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Chris Lattner 430be971a9 some libc++ notes. 
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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"
"http://www.w3.org/TR/html4/strict.dtd">
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
<meta encoding="utf8">
<link rel="stylesheet" href="llvm.css" type="text/css">
<title>LLVM 2.9 Release Notes</title>
</head>
<body>
<h1 class="doc_title">LLVM 2.9 Release Notes</h1>
<img align=right src="http://llvm.org/img/DragonSmall.png"
width="136" height="136" alt="LLVM Dragon Logo">
<ol>
<li><a href="#intro">Introduction</a></li>
<li><a href="#subproj">Sub-project Status Update</a></li>
<li><a href="#externalproj">External Projects Using LLVM 2.9</a></li>
<li><a href="#whatsnew">What's New in LLVM 2.9?</a></li>
<li><a href="GettingStarted.html">Installation Instructions</a></li>
<li><a href="#knownproblems">Known Problems</a></li>
<li><a href="#additionalinfo">Additional Information</a></li>
</ol>
<div class="doc_author">
<p>Written by the <a href="http://llvm.org">LLVM Team</a></p>
</div>
<h1 style="color:red">These are in-progress notes for the upcoming LLVM 2.9
release.<br>
You may prefer the
<a href="http://llvm.org/releases/2.8/docs/ReleaseNotes.html">LLVM 2.8
Release Notes</a>.</h1>
<!-- *********************************************************************** -->
<h1>
<a name="intro">Introduction</a>
</h1>
<!-- *********************************************************************** -->
<div class="doc_text">
<p>This document contains the release notes for the LLVM Compiler
Infrastructure, release 2.9. Here we describe the status of LLVM, including
major improvements from the previous release and significant known problems.
All LLVM releases may be downloaded from the <a
href="http://llvm.org/releases/">LLVM releases web site</a>.</p>
<p>For more information about LLVM, including information about the latest
release, please check out the <a href="http://llvm.org/">main LLVM
web site</a>. If you have questions or comments, the <a
href="http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev">LLVM Developer's
Mailing List</a> is a good place to send them.</p>
<p>Note that if you are reading this file from a Subversion checkout or the
main LLVM web page, this document applies to the <i>next</i> release, not the
current one. To see the release notes for a specific release, please see the
<a href="http://llvm.org/releases/">releases page</a>.</p>
</div>
<!-- Features that need text if they're finished for 3.1:
ARM EHABI
combiner-aa?
strong phi elim
loop dependence analysis
CorrelatedValuePropagation
lib/Transforms/IPO/MergeFunctions.cpp => consider for 3.1.
-->
<!-- *********************************************************************** -->
<h1>
<a name="subproj">Sub-project Status Update</a>
</h1>
<!-- *********************************************************************** -->
<div class="doc_text">
<p>
The LLVM 2.9 distribution currently consists of code from the core LLVM
repository (which roughly includes the LLVM optimizers, code generators
and supporting tools), the Clang repository and the llvm-gcc repository. In
addition to this code, the LLVM Project includes other sub-projects that are in
development. Here we include updates on these subprojects.
</p>
</div>
<!--=========================================================================-->
<h2>
<a name="clang">Clang: C/C++/Objective-C Frontend Toolkit</a>
</h2>
<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, Objective-C, C++ and Objective-C++ on x86
(32- and 64-bit), and for darwin/arm targets.</p>
<p>In the LLVM 2.9 time-frame, the Clang team has made many improvements in C,
C++ and Objective-C support. C++ support is now generally rock solid, has
been exercised on a broad variety of code, and has several new <a
href="http://clang.llvm.org/cxx_status.html#cxx0x">C++'0x features</a>
implemented (such as rvalue references and variadic templates). LLVM 2.9 has
also brought in a large range of bug fixes and minor features (e.g. __label__
support), and is much more compatible with the Linux Kernel.</p>
<p>If Clang rejects your code that is built with another compiler, please take a
look at the <a href="http://clang.llvm.org/compatibility.html">language
compatibility</a> guide to make sure the issue isn't intentional or a known
issue.
