https://github.com/epiqc/ScaffCC
Tip revision: 861c8b60980b0f4d36b101b45346a2e64b0fa390 authored by Pranav Gokhale on 05 February 2018, 05:29:11 UTC
update documentation and release notes
update documentation and release notes
Tip revision: 861c8b6
ReleaseNotes.html
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<h1>LLVM 3.1 Release Notes</h1>
<div>
<img style="float:right" src="http://llvm.org/img/DragonSmall.png"
width="136" height="136" alt="LLVM Dragon Logo">
</div>
<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 3.1</a></li>
<li><a href="#whatsnew">What's New in LLVM?</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 3.1
release.<br>
You may prefer the
<a href="http://llvm.org/releases/3.0/docs/ReleaseNotes.html">LLVM 3.0
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 3.1. Here we describe the status of LLVM, including
major improvements from the previous release, improvements in various
subprojects of LLVM, and some of the current users of the code.
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>
<!-- *********************************************************************** -->
<h2>
<a name="subproj">Sub-project Status Update</a>
</h2>
<!-- *********************************************************************** -->
<div>
<p>The LLVM 3.1 distribution currently consists of code from the core LLVM
repository (which roughly includes the LLVM optimizers, code generators and
supporting tools), and the Clang 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 3.1 time-frame, the Clang team has made many improvements:</p>
<ul>
<li>C++11 support is greatly expanded including lambdas, initializer lists, constexpr, user-defined literals, and atomics.</li>
<li>...</li>
</ul>
<p>For more details about the changes to Clang since the 2.9 release, see the
<a href="http://clang.llvm.org/docs/ReleaseNotes.html">Clang release notes</a>
</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. It works with gcc-4.5 and gcc-4.6
(and partially with gcc-4.7), can target the x86-32/x86-64 and ARM processor
families, and has been successfully used on the Darwin, FreeBSD, KFreeBSD,
Linux and OpenBSD platforms. It fully supports Ada, C, C++ and Fortran. It
has partial support for Go, Java, Obj-C and Obj-C++.</p>
<p>The 3.1 release has the following notable changes:</p>
<ul>
<li>Partial support for gcc-4.7. Ada support is poor, but other languages work
fairly well.</li>
<li>Support for ARM processors. Some essential gcc headers that are needed to
build DragonEgg for ARM are not installed by gcc. To work around this,
copy the missing headers from the gcc source tree.</li>
<li>Better optimization for Fortran by exploiting the fact that Fortran scalar
arguments have 'restrict' semantics.</li>
<li>Better optimization for all languages by passing information about type
aliasing and type ranges to the LLVM optimizers.</li>
<li>A regression test-suite was added.</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>....</p>
</div>
<!--=========================================================================-->
<h3>
<a name="lldb">LLDB: Low Level Debugger</a>
</h3>
<div>
<p>LLDB is a ground-up implementation of a command line debugger, as well as a
debugger API that can be used from other applications. LLDB makes use of the
Clang parser to provide high-fidelity expression parsing (particularly for
C++) and uses the LLVM JIT for target support.</p>
<p>...</p>
</div>
<!--=========================================================================-->
<h3>
<a name="libc++">libc++: C++ Standard Library</a>
</h3>
<div>
<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>
<p>...</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.
<p>In the LLVM 3.1 time-frame, VMKit has had significant improvements on both
runtime and startup performance:</p>
<ul>
<li>...</li>
</ul>
</div>
<!--=========================================================================-->
<h3>
<a name="Polly">Polly: Polyhedral Optimizer</a>
</h3>
<div>
<p><a href="http://polly.llvm.org/">Polly</a> is an <em>experimental</em>
optimizer for data locality and parallelism. It currently provides high-level
loop optimizations and automatic parallelisation (using the OpenMP run time).
Work in the area of automatic SIMD and accelerator code generation was
started.
<p>Within the LLVM 3.1 time-frame there were the following highlights:</p>
<ul>
<li>Polly became an official LLVM project</li>
<li>Polly can be loaded directly into clang (Enabled by '-O3 -mllvm -polly'
)</li>
<li>An automatic scheduling optimizer (derived from <a
href="http://pluto-compiler.sourceforge.net/">Pluto</a>) was integrated. It
performs loop transformations to optimize for data-locality and parallelism.
The transformations include, but are not limited to interchange, fusion,
fission, skewing and tiling.
</li>
</ul>
</div>
</div>
<!-- *********************************************************************** -->
<h2>
<a name="externalproj">External Open Source Projects Using LLVM 3.1</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 3.1.</p>
<h3>Pure</h3>
<p>Pure (http://pure-lang.googlecode.com/) 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 0.54 has been tested and is known to work with LLVM 3.1 (and
continues to work with older LLVM releases >= 2.5).</p>
</div>
<!-- *********************************************************************** -->
<h2>
<a name="whatsnew">What's New in LLVM 3.1?</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>
<!-- 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.
Integrated assembler on by default for arm/thumb?
-->
<!-- Near dead:
Analysis/RegionInfo.h + Dom Frontiers
SparseBitVector: used in LiveVar.
llvm/lib/Archive - replace with lib object?
