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Adding a note about IR generation to the LLVM FAQ.

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Gordon Henriksen 2008-02-22 20:58:29 +00:00
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@ -59,6 +59,9 @@
<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 an LLVM compiler for my language.
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
@ -413,6 +416,57 @@ using <tt>llvm-gcc</tt> instead.</p>
<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 an LLVM compiler for my language. 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. 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>