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Chris Lattner b7e6b1ab70 many edits, patch by Kelly Wilson! 
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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"
"http://www.w3.org/TR/html4/strict.dtd">
<html>
<head>
<title>Kaleidoscope: Conclusion and other useful LLVM tidbits</title>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
<meta name="author" content="Chris Lattner">
<link rel="stylesheet" href="../llvm.css" type="text/css">
</head>
<body>
<div class="doc_title">Kaleidoscope: Conclusion and other useful LLVM
tidbits</div>
<ul>
<li><a href="index.html">Up to Tutorial Index</a></li>
<li>Chapter 8
<ol>
<li><a href="#conclusion">Tutorial Conclusion</a></li>
<li><a href="#llvmirproperties">Properties of LLVM IR</a>
<ul>
<li><a href="#targetindep">Target Independence</a></li>
<li><a href="#safety">Safety Guarantees</a></li>
<li><a href="#langspecific">Language-Specific Optimizations</a></li>
</ul>
</li>
<li><a href="#tipsandtricks">Tips and Tricks</a>
<ul>
<li><a href="#offsetofsizeof">Implementing portable
offsetof/sizeof</a></li>
<li><a href="#gcstack">Garbage Collected Stack Frames</a></li>
</ul>
</li>
</ol>
</li>
</ul>
<div class="doc_author">
<p>Written by <a href="mailto:sabre@nondot.org">Chris Lattner</a></p>
</div>
<!-- *********************************************************************** -->
<div class="doc_section"><a name="conclusion">Tutorial Conclusion</a></div>
<!-- *********************************************************************** -->
<div class="doc_text">
<p>Welcome to the the final chapter of the "<a href="index.html">Implementing a
language with LLVM</a>" tutorial. In the course of this tutorial, we have grown
our little Kaleidoscope language from being a useless toy, to being a
semi-interesting (but probably still useless) toy. :)</p>
<p>It is interesting to see how far we've come, and how little code it has
taken. We built the entire lexer, parser, AST, code generator, and an
interactive run-loop (with a JIT!) by-hand in under 700 lines of
(non-comment/non-blank) code.</p>
<p>Our little language supports a couple of interesting features: it supports
user defined binary and unary operators, it uses JIT compilation for immediate
evaluation, and it supports a few control flow constructs with SSA construction.
</p>
<p>Part of the idea of this tutorial was to show you how easy and fun it can be
to define, build, and play with languages. Building a compiler need not be a
scary or mystical process! Now that you've seen some of the basics, I strongly
encourage you to take the code and hack on it. For example, try adding:</p>
<ul>
<li><b>global variables</b> - While global variables have questional value in
modern software engineering, they are often useful when putting together quick
little hacks like the Kaleidoscope compiler itself. Fortunately, our current
setup makes it very easy to add global variables: just have value lookup check
to see if an unresolved variable is in the global variable symbol table before
rejecting it. To create a new global variable, make an instance of the LLVM
<tt>GlobalVariable</tt> class.</li>
<li><b>typed variables</b> - Kaleidoscope currently only supports variables of
type double. This gives the language a very nice elegance, because only
supporting one type means that you never have to specify types. Different
languages have different ways of handling this. The easiest way is to require
the user to specify types for every variable definition, and record the type
of the variable in the symbol table along with its Value*.</li>
<li><b>arrays, structs, vectors, etc</b> - Once you add types, you can start
extending the type system in all sorts of interesting ways. Simple arrays are
very easy and are quite useful for many different applications. Adding them is
mostly an exercise in learning how the LLVM <a
href="../LangRef.html#i_getelementptr">getelementptr</a> instruction works: it
is so nifty/unconventional, it <a
href="../GetElementPtr.html">has its own FAQ</a>! If you add support
for recursive types (e.g. linked lists), make sure to read the <a
href="../ProgrammersManual.html#TypeResolve">section in the LLVM
Programmer's Manual</a> that describes how to construct them.</li>
<li><b>standard runtime</b> - Our current language allows the user to access
arbitrary external functions, and we use it for things like "printd" and
"putchard". As you extend the language to add higher-level constructs, often
these constructs make the most sense if they are lowered to calls into a
language-supplied runtime. For example, if you add hash tables to the language,
it would probably make sense to add the routines to a runtime, instead of
inlining them all the way.</li>
<li><b>memory management</b> - Currently we can only access the stack in
Kaleidoscope. It would also be useful to be able to allocate heap memory,
either with calls to the standard libc malloc/free interface or with a garbage
collector. If you would like to use garbage collection, note that LLVM fully
supports <a href="../GarbageCollection.html">Accurate Garbage Collection</a>
including algorithms that move objects and need to scan/update the stack.</li>
<li><b>debugger support</b> - LLVM supports generation of <a
href="../SourceLevelDebugging.html">DWARF Debug info</a> which is understood by
common debuggers like GDB. Adding support for debug info is fairly
straightforward. The best way to understand it is to compile some C/C++ code
with "<tt>llvm-gcc -g -O0</tt>" and taking a look at what it produces.</li>
<li><b>exception handling support</b> - LLVM supports generation of <a
href="../ExceptionHandling.html">zero cost exceptions</a> which interoperate
with code compiled in other languages. You could also generate code by
implicitly making every function return an error value and checking it. You
could also make explicit use of setjmp/longjmp. There are many different ways
to go here.</li>
<li><b>object orientation, generics, database access, complex numbers,
geometric programming, ...</b> - Really, there is
no end of crazy features that you can add to the language.</li>
<li><b>unusual domains</b> - We've been talking about applying LLVM to a domain
that many people are interested in: building a compiler for a specific language.
