Gitiles
Code Review Sign In
gerrit-public.fairphone.software / platform / external / llvm / c0892285e7a436c949f0b09777c9f971e60c2c90 / . / docs / tutorial / JITTutorial1.html
blob: 5228982ead848c3c52a90a7fbdec7d058e376ef0 [file] [log] [blame]
Owen Anderson75bbde82007-10-20 05:23:06 +0000[diff] [blame]1<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"
2 "http://www.w3.org/TR/html4/strict.dtd">
3
4<html>
5<head>
6 <title>LLVM Tutorial 1: A First Function</title>
7 <meta http-equiv="Content-Type" content="text/html; charset=utf-8">
8 <meta name="author" content="Owen Anderson">
9 <meta name="description"
10 content="LLVM Tutorial 1: A First Function.">
11 <link rel="stylesheet" href="../llvm.css" type="text/css">
12</head>
13
14<body>
15
16<div class="doc_title"> LLVM Tutorial 1: A First Function </div>
17
18<div class="doc_author">
19 <p>Written by <a href="mailto:owen@apple.com">Owen Anderson</a></p>
20</div>
21
Owen Anderson334b8732007-10-22 06:35:07 +0000[diff] [blame]22<!-- *********************************************************************** -->
Owen Anderson334b8732007-10-22 06:35:07 +0000[diff] [blame]23<div class="doc_section"><a name="intro">A First Function</a></div>
24<!-- *********************************************************************** -->
25
Owen Anderson75bbde82007-10-20 05:23:06 +0000[diff] [blame]26<div class="doc_text">
27
Chris Lattnerdd684e22008-02-10 19:11:04 +0000[diff] [blame]28<p>For starters, let's consider a relatively straightforward function that takes three integer parameters and returns an arithmetic combination of them. This is nice and simple, especially since it involves no control flow:</p>
Owen Anderson75bbde82007-10-20 05:23:06 +0000[diff] [blame]29
30<div class="doc_code">
31<pre>
Owen Anderson22340ef2007-10-20 05:41:39 +0000[diff] [blame]32int mul_add(int x, int y, int z) {
33 return x * y + z;
34}
Owen Anderson75bbde82007-10-20 05:23:06 +0000[diff] [blame]35</pre>
36</div>
37
38<p>As a preview, the LLVM IR we’re going to end up generating for this function will look like:</p>
39
40<div class="doc_code">
41<pre>
Owen Anderson22340ef2007-10-20 05:41:39 +0000[diff] [blame]42define i32 @mul_add(i32 %x, i32 %y, i32 %z) {
43entry:
44 %tmp = mul i32 %x, %y
45 %tmp2 = add i32 %tmp, %z
46 ret i32 %tmp2
47}
Owen Anderson75bbde82007-10-20 05:23:06 +0000[diff] [blame]48</pre>
49</div>
50
Owen Anderson56c4d7f2007-10-20 06:12:33 +0000[diff] [blame]51<p>If you're unsure what the above code says, skim through the <a href="../LangRef.html">LLVM Language Reference Manual</a> and convince yourself that the above LLVM IR is actually equivalent to the original function. Once you’re satisfied with that, let’s move on to actually generating it programmatically!</p>
Owen Anderson75bbde82007-10-20 05:23:06 +0000[diff] [blame]52
Owen Andersona7e5ef42007-10-25 06:49:29 +0000[diff] [blame]53<p>Of course, before we can start, we need to <code>#include</code> the appropriate LLVM header files:</p>
54
55<div class="doc_code">
56<pre>
57#include &lt;llvm/Module.h&gt;
58#include &lt;llvm/Function.h&gt;
59#include &lt;llvm/PassManager.h&gt;
Owen Andersona35e1ab2007-11-06 01:26:56 +0000[diff] [blame]60#include &lt;llvm/CallingConv.h&gt;
Owen Andersona7e5ef42007-10-25 06:49:29 +0000[diff] [blame]61#include &lt;llvm/Analysis/Verifier.h&gt;
62#include &lt;llvm/Assembly/PrintModulePass.h&gt;
Duncan Sandsa989c7e2008-04-13 06:22:09 +0000[diff] [blame]63#include &lt;llvm/Support/IRBuilder.h&gt;
Owen Andersona7e5ef42007-10-25 06:49:29 +0000[diff] [blame]64</pre>
65</div>
Owen Anderson75bbde82007-10-20 05:23:06 +0000[diff] [blame]66
67<p>Now, let’s get started on our real program. Here’s what our basic <code>main()</code> will look like:</p>
68
69<div class="doc_code">
70<pre>
71using namespace llvm;
72
73Module* makeLLVMModule();
74
75int main(int argc, char**argv) {
76 Module* Mod = makeLLVMModule();
77
78 verifyModule(*Mod, PrintMessageAction);