</p>
<ul>
</ul>
</div>
<!--=========================================================================-->
<h2>
<a name="dragonegg">DragonEgg: GCC front-ends, LLVM back-end</a>
</h2>
<div class="doc_text">
<p>
<a href="http://dragonegg.llvm.org/">DragonEgg</a> is a
<a href="http://gcc.gnu.org/wiki/plugins">gcc plugin</a> that replaces GCC's
optimizers and code generators with LLVM's.
Currently it requires a patched version of gcc-4.5.
The plugin can target the x86-32 and x86-64 processor families and has been
used successfully on the Darwin, FreeBSD and Linux platforms.
The Ada, C, C++ and Fortran languages work well.
The plugin is capable of compiling plenty of Obj-C, Obj-C++ and Java but it is
not known whether the compiled code actually works or not!
</p>
<p>
The 2.9 release has the following notable changes:
<ul>
<li>The plugin is much more stable when compiling Fortran.</li>
<li>Inline assembly where an asm output is tied to an input of a different size
is now supported in many more cases.</li>
<li>Basic support for the __float128 type was added. It is now possible to
generate LLVM IR from programs using __float128 but code generation does not
work yet.</li>
<li>Compiling Java programs no longer systematically crashes the plugin.</li>
</ul>
</div>
<!--=========================================================================-->
<h2>
<a name="compiler-rt">compiler-rt: Compiler Runtime Library</a>
</h2>
<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>In the LLVM 2.9 timeframe, compiler_rt has had several minor changes for
better ARM support, and a fairly major license change. All of the code in the
compiler-rt project is now <a href="DeveloperPolicy.html#license">dual
licensed</a> under MIT and UIUC license, which allows you to use compiler-rt
in applications without the binary copyright reproduction clause. If you
prefer the LLVM/UIUC license, you are free to continue using it under that
license as well.</p>
</div>
<!--=========================================================================-->
<h2>
<a name="lldb">LLDB: Low Level Debugger</a>
</h2>
<div class="doc_text">
<p>
<a href="http://lldb.llvm.org/">LLDB</a> is a brand new member of the LLVM
umbrella of projects. LLDB is a next generation, high-performance debugger. It
is built as a set of reusable components which highly leverage existing
libraries in the larger LLVM Project, such as the Clang expression parser, the
LLVM disassembler and the LLVM JIT.</p>
<p>
LLDB is has advanced by leaps and bounds in the 2.9 timeframe. It is
dramatically more stable and useful, and includes both a new <a
href="http://lldb.llvm.org/tutorial.html">tutorial</a> and a <a
href="http://lldb.llvm.org/lldb-gdb.html">side-by-side comparison with
GDB</a>.</p>
</div>
<!--=========================================================================-->
<h2>
<a name="libc++">libc++: C++ Standard Library</a>
</h2>
<div class="doc_text">
<p>
<a href="http://libcxx.llvm.org/">libc++</a> is another new member of the LLVM
family. It is an implementation of the C++ standard library, written from the
ground up to specifically target the forthcoming C++'0X standard and focus on
delivering great performance.</p>
<p>
In the LLVM 2.9 timeframe, libc++ has had numerous bugs fixed, and is now being
co-developed with Clang's C++'0x mode.</p>
<p>
Like compiler_rt, libc++ is now <a href="DeveloperPolicy.html#license">dual
licensed</a> under the MIT and UIUC license, allowing it to be used more
permissively.
</p>
</div>
<!--=========================================================================-->
<!--
<h2>
<a name="klee">KLEE: A Symbolic Execution Virtual Machine</a>
</h2>
<div class="doc_text">
<p>
<a href="http://klee.llvm.org/">KLEE</a> is a symbolic execution framework for
programs in LLVM bitcode form. KLEE tries to symbolically evaluate "all" paths
through the application and records state transitions that lead to fault
states. This allows it to construct testcases that lead to faults and can even
be used to verify some algorithms.
