition, many APIs have changed in this release. Some of the major LLVM
|
|
API changes are:
|
|
|
|
We've added a new interface for allowing IR-level passes to access
|
|
target-specific information. A new IR-level pass, called
|
|
``TargetTransformInfo`` provides a number of low-level interfaces. LSR and
|
|
LowerInvoke already use the new interface.
|
|
|
|
The ``TargetData`` structure has been renamed to ``DataLayout`` and moved to
|
|
``VMCore`` to remove a dependency on ``Target``.
|
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|
#. ...
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|
Tools Changes
|
|
-------------
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In addition, some tools have changed in this release. Some of the changes are:
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#. ...
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Python Bindings
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---------------
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Officially supported Python bindings have been added! Feature support is far
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from complete. The current bindings support interfaces to:
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#. ...
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Known Problems
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|
==============
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LLVM is generally a production quality compiler, and is used by a broad range
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of applications and shipping in many products. That said, not every subsystem
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|
is as mature as the aggregate, particularly the more obscure1 targets. If you
|
|
run into a problem, please check the `LLVM bug database
|
|
<http://llvm.org/bugs/>`_ and submit a bug if there isn't already one or ask on
|
|
the `LLVMdev list <http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev>`_.
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Known problem areas include:
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|
|
#. The MSP430 and XCore backends are experimental.
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#. The integrated assembler, disassembler, and JIT is not supported by several
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|
targets. If an integrated assembler is not supported, then a system
|
|
assembler is required. For more details, see the
|
|
:ref:`target-feature-matrix`.
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Additional Information
|
|
======================
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|
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|
A wide variety of additional information is available on the `LLVM web page
|
|
<http://llvm.org/>`_, in particular in the `documentation
|
|
<http://llvm.org/docs/>`_ 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/docs/`` directory in the LLVM tree.
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|
If you have any questions or comments about LLVM, please feel free to contact
|
|
us via the `mailing lists <http://llvm.org/docs/#maillist>`_.
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