79
80 PassManager PM;
81 PM.add(new PrintModulePass(&amp;llvm::cout));
82 PM.run(*Mod);
83
84 return 0;
85}
86</pre>
87</div>
88
Chris Lattnerdd684e22008-02-10 19:11:04 +0000[diff] [blame]89<p>The first segment is pretty simple: it creates an LLVM “module.” In LLVM, a module represents a single unit of code that is to be processed together. A module contains things like global variables, function declarations, and implementations. Here we’ve declared a <code>makeLLVMModule()</code> function to do the real work of creating the module. Don’t worry, we’ll be looking at that one next!</p>
Owen Anderson75bbde82007-10-20 05:23:06 +0000[diff] [blame]90
91<p>The second segment runs the LLVM module verifier on our newly created module. While this probably isn’t really necessary for a simple module like this one, it’s always a good idea, especially if you’re generating LLVM IR based on some input. The verifier will print an error message if your LLVM module is malformed in any way.</p>
92
Bill Wendlingc0892282008-05-19 00:05:30 +0000[diff] [blame^]93<p>Finally, we instantiate an LLVM <code>PassManager</code> and run
94the <code>PrintModulePass</code> on our module. LLVM uses an explicit pass
95infrastructure to manage optimizations and various other things.
96A <code>PassManager</code>, as should be obvious from its name, manages passes:
97it is responsible for scheduling them, invoking them, and ensuring the proper
98disposal after we’re done with them. For this example, we’re just using a
99trivial pass that prints out our module in textual form.</p>
Owen Anderson75bbde82007-10-20 05:23:06 +0000[diff] [blame]100
Bill Wendlingc0892282008-05-19 00:05:30 +0000[diff] [blame^]101<p>Now onto the interesting part: creating and populating a module. Here’s the
102first chunk of our <code>makeLLVMModule()</code>:</p>
Owen Anderson75bbde82007-10-20 05:23:06 +0000[diff] [blame]103
104<div class="doc_code">
105<pre>
106Module* makeLLVMModule() {
107 // Module Construction
108 Module* mod = new Module("test");
109</pre>
110</div>
111
112<p>Exciting, isn’t it!? All we’re doing here is instantiating a module and giving it a name. The name isn’t particularly important unless you’re going to be dealing with multiple modules at once.</p>
113
114<div class="doc_code">
115<pre>
Chris Lattnerdd684e22008-02-10 19:11:04 +0000[diff] [blame]116 Constant* c = mod-&gt;getOrInsertFunction("mul_add",
Owen Anderson9b3cf312007-10-22 06:29:31 +0000[diff] [blame]117 /*ret type*/ IntegerType::get(32),
118 /*args*/ IntegerType::get(32),
119 IntegerType::get(32),
120 IntegerType::get(32),
121 /*varargs terminated with null*/ NULL);
Owen Anderson75bbde82007-10-20 05:23:06 +0000[diff] [blame]122
Owen Anderson64255002007-10-20 05:40:47 +0000[diff] [blame]123 Function* mul_add = cast&lt;Function&gt;(c);
Chris Lattnerdd684e22008-02-10 19:11:04 +0000[diff] [blame]124 mul_add-&gt;setCallingConv(CallingConv::C);
Owen Anderson75bbde82007-10-20 05:23:06 +0000[diff] [blame]125</pre>
126</div>
127
Chris Lattnerdd684e22008-02-10 19:11:04 +0000[diff] [blame]128<p>We construct our <code>Function</code> by calling <code>getOrInsertFunction()</code> on our module, passing in the name, return type, and argument types of the function. In the case of our <code>mul_add</code> function, that means one 32-bit integer for the return value and three 32-bit integers for the arguments.</p>
Owen Anderson75bbde82007-10-20 05:23:06 +0000[diff] [blame]129
Chris Lattnerdd684e22008-02-10 19:11:04 +0000[diff] [blame]130<p>You'll notice that <code>getOrInsertFunction()</code> doesn't actually return a <code>Function*</code>. This is because <code>getOrInsertFunction()</code> will return a cast of the existing function if the function already existed with a different prototype. Since we know that there's not already a <code>mul_add</code> function, we can safely just cast <code>c</code> to a <code>Function*</code>.