</p>
<p>UPDATE!</p>
</div>-->
<!-- *********************************************************************** -->
<h1>
<a name="externalproj">External Open Source Projects Using LLVM 2.9</a>
</h1>
<!-- *********************************************************************** -->
<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 2.9.</p>
</div>
<!-- *********************************************************************** -->
<h1>
<a name="whatsnew">What's New in LLVM 2.9?</a>
</h1>
<!-- *********************************************************************** -->
<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>
<!--=========================================================================-->
<h2>
<a name="majorfeatures">Major New Features</a>
</h2>
<div class="doc_text">
<p>LLVM 2.9 includes several major new capabilities:</p>
<ul>
<li>
TBAA: On by default in clang. Disable it with -fno-strict-aliasing.
Could be more aggressive for structs.
</li>
<li>New Nvidia PTX backend, not generally useful in 2.9 though.</li>
<li>
Much better debug info generated, particularly in optimized code situations.
</li>
<li>
inline asm multiple alternative constraint support.
</li>
<li>
New naming rules in coding standards: CodingStandards.html#ll_naming
</li>
</ul>
</div>
<!--=========================================================================-->
<h2>
<a name="coreimprovements">LLVM IR and Core Improvements</a>
</h2>
<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>udiv, ashr, lshr, shl now have exact and nuw/nsw bits:
PR8862 / LangRef.html</li>
unnamed_addr + PR8927
new 'hotpatch' attribute: LangRef.html#fnattrs
</ul>
</div>
<!--=========================================================================-->
<h2>
<a name="optimizer">Optimizer Improvements</a>
</h2>
<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>LTO has been improved to use MC for parsing inline asm and now
can build large programs like Firefox 4 on both OS X and Linux.</li>
LoopIdiom: memset/memcpy formation and memset_pattern on darwin. Build with
-ffreestanding or -fno-builtin if your memcpy is being compiled into infinite
recursion.
TargetLibraryInfo
EarlyCSE pass.
LoopInstSimplify pass.
New <a href="WritingAnLLVMPass.html#RegionPass">RegionPass</a> infrastructure
for region-based optimizations.
Can optimize printf to iprintf when no floating point is used, for embedded
targets with smaller iprintf implementation.
Speedups to various mid-level passes:
GVN is much faster on functions with deep dominator trees / lots of BBs.
DomTree and DominatorFrontier are much faster to compute, and preserved by
more passes (so they are computed less often)
SRoA is also much faster and doesn't use DominanceFrontier.
DSE is more aggressive with stores of different types: e.g. a large store
following a small one to the same address.
We now optimize various idioms for overflow detection into check of the flag
register on various CPUs, e.g.:
unsigned long t = a+b;
if (t &lt; a) ...
into:
addq %rdi, %rbx
jno LBB0_2
</ul>
<!--
<p>In addition to these features that are done in 2.8, there is preliminary
support in the release for Type Based Alias Analysis
Preliminary work on TBAA but not usable in 2.8.
New CorrelatedValuePropagation pass, not on by default in 2.8 yet.
-->
</div>
<!--=========================================================================-->
<h2>
<a name="mc">MC Level Improvements</a>
</h2>
<div class="doc_text">
<p>
The LLVM Machine Code (aka MC) subsystem was created to solve a number
of problems in the realm of assembly, disassembly, object file format handling,
and a number of other related areas that CPU instruction-set level tools work
in.</p>
<ul>
<li>MC is now used by default for ELF systems on x86 and
x86-64.</li>
<li>MC supports and CodeGen uses the <tt>.loc</tt> directives for
producing line number debug info. This produces more compact line
tables.</li>
<li>MC supports the <tt>.cfi_*</tt> directives for producing DWARF
frame information, but it is still not used by CodeGen by default.</li>
<li>COFF support?</li>
MC Assembler: X86 now generates much better diagnostics for common errors,
is much faster at matching instructions, is much more bug-compatible with
the GAS assembler, and is now generally useful for a broad range of X86
assembly.