However, there are many other domains that can use compiler technology that are
not typically considered. For example, LLVM has been used to implement OpenGL
graphics acceleration, translate C++ code to ActionScript, and many other
cute and clever things. Maybe you will be the first to JIT compile a regular
expression interpreter into native code with LLVM?</li>
</ul>
<p>
Have fun - try doing something crazy and unusual. Building a language like
everyone else always has, is much less fun than trying something a little crazy
or off the wall and seeing how it turns out. If you get stuck or want to talk
about it, feel free to email the <a
href="http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev">llvmdev mailing
list</a>: it has lots of people who are interested in languages and are often
willing to help out.
</p>
<p>Before we end this tutorial, I want to talk about some "tips and tricks" for generating
LLVM IR. These are some of the more subtle things that may not be obvious, but
are very useful if you want to take advantage of LLVM's capabilities.</p>
</div>
<!-- *********************************************************************** -->
<div class="doc_section"><a name="llvmirproperties">Properties of the LLVM
IR</a></div>
<!-- *********************************************************************** -->
<div class="doc_text">
<p>We have a couple common questions about code in the LLVM IR form - lets just
get these out of the way right now, shall we?</p>
</div>
<!-- ======================================================================= -->
<div class="doc_subsubsection"><a name="targetindep">Target
Independence</a></div>
<!-- ======================================================================= -->
<div class="doc_text">
<p>Kaleidoscope is an example of a "portable language": any program written in
Kaleidoscope will work the same way on any target that it runs on. Many other
languages have this property, e.g. lisp, java, haskell, javascript, python, etc
(note that while these languages are portable, not all their libraries are).</p>
<p>One nice aspect of LLVM is that it is often capable of preserving target
independence in the IR: you can take the LLVM IR for a Kaleidoscope-compiled
program and run it on any target that LLVM supports, even emitting C code and
compiling that on targets that LLVM doesn't support natively. You can trivially
tell that the Kaleidoscope compiler generates target-independent code because it
never queries for any target-specific information when generating code.</p>
<p>The fact that LLVM provides a compact, target-independent, representation for
code gets a lot of people excited. Unfortunately, these people are usually
thinking about C or a language from the C family when they are asking questions
about language portability. I say "unfortunately", because there is really no
way to make (fully general) C code portable, other than shipping the source code
around (and of course, C source code is not actually portable in general
either - ever port a really old application from 32- to 64-bits?).</p>
<p>The problem with C (again, in its full generality) is that it is heavily
laden with target specific assumptions. As one simple example, the preprocessor
often destructively removes target-independence from the code when it processes
the input text:</p>
<div class="doc_code">
<pre>
#ifdef __i386__
int X = 1;
#else
int X = 42;
#endif
</pre>
</div>
<p>While it is possible to engineer more and more complex solutions to problems
like this, it cannot be solved in full generality in a way that is better than shipping
the actual source code.</p>
<p>That said, there are interesting subsets of C that can be made portable. If
you are willing to fix primitive types to a fixed size (say int = 32-bits,
and long = 64-bits), don't care about ABI compatibility with existing binaries,
and are willing to give up some other minor features, you can have portable
code. This can make sense for specialized domains such as an
in-kernel language.</p>
</div>
<!-- ======================================================================= -->
<div class="doc_subsubsection"><a name="safety">Safety Guarantees</a></div>
<!-- ======================================================================= -->
<div class="doc_text">
<p>Many of the languages above are also "safe" languages: it is impossible for
a program written in Java to corrupt its address space and crash the process
(assuming the JVM has no bugs).