Owen Anderson64255002007-10-20 05:40:47 +0000[diff] [blame]131
Bill Wendlingc0892282008-05-19 00:05:30 +0000[diff] [blame^]132<p>In addition, we set the calling convention for our new function to be the C
133calling convention. This isn’t strictly necessary, but it ensures that our new
134function will interoperate properly with C code, which is a good thing.</p>
Owen Anderson75bbde82007-10-20 05:23:06 +0000[diff] [blame]135
136<div class="doc_code">
137<pre>
Chris Lattnerdd684e22008-02-10 19:11:04 +0000[diff] [blame]138 Function::arg_iterator args = mul_add-&gt;arg_begin();
Owen Anderson75bbde82007-10-20 05:23:06 +0000[diff] [blame]139 Value* x = args++;
Chris Lattnerdd684e22008-02-10 19:11:04 +0000[diff] [blame]140 x-&gt;setName("x");
Owen Anderson75bbde82007-10-20 05:23:06 +0000[diff] [blame]141 Value* y = args++;
Chris Lattnerdd684e22008-02-10 19:11:04 +0000[diff] [blame]142 y-&gt;setName("y");
Owen Anderson75bbde82007-10-20 05:23:06 +0000[diff] [blame]143 Value* z = args++;
Chris Lattnerdd684e22008-02-10 19:11:04 +0000[diff] [blame]144 z-&gt;setName("z");
Owen Anderson75bbde82007-10-20 05:23:06 +0000[diff] [blame]145</pre>
146</div>
147
Chris Lattnerdd684e22008-02-10 19:11:04 +0000[diff] [blame]148<p>While we’re setting up our function, let’s also give names to the parameters. This also isn’t strictly necessary (LLVM will generate names for them if you don’t specify them), but it’ll make looking at our output somewhat more pleasant. To name the parameters, we iterate over the arguments of our function and call <code>setName()</code> on them. We’ll also keep the pointer to <code>x</code>, <code>y</code>, and <code>z</code> around, since we’ll need them when we get around to creating instructions.</p>
Owen Anderson75bbde82007-10-20 05:23:06 +0000[diff] [blame]149
Chris Lattnerdd684e22008-02-10 19:11:04 +0000[diff] [blame]150<p>Great! We have a function now. But what good is a function if it has no body? Before we start working on a body for our new function, we need to recall some details of the LLVM IR. The IR, being an abstract assembly language, represents control flow using jumps (we call them branches), both conditional and unconditional. The straight-line sequences of code between branches are called basic blocks, or just blocks. To create a body for our function, we fill it with blocks:</p>
Owen Anderson75bbde82007-10-20 05:23:06 +0000[diff] [blame]151
152<div class="doc_code">
153<pre>
Gabor Greife794fc32008-04-19 22:25:09 +0000[diff] [blame]154 BasicBlock* block = BasicBlock::Create("entry", mul_add);
Duncan Sandsa989c7e2008-04-13 06:22:09 +0000[diff] [blame]155 IRBuilder builder(block);
Owen Anderson75bbde82007-10-20 05:23:06 +0000[diff] [blame]156</pre>
157</div>
158
Duncan Sandsa989c7e2008-04-13 06:22:09 +0000[diff] [blame]159<p>We create a new basic block, as you might expect, by calling its constructor. All we need to tell it is its name and the function to which it belongs. In addition, we’re creating an <code>IRBuilder</code> object, which is a convenience interface for creating instructions and appending them to the end of a block. Instructions can be created through their constructors as well, but some of their interfaces are quite complicated. Unless you need a lot of control, using <code>IRBuilder</code> will make your life simpler.</p>