ELF MC support: on by default in clang. There are still known missing features
for human written assembly.
Some basic <a href="CodeGenerator.html#mc">internals documentation</a> for MC.
MC Assembler support for .file and .loc.
tblgen support for assembler aliases: <a
href="CodeGenerator.html#na_instparsing">MnemonicAlias and InstAlias</a>
Win32 PE-COFF support in the MC assembler has made a lot of progress in the 2.9
timeframe, but is still not generally useful. Please see
"http://llvm.org/bugs/showdependencytree.cgi?id=9100&amp;hide_resolved=1" for open bugs?
lib/Object and llvm-objdump
Experimental format independent object file manipulation library.
* Supports PE/COFF and ELF.
* llvm-nm extended to work with object files. Exactly matches
binutils-nm for the files I've tested.
* llvm-objdump added with support for disassembly (no relocations displayed).
</ul>
<p>For more information, please see the <a
href="http://blog.llvm.org/2010/04/intro-to-llvm-mc-project.html">Intro to the
LLVM MC Project Blog Post</a>.
</p>
</div>
<!--=========================================================================-->
<h2>
<a name="codegen">Target Independent Code Generator Improvements</a>
</h2>
<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>
<!-- SplitKit -->
<li>The pre-register-allocation (preRA) instruction scheduler models register
pressure much more accurately in some cases. This allows the adoption of more
aggressive scheduling heuristics.
</li>
LiveDebugVariables is a new pass that keeps track of debugging information for
user variables that are kept in registers in optimized builds.
Scheduler now models operand latency and pipeline forwarding.
Major regalloc rewrite, not on by default for 2.9 and not advised to use it.
* New basic register allocator that can be used as a safe fallback when
debugging. Enable with -regalloc=basic.
* New infrastructure for live range splitting. SplitKit can break a live
interval into smaller pieces while preserving SSA form, and SpillPlacement
can help find the best split points. This is a work in progress so the API
is changing quickly.
* The inline spiller has learned to clean up after live range splitting. It
can hoist spills out of loops, and it can eliminate redundant spills.
Rematerialization works with live range splitting.
* New greedy register allocator using live range splitting. This will be the
default register allocator in the next LLVM release, but it is not turned on
by default in 2.9.
</ul>
</div>
<!--=========================================================================-->
<h2>
<a name="x86">X86-32 and X86-64 Target Improvements</a>
</h2>
<div class="doc_text">
<p>New features and major changes in the X86 target include:
</p>
<ul>
<li>
X86: Reimplemented all of MMX to introduce a new LLVM IR x86_mmx type. Now
random types like &lt;2 x i32&gt; are not iseld to mmx without emms. The
-disable-mmx flag is gone now.
</li>
<li>
X86 support for FS/GS relative loads and stores using address space 256/257 are
reliable now.
</li>
<li>
X86: Much better codegen for several cases using adc/sbb instead of cmovs for
conditional increment and other idioms.
</li>
<li>
The X86 backend has adopted a new preRA scheduling
mode, "list-ilp", to shorten the height of instruction schedules
without inducing register spills.
</li>
MC assembler support for 3dNow! and 3DNowA instructions.
<li>Several bugs have been fixed for Windows x64 code generator.</li>
</ul>
</div>
<!--=========================================================================-->
<h2>
<a name="ARM">ARM Target Improvements</a>
</h2>
<div class="doc_text">
<p>New features of the ARM target include:
</p>
<ul>
<li>ARM Fast ISel</li>
<li>ARM: New code placement pass.</li>
<li>ARM: Improved code generation for Cortex-A8 and Cortex-A9 CPUs.</li>
<li>ARM: __builtin_prefetch turns into prefetch instructions.</li>
<li>Countless ARM microoptimizations.</li>
<li> The ARM backend preRA scheduler now models machine resources at cycle
granularity. This allows the scheduler to both accurately model
instruction latency and avoid overcommitting functional units.</li>
</ul>
</div>
<!--=========================================================================-->
<h2>
<a name="OtherTS">Other Target Specific Improvements</a>
</h2>
<div class="doc_text">
<ul>
PPC: Switched to MCInstPrinter, and MCCodeEmitter. Ready to implement support
for directly writing out mach-o object files, but noone seems interested.