Safety is an interesting property that requires a combination of language
design, runtime support, and often operating system support.</p>
<p>It is certainly possible to implement a safe language in LLVM, but LLVM IR
does not itself guarantee safety. The LLVM IR allows unsafe pointer casts,
use after free bugs, buffer over-runs, and a variety of other problems. Safety
needs to be implemented as a layer on top of LLVM and, conveniently, several
groups have investigated this. Ask on the <a
href="http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev">llvmdev mailing
list</a> if you are interested in more details.</p>
</div>
<!-- ======================================================================= -->
<div class="doc_subsubsection"><a name="langspecific">Language-Specific
Optimizations</a></div>
<!-- ======================================================================= -->
<div class="doc_text">
<p>One thing about LLVM that turns off many people is that it does not solve all
the world's problems in one system (sorry 'world hunger', someone else will have
to solve you some other day). One specific complaint is that people perceive
LLVM as being incapable of performing high-level language-specific optimization:
LLVM "loses too much information".</p>
<p>Unfortunately, this is really not the place to give you a full and unified
version of "Chris Lattner's theory of compiler design". Instead, I'll make a
few observations:</p>
<p>First, you're right that LLVM does lose information. For example, as of this
writing, there is no way to distinguish in the LLVM IR whether an SSA-value came
from a C "int" or a C "long" on an ILP32 machine (other than debug info). Both
get compiled down to an 'i32' value and the information about what it came from
is lost. The more general issue here, is that the LLVM type system uses
"structural equivalence" instead of "name equivalence". Another place this
surprises people is if you have two types in a high-level language that have the
same structure (e.g. two different structs that have a single int field): these
types will compile down into a single LLVM type and it will be impossible to
tell what it came from.</p>
<p>Second, while LLVM does lose information, LLVM is not a fixed target: we
continue to enhance and improve it in many different ways. In addition to
adding new features (LLVM did not always support exceptions or debug info), we
also extend the IR to capture important information for optimization (e.g.
whether an argument is sign or zero extended, information about pointers
aliasing, etc). Many of the enhancements are user-driven: people want LLVM to
include some specific feature, so they go ahead and extend it.</p>
<p>Third, it is <em>possible and easy</em> to add language-specific
optimizations, and you have a number of choices in how to do it. As one trivial
example, it is easy to add language-specific optimization passes that
"know" things about code compiled for a language. In the case of the C family,
there is an optimization pass that "knows" about the standard C library
functions. If you call "exit(0)" in main(), it knows that it is safe to
optimize that into "return 0;" because C specifies what the 'exit'
function does.</p>
<p>In addition to simple library knowledge, it is possible to embed a variety of
other language-specific information into the LLVM IR. If you have a specific
need and run into a wall, please bring the topic up on the llvmdev list. At the
very worst, you can always treat LLVM as if it were a "dumb code generator" and
implement the high-level optimizations you desire in your front-end, on the
language-specific AST.
</p>
</div>
<!-- *********************************************************************** -->
<div class="doc_section"><a name="tipsandtricks">Tips and Tricks</a></div>
<!-- *********************************************************************** -->
<div class="doc_text">
<p>There is a variety of useful tips and tricks that you come to know after
working on/with LLVM that aren't obvious at first glance. Instead of letting
everyone rediscover them, this section talks about some of these issues.</p>
</div>
<!-- ======================================================================= -->
<div class="doc_subsubsection"><a name="offsetofsizeof">Implementing portable
offsetof/sizeof</a></div>
<!-- ======================================================================= -->
<div class="doc_text">
<p>One interesting thing that comes up, if you are trying to keep the code
generated by your compiler "target independent", is that you often need to know
the size of some LLVM type or the offset of some field in an llvm structure.
For example, you might need to pass the size of a type into a function that
allocates memory.</p>
<p>Unfortunately, this can vary widely across targets: for example the width of
a pointer is trivially target-specific. However, there is a <a
href="http://nondot.org/sabre/LLVMNotes/SizeOf-OffsetOf-VariableSizedStructs.txt">clever
way to use the getelementptr instruction</a> that allows you to compute this
in a portable way.</p>
</div>
<!-- ======================================================================= -->
<div class="doc_subsubsection"><a name="gcstack">Garbage Collected
Stack Frames</a></div>
<!-- ======================================================================= -->
<div class="doc_text">
<p>Some languages want to explicitly manage their stack frames, often so that
they are garbage collected or to allow easy implementation of closures. There
are often better ways to implement these features than explicit stack frames,
but <a
href="http://nondot.org/sabre/LLVMNotes/ExplicitlyManagedStackFrames.txt">LLVM
does support them,</a> if you want. It requires your front-end to convert the
code into <a
href="http://en.wikipedia.org/wiki/Continuation-passing_style">Continuation
Passing Style</a> and the use of tail calls (which LLVM also supports).</p>
</div>
<!-- *********************************************************************** -->
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