Owen Anderson75bbde82007-10-20 05:23:06 +0000[diff] [blame]160
161<div class="doc_code">
162<pre>
163 Value* tmp = builder.CreateBinOp(Instruction::Mul,
164 x, y, "tmp");
165 Value* tmp2 = builder.CreateBinOp(Instruction::Add,
166 tmp, z, "tmp2");
167
168 builder.CreateRet(tmp2);
Owen Anderson9b3cf312007-10-22 06:29:31 +0000[diff] [blame]169
170 return mod;
Owen Anderson75bbde82007-10-20 05:23:06 +0000[diff] [blame]171}
172</pre>
173</div>
174
Duncan Sandsa989c7e2008-04-13 06:22:09 +0000[diff] [blame]175<p>The final step in creating our function is to create the instructions that make it up. Our <code>mul_add</code> function is composed of just three instructions: a multiply, an add, and a return. <code>IRBuilder</code> gives us a simple interface for constructing these instructions and appending them to the “entry” block. Each of the calls to <code>IRBuilder</code> returns a <code>Value*</code> that represents the value yielded by the instruction. You’ll also notice that, above, <code>x</code>, <code>y</code>, and <code>z</code> are also <code>Value*</code>’s, so it’s clear that instructions operate on <code>Value*</code>’s.</p>
Owen Anderson75bbde82007-10-20 05:23:06 +0000[diff] [blame]176
Chris Lattnerdd684e22008-02-10 19:11:04 +0000[diff] [blame]177<p>And that’s it! Now you can compile and run your code, and get a wonderful textual print out of the LLVM IR we saw at the beginning. To compile, use the following command line as a guide:</p>
Owen Anderson75bbde82007-10-20 05:23:06 +0000[diff] [blame]178
Owen Andersona7e5ef42007-10-25 06:49:29 +0000[diff] [blame]179<div class="doc_code">
180<pre>
Owen Andersonbbc4e3d2008-03-24 21:38:01 +0000[diff] [blame]181# c++ -g tut1.cpp `llvm-config --cxxflags --ldflags --libs core` -o tut1
Chris Lattnerdd684e22008-02-10 19:11:04 +0000[diff] [blame]182# ./tut1
Owen Andersona7e5ef42007-10-25 06:49:29 +0000[diff] [blame]183</pre>
184</div>
185
186<p>The <code>llvm-config</code> utility is used to obtain the necessary GCC-compatible compiler flags for linking with LLVM. For this example, we only need the 'core' library. We'll use others once we start adding optimizers and the JIT engine.</p>
Owen Anderson75bbde82007-10-20 05:23:06 +0000[diff] [blame]187
Chris Lattnerdd684e22008-02-10 19:11:04 +0000[diff] [blame]188<a href="JITTutorial2.html">Next: A More Complicated Function</a>
Owen Anderson75bbde82007-10-20 05:23:06 +0000[diff] [blame]189</div>
190
Owen Anderson8e75bf72007-10-25 06:45:01 +0000[diff] [blame]191<!-- *********************************************************************** -->
192<hr>
193<address>
194 <a href="http://jigsaw.w3.org/css-validator/check/referer"><img
195 src="http://jigsaw.w3.org/css-validator/images/vcss" alt="Valid CSS!"></a>
196 <a href="http://validator.w3.org/check/referer"><img
197 src="http://www.w3.org/Icons/valid-html401" alt="Valid HTML 4.01!"></a>
198
199 <a href="mailto:owen@apple.com">Owen Anderson</a><br>
200 <a href="http://llvm.org">The LLVM Compiler Infrastructure</a><br>
201 Last modified: $Date: 2007-10-17 11:05:13 -0700 (Wed, 17 Oct 2007) $
202</address>
203
Owen Anderson75bbde82007-10-20 05:23:06 +0000[diff] [blame]204</body>
Duncan Sands1c5ae822007-11-05 15:15:50 +0000[diff] [blame]205</html>