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- <title>Kaleidoscope: Conclusion, ideas for extensions, and other useful
- tidbits</title>
+ <title>Kaleidoscope: Conclusion and other useful LLVM tidbits</title>
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<meta name="author" content="Chris Lattner">
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-<div class="doc_title">Kaleidoscope: Conclusion</div>
+<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="intro">Tutorial Conclusion</a></div>
+<div class="doc_section"><a name="conclusion">Tutorial Conclusion</a></div>
<!-- *********************************************************************** -->
<div class="doc_text">
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.
-The getelementptr instruction is so nifty/unconventional, it <a
-href="../GetElementPtr.html">has its own FAQ</a>!).</li>
+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 amount of sense to be lowered into calls into a
+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 choose to use garbage collection, note that LLVM fully
+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
-straight-forward. The best way to understand it is to compile some C/C++ code
+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
<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
-and off the wall and seeing how it turns out. If you get stuck or want to talk
+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, I want to talk about some "tips and tricks" for generating
+<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 LLVM
+<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>
+<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>
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 language
+<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
+<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
</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 better than shipping
+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 even make real sense for specialized domains such as an
+code. This can make sense for specialized domains such as an
in-kernel language.</p>
</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.
+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>
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
+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
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
-do some specific feature, so they go ahead and extend it to do so.</p>
+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 <em>is certainly possible</em> to add language-specific
+<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 possible to add language-specific optimization passes that
-"known" things about code compiled for a language. In the case of the C family,
-there is an optimziation pass that "knows" about the standard C library
+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;" for example, because C specifies what the 'exit'
+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
+implement the high-level optimizations you desire in your front-end, on the
language-specific AST.
</p>
<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
+<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>
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 if you want</a>. It requires your front-end to convert the
+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 use of tail calls (which LLVM also supports).</p>
+Passing Style</a> and the use of tail calls (which LLVM also supports).</p>
</div>
projects / oota-llvm.git / blobdiff