MicroBlaze: major updates for aggressive delay slot filler, MC-based assembly
printing, assembly instruction parsing, ELF .o file emission, and MC
instruction disassembler.
SPARC: Many improvements, including using the Y registers for multiplications
and addition of a simple delay slot filler.
</ul>
</div>
<!--=========================================================================-->
<h2>
<a name="changes">Major Changes and Removed Features</a>
</h2>
<div class="doc_text">
<p>If you're already an LLVM user or developer with out-of-tree changes based
on LLVM 2.8, this section lists some "gotchas" that you may run into upgrading
from the previous release.</p>
<ul>
last release for llvm-gcc
- DIBuilder provides simpler interface for front ends like Clang to encode debug info in LLVM IR.
- This interface hides implementation details (e.g. DIDerivedType, existence of compile unit etc..) that any front end should not know about.
For example, DIFactory DebugFactory;
Ty = DebugFactory.CreateDerivedType(DW_TAG_volatile_type,
findRegion(TYPE_CONTEXT(type)),
StringRef(),
getOrCreateFile(main_input_filename),
0 /*line no*/,
NodeSizeInBits(type),
NodeAlignInBits(type),
0 /*offset */,
0 /* flags */,
MainTy);
can be replaced by
DbgTy = DBuilder.createQualifiedType(DW_TAG_volatile_type, MainTy);
DIFactory is gone now.
LoopIndexSplit pass was removed, unmaintained.
LiveValues, SimplifyHalfPowrLibCalls, and GEPSplitter were removed.
Removed the PartialSpecialization pass, it was unmaintained and buggy.
DIFactory removed, use DIBuilder instead.
Triple::normalize is new, llvm triples are always stored in normalized form internally.
Triple x86_64--mingw64 is obsoleted. Use x86_64--mingw32 instead.
PointerTracking has been removed from mainline, moved to ClamAV.
</ul>
</div>
<!--=========================================================================-->
<h2>
<a name="api_changes">Internal API Changes</a>
</h2>
<div class="doc_text">
<p>In addition, many APIs have changed in this release. Some of the major
LLVM API changes are:</p>
<ul>
include/llvm/System merged into include/llvm/Support.
APInt API changes, see PR5207.
MVT::Flag renamed to MVT::Glue
error_code + libsystem + PathV2 changes
The system_error header from C++0x was added.
* Use if (error_code ec = function()) to check for error conditions
from functions which return it.
* error_code::message returns a human readable description of the error.
PathV1 has been deprecated in favor of PathV2 (sorry I didn't finish
this before the release).
* No Path class, use a r-value convertible to a twine instead.
* Assumes all paths are UTF-8.
</ul>
</div>
<!-- *********************************************************************** -->
<h1>
<a name="knownproblems">Known Problems</a>
</h1>
<!-- *********************************************************************** -->
<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>
</div>
<!-- ======================================================================= -->
<h2>
<a name="experimental">Experimental features included with this release</a>
</h2>
<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 Alpha, Blackfin, CellSPU, MicroBlaze, MSP430, MIPS, PTX, SystemZ
and XCore backends are experimental.</li>
<li><tt>llc</tt> "<tt>-filetype=obj</tt>" is experimental on all targets
other than darwin-i386 and darwin-x86_64. FIXME: Not true on ELF anymore?</li>
</ul>
</div>
<!-- ======================================================================= -->
<h2>
<a name="x86-be">Known problems with the X86 back-end</a>
</h2>
<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 X86-64 backend does not yet support the LLVM IR instruction
<tt>va_arg</tt>. Currently, front-ends support variadic
argument constructs on X86-64 by lowering them manually.</li>
<li>Windows x64 (aka Win64) code generator has a few issues.