-->
<p>LLVM 3.1 includes several major changes and big features:</p>
<ul>
<li><a href="../tools/clang/docs/AddressSanitizer.html">AddressSanitizer</a>,
a fast memory error detector.</li>
<li><a href="CodeGenerator.html#machineinstrbundle">MachineInstr Bundles</a>,
Support to model instruction bundling / packing.</li>
<li><a href="#armintegratedassembler">ARM Integrated Assembler</a>,
A full featured assembler and direct-to-object support for ARM.</li>
<li><a href="#blockplacement">Basic Block Placement</a>
Probability driven basic block placement.</li>
<li>....</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>IR support for half float</li>
<li>IR support for vectors of pointers, including vector GEPs.</li>
<li>Module flags have been introduced. They convey information about the
module as a whole to LLVM subsystems.</li>
<li>Loads can now have range metadata attached to them to describe the
possible values being loaded.</li>
<li>Inline cost heuristics have been completely overhauled and now closely
model constant propagation through call sites, disregard trivially dead
code costs, and can model C++ STL iterator patterns.</li>
<li>....</li>
</ul>
</div>
<!--=========================================================================-->
<h3>
<a name="optimizer">Optimizer Improvements</a>
</h3>
<div>
<p>In addition to many minor performance tweaks and bug fixes, this
release includes a few major enhancements and additions to the
optimizers:</p>
<ul>
<li>The loop unroll pass now is able to unroll loops with run-time trip counts.
This feature is turned off by default, and is enabled with the
<code>-unroll-runtime</code> flag.</li>
<li>A new basic-block autovectorization pass is available. Pass
<code>-vectorize</code> to run this pass along with some associated
post-vectorization cleanup passes. For more information, see the EuroLLVM
2012 slides: <a href="http://llvm.org/devmtg/2012-04-12/Slides/Hal_Finkel.pdf">
Autovectorization with LLVM</a>.</li>
<li>....</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. 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>
<ul>
<li>....</li>
</ul>
</div>
<!--=========================================================================-->
<h3>
<a name="codegen">Target Independent Code Generator Improvements</a>
</h3>
<div>
<p>We have changed the way that the Type Legalizer legalizes vectors. The type
legalizer now attempts to promote integer elements. This enabled the
implementation of vector-select. Additionally, we see a performance boost on
workloads which use vectors of chars and shorts, since they are now promoted
to 32-bit types, which are better supported by the SIMD instruction set.
Floating point types are still widened as before.</p>
<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>TableGen can now synthesize register classes that are only needed to
represent combinations of constraints from instructions and sub-registers.
The synthetic register classes inherit most of their properties form their
closest user-defined super-class.</li>
<li><code>MachineRegisterInfo</code> now allows the reserved registers to be
frozen when register allocation starts. Target hooks should use the
<code>MRI->canReserveReg(FramePtr)</code> method to avoid accidentally
disabling frame pointer elimination during register allocation.</li>
<li>A new kind of <code>MachineOperand</code> provides a compact
representation of large clobber lists on call instructions. The register
mask operand references a bit mask of preserved registers. Everything else
is clobbered.</li>
</ul>
<p> We added new TableGen infrastructure to support bundling for
Very Long Instruction Word (VLIW) architectures. TableGen can now
automatically generate a deterministic finite automaton from a VLIW
target's schedule description which can be queried to determine
legal groupings of instructions in a bundle.</p>
<p> We have added a new target independent VLIW packetizer based on the
DFA infrastructure to group machine instructions into bundles.</p>
</div>
<h4>
<a name="blockplacement">Basic Block Placement</a>
</h4>
<div>
<p>A probability based block placement and code layout algorithm was added to
LLVM's code generator. This layout pass supports probabilities derived from
static heuristics as well as source code annotations such as
<code>__builtin_expect</code>.</p>
</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>Bug fixes and improved support for AVX1</li>
<li>Support for AVX2 (still incomplete at this point)</li>
<li>Call instructions use the new register mask operands for faster compile
times and better support for different calling conventions. The old WINCALL
instructions are no longer needed.</li>
<li>DW2 Exception Handling is enabled on Cygwin and MinGW.</li>
<li>Support for implicit TLS model used with MS VC runtime</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 constant island pass now supports basic block and constant pool entry
alignments greater than 4 bytes.</li>
<li>On Darwin, the ARM target now has a full-featured integrated assembler.
