<li><a href="#felangs">Source Languages</a>
<ol>
<li><a href="#langs">What source languages are supported?</a></li>
+ <li><a href="#langirgen">I'd like to write a self-hosting LLVM compiler. How
+ should I interface with the LLVM middle-end optimizers and back-end code
+ generators?</a></div>
<li><a href="#langhlsupp">What support is there for higher level source
language constructs for building a compiler?</a></li>
<li><a href="GetElementPtr.html">I don't understand the GetElementPtr
<li><a href="#cfe_code">Questions about code generated by the GCC front-end</a>
<ol>
- <li><a href="#__main">What is this <tt>__main()</tt> call that gets inserted into
- <tt>main()</tt>?</a></li>
<li><a href="#iosinit">What is this <tt>llvm.global_ctors</tt> and
<tt>_GLOBAL__I__tmp_webcompile...</tt> stuff that happens when I
#include <iostream>?</a></li>
<p>The PyPy developers are working on integrating LLVM into the PyPy backend
so that PyPy language can translate to LLVM.</p>
</div>
+
+<div class="question"><p><a name="langirgen">
+ I'd like to write a self-hosting LLVM compiler. How should I interface with
+ the LLVM middle-end optimizers and back-end code generators?
+</a></p></div>
+<div class="answer">
+ <p>Your compiler front-end will communicate with LLVM by creating a module in
+ the LLVM intermediate representation (IR) format. Assuming you want to
+ write your language's compiler in the language itself (rather than C++),
+ there are 3 major ways to tackle generating LLVM IR from a front-end:</p>
+ <ul>
+ <li>
+ <strong>Call into the LLVM libraries code using your language's FFI
+ (foreign function interface).</strong>
+ <ul>
+ <li><em>for:</em> best tracks changes to the LLVM IR, .ll syntax,
+ and .bc format</li>
+ <li><em>for:</em> enables running LLVM optimization passes without a
+ emit/parse overhead</li>
+ <li><em>for:</em> adapts well to a JIT context</li>
+ <li><em>against:</em> lots of ugly glue code to write</li>
+ </ul>
+ </li>
+ <li>
+ <strong>Emit LLVM assembly from your compiler's native language.</strong>
+ <ul>
+ <li><em>for:</em> very straightforward to get started</li>
+ <li><em>against:</em> the .ll parser is slower than the bitcode reader
+ when interfacing to the middle end</li>
+ <li><em>against:</em> you'll have to re-engineer the LLVM IR object
+ model and asm writer in your language</li>
+ <li><em>against:</em> it may be harder to track changes to the IR</li>
+ </ul>
+ </li>
+ <li>
+ <strong>Emit LLVM bitcode from your compiler's native language.</strong>
+ <ul>
+ <li><em>for:</em> can use the more-efficient bitcode reader when
+ interfacing to the middle end</li>
+ <li><em>against:</em> you'll have to re-engineer the LLVM IR object
+ model and bitcode writer in your language</li>
+ <li><em>against:</em> it may be harder to track changes to the IR</li>
+ </ul>
+ </li>
+ </ul>
+ <p>If you go with the first option, the C bindings in include/llvm-c should
+ help a lot, since most languages have strong support for interfacing with
+ C. The most common hurdle with calling C from managed code is interfacing
+ with the garbage collector. The C interface was designed to require very
+ little memory management, and so is straightforward in this regard.</p>
+</div>
+
<div class="question"><p><a name="langhlsupp">
What support is there for a higher level source language constructs for
building a compiler?</a></p>
<a name="cfe_code">Questions about code generated by the GCC front-end</a>
</div>
-<div class="question"><p>
-<a name="__main"></a>
-What is this <tt>__main()</tt> call that gets inserted into <tt>main()</tt>?
-</p></div>
-
-<div class="answer">
-<p>
-The <tt>__main</tt> call is inserted by the C/C++ compiler in order to guarantee
-that static constructors and destructors are called when the program starts up
-and shuts down. In C, you can create static constructors and destructors by
-using GCC extensions, and in C++ you can do so by creating a global variable
-whose class has a ctor or dtor.
-</p>
-
-<p>
-The actual implementation of <tt>__main</tt> lives in the
-<tt>llvm/runtime/GCCLibraries/crtend/</tt> directory in the source-base, and is
-linked in automatically when you link the program.
-</p>
-</div>
-
-<!--=========================================================================-->
-
<div class="question">
<a name="iosinit"></a>
<p> What is this <tt>llvm.global_ctors</tt> and
projects / oota-llvm.git / blobdiff