<ul>
<li>llvm-gcc cannot build the mingw-w64 runtime currently
due to lack of support for the 'u' inline assembly
constraint and for X87 floating point inline assembly.</li>
<li>On mingw-w64, you will see unresolved symbol <tt>__chkstk</tt>
due to <a href="http://llvm.org/bugs/show_bug.cgi?id=8919">Bug 8919</a>.
It is fixed in <a href="http://lists.cs.uiuc.edu/pipermail/llvm-commits/Week-of-Mon-20110321/118499.html">r128206</a>.</li>
<li>Miss-aligned MOVDQA might crash your program. It is due to
<a href="http://llvm.org/bugs/show_bug.cgi?id=9483">Bug 9483</a>,
lack of handling aligned internal globals.</li>
</ul>
</li>
</ul>
</div>
<!-- ======================================================================= -->
<h2>
<a name="ppc-be">Known problems with the PowerPC back-end</a>
</h2>
<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>
<!-- ======================================================================= -->
<h2>
<a name="arm-be">Known problems with the ARM back-end</a>
</h2>
<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>
<!-- ======================================================================= -->
<h2>
<a name="sparc-be">Known problems with the SPARC back-end</a>
</h2>
<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>
<!-- ======================================================================= -->
<h2>
<a name="mips-be">Known problems with the MIPS back-end</a>
</h2>
<div class="doc_text">
<ul>
<li>64-bit MIPS targets are not supported yet.</li>
</ul>
</div>
<!-- ======================================================================= -->
<h2>
<a name="alpha-be">Known problems with the Alpha back-end</a>
</h2>
<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>
<!-- ======================================================================= -->
<h2>
<a name="c-be">Known problems with the C back-end</a>
</h2>
<div class="doc_text">
<p>The C backend has numerous problems and is not being actively maintained.
Depending on it for anything serious is not advised.</p>
<ul>
<li><a href="http://llvm.org/PR802">The C backend has only basic support for
inline assembly code</a>.</li>
<li><a href="http://llvm.org/PR1658">The C backend violates the ABI of common
C++ programs</a>, preventing intermixing between C++ compiled by the CBE and
C++ code compiled with <tt>llc</tt> or native compilers.</li>
<li>The C backend does not support all exception handling constructs.</li>
<li>The C backend does not support arbitrary precision integers.</li>
</ul>
</div>
<!-- ======================================================================= -->
<h2>
<a name="llvm-gcc">Known problems with the llvm-gcc front-end</a>
</h2>
<div class="doc_text">
<p>llvm-gcc is generally very stable for the C family of languages. The only
major language feature of GCC not supported by llvm-gcc is the
<tt>__builtin_apply</tt> family of builtins. However, some extensions
are only supported on some targets. For example, trampolines are only
supported on some targets (these are used when you take the address of a
nested function).</p>
<p>Fortran support generally works, but there are still several unresolved bugs
in <a href="http://llvm.org/bugs/">Bugzilla</a>. Please see the
tools/gfortran component for details. Note that llvm-gcc is missing major
Fortran performance work in the frontend and library that went into GCC after
4.2. If you are interested in Fortran, we recommend that you consider using
<a href="#dragonegg">dragonegg</a> instead.</p>
<p>The llvm-gcc 4.2 Ada compiler has basic functionality, but is no longer being
actively maintained. If you are interested in Ada, we recommend that you
consider using <a href="#dragonegg">dragonegg</a> instead.</p>
</div>
<!-- *********************************************************************** -->
<h1>
<a name="additionalinfo">Additional Information</a>
</h1>
<!-- *********************************************************************** -->
<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>
<!-- *********************************************************************** -->
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