</li>
</ul>
<h4>
<a name="armintegratedassembler">ARM Integrated Assembler</a>
</h4>
<div>
<p>The ARM target now includes a full featured macro assembler, including
direct-to-object module support for clang. The assembler is currently enabled
by default for Darwin only pending testing and any additional necessary
platform specific support for Linux.</p>
<p>Full support is included for Thumb1, Thumb2 and ARM modes, along with
subtarget and CPU specific extensions for VFP2, VFP3 and NEON.</p>
<p>The assembler is Unified Syntax only (see ARM Architecural Reference Manual
for details). While there is some, and growing, support for pre-unfied (divided)
syntax, there are still significant gaps in that support.</p>
</div>
</div>
<!--=========================================================================-->
<h3>
<a name="MIPS">MIPS Target Improvements</a>
</h3>
<div>
<p>This release has seen major new work on just about every aspect of the MIPS
backend. Some of the major new features include:</p>
<ul>
<li>....</li>
</ul>
</div>
<!--=========================================================================-->
<h3>
<a name="OtherTS">Other Target Specific Improvements</a>
</h3>
<div>
<p>Support for Qualcomm's Hexagon VLIW processor has been added.</p>
<ul>
<li>....</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 3.1, this section lists some "gotchas" that you may run into upgrading
from the previous release.</p>
<ul>
<li>LLVM 3.1 removes support for reading LLVM 2.9 bitcode files. Going
forward, we aim for all future versions of LLVM to read bitcode files and
<tt>.ll</tt> files produced by LLVM 3.0 and later.</li>
<li>The <tt>unwind</tt> instruction is now gone. With the introduction of the
new exception handling system in LLVM 3.0, the <tt>unwind</tt> instruction
became obsolete.</li>
<li>LLVM 3.0 and earlier automatically added the returns_twice fo functions
like setjmp based on the name. This functionality was removed in 3.1.
This affects Clang users, if -ffreestanding is used.</li>
<li>....</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>Target specific options have been moved from global variables to members
on the new <code>TargetOptions</code> class, which is local to each
<code>TargetMachine</code>. As a consequence, the associated flags will
no longer be accepted by <tt>clang -mllvm</tt>. This includes:
<ul>
<li><code>llvm::PrintMachineCode</code></li>
<li><code>llvm::NoFramePointerElim</code></li>
<li><code>llvm::NoFramePointerElimNonLeaf</code></li>
<li><code>llvm::DisableFramePointerElim(const MachineFunction &)</code></li>
<li><code>llvm::LessPreciseFPMADOption</code></li>
<li><code>llvm::LessPrecideFPMAD()</code></li>
<li><code>llvm::NoExcessFPPrecision</code></li>
<li><code>llvm::UnsafeFPMath</code></li>
<li><code>llvm::NoInfsFPMath</code></li>
<li><code>llvm::NoNaNsFPMath</code></li>
<li><code>llvm::HonorSignDependentRoundingFPMathOption</code></li>
<li><code>llvm::HonorSignDependentRoundingFPMath()</code></li>
<li><code>llvm::UseSoftFloat</code></li>
<li><code>llvm::FloatABIType</code></li>
<li><code>llvm::NoZerosInBSS</code></li>
<li><code>llvm::JITExceptionHandling</code></li>
<li><code>llvm::JITEmitDebugInfo</code></li>
<li><code>llvm::JITEmitDebugInfoToDisk</code></li>
<li><code>llvm::GuaranteedTailCallOpt</code></li>
<li><code>llvm::StackAlignmentOverride</code></li>
<li><code>llvm::RealignStack</code></li>
<li><code>llvm::DisableJumpTables</code></li>
<li><code>llvm::EnableFastISel</code></li>
<li><code>llvm::getTrapFunctionName()</code></li>
<li><code>llvm::EnableSegmentedStacks</code></li>
</ul></li>
<li>The MDBuilder class has been added to simplify the creation of
metadata.</li>
<li>....</li>
</ul>
</div>
<!--=========================================================================-->
<h3>
<a name="tools_changes">Tools Changes</a>
</h3>
<div>
<p>In addition, some tools have changed in this release. Some of the changes
are:</p>
<ul>
<li>llvm-stress is a command line tool for generating random .ll files to fuzz
different LLVM components. </li>
<li>llvm-ld has been removed. Use llvm-link or Clang instead.</li>
<li>....</li>
</ul>
<ul>
<li>....</li>
</ul>
</div>
</div>
<!-- *********************************************************************** -->
<h2>
<a name="knownproblems">Known Problems</a>
</h2>
<!-- *********************************************************************** -->
<div>
<p>LLVM is generally a production quality compiler, and is used by a broad range
of applications and shipping in many products. That said, not every
subsystem is as mature as the aggregate, particularly the more obscure
targets. 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 or ask on the <a
href="http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev">LLVMdev
list</a>.</p>
<p>Known problem areas include:</p>
<ul>
<li>The Alpha, Blackfin, CellSPU, MSP430, PTX, SystemZ and
XCore backends are experimental, and the Alpha, Blackfin and SystemZ
targets have already been removed from mainline.</li>
<li>The integrated assembler, disassembler, and JIT is not supported by
several targets. If an integrated assembler is not supported, then a
system assembler is required. For more details, see the <a
href="CodeGenerator.html#targetfeatures">Target Features Matrix</a>.
</li>
<li>The C backend has numerous problems and is not being actively maintained.
Depending on it for anything serious is not advised.</li>
</ul>
</div>
<!-- *********************************************************************** -->
<h2>
<a name="additionalinfo">Additional Information</a>
</h2>
<!-- *********************************************************************** -->
<div>
<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>
<!-- *********************************************************************** -->
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