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6 <title>LLVM Tutorial 1: A First Function</title>
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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 Anderson3295cfd2007-10-22 06:35:07 +0000[diff] [blame]22<!-- *********************************************************************** -->
Owen Anderson3295cfd2007-10-22 06:35:07 +0000[diff] [blame]23<div class="doc_section"><a name="intro">A First Function</a></div>
24<!-- *********************************************************************** -->
25
Owen Andersonb207ff22007-10-20 05:23:06 +0000[diff] [blame]26<div class="doc_text">
27
Chris Lattnera1ad2bf2008-02-10 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 Andersonb207ff22007-10-20 05:23:06 +0000[diff] [blame]29
30<div class="doc_code">
31<pre>
Owen Anderson9f2be012007-10-20 05:41:39 +0000[diff] [blame]32int mul_add(int x, int y, int z) {
33 return x * y + z;
34}
Owen Andersonb207ff22007-10-20 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 Anderson9f2be012007-10-20 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 Andersonb207ff22007-10-20 05:23:06 +0000[diff] [blame]48</pre>
49</div>
50
Owen Anderson18be8742007-10-20 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 Andersonb207ff22007-10-20 05:23:06 +0000[diff] [blame]52
Owen Anderson4c209da2007-10-25 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 Anderson0d282382007-11-06 01:26:56 +0000[diff] [blame]60#include &lt;llvm/CallingConv.h&gt;
Owen Anderson4c209da2007-10-25 06:49:29 +0000[diff] [blame]61#include &lt;llvm/Analysis/Verifier.h&gt;
62#include &lt;llvm/Assembly/PrintModulePass.h&gt;
Duncan Sandsa07136e2008-04-13 06:22:09 +0000[diff] [blame]63#include &lt;llvm/Support/IRBuilder.h&gt;
Owen Anderson4c209da2007-10-25 06:49:29 +0000[diff] [blame]64</pre>
65</div>
Owen Andersonb207ff22007-10-20 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 Lattnera1ad2bf2008-02-10 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 Andersonb207ff22007-10-20 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
93<p>Finally, we instantiate an LLVM <code>PassManager</code> and run the <code>PrintModulePass</code> on our module. LLVM uses an explicit pass infrastructure to manage optimizations and various other things. A <code>PassManager</code>, as should be obvious from its name, manages passes: it is responsible for scheduling them, invoking them, and insuring the proper disposal after we’re done with them. For this example, we’re just using a trivial pass that prints out our module in textual form.</p>
94
Duncan Sandsf8bc4062007-11-05 15:15:50 +0000[diff] [blame]95<p>Now onto the interesting part: creating and populating a module. Here’s the first chunk of our <code>makeLLVMModule()</code>:</p>
Owen Andersonb207ff22007-10-20 05:23:06 +0000[diff] [blame]96
97<div class="doc_code">
98<pre>
99Module* makeLLVMModule() {
100 // Module Construction
101 Module* mod = new Module("test");
102</pre>
103</div>
104
105<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>
106
107<div class="doc_code">
108<pre>
Chris Lattnera1ad2bf2008-02-10 19:11:04 +0000[diff] [blame]109 Constant* c = mod-&gt;getOrInsertFunction("mul_add",
Owen Andersonbbed0432007-10-22 06:29:31 +0000[diff] [blame]110 /*ret type*/ IntegerType::get(32),
111 /*args*/ IntegerType::get(32),
112 IntegerType::get(32),
113 IntegerType::get(32),
114 /*varargs terminated with null*/ NULL);
Owen Andersonb207ff22007-10-20 05:23:06 +0000[diff] [blame]115
Owen Andersonf747ab12007-10-20 05:40:47 +0000[diff] [blame]116 Function* mul_add = cast&lt;Function&gt;(c);
Chris Lattnera1ad2bf2008-02-10 19:11:04 +0000[diff] [blame]117 mul_add-&gt;setCallingConv(CallingConv::C);
Owen Andersonb207ff22007-10-20 05:23:06 +0000[diff] [blame]118</pre>
119</div>
120
Chris Lattnera1ad2bf2008-02-10 19:11:04 +0000[diff] [blame]121<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 Andersonb207ff22007-10-20 05:23:06 +0000[diff] [blame]122
Chris Lattnera1ad2bf2008-02-10 19:11:04 +0000[diff] [blame]123<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 Andersonf747ab12007-10-20 05:40:47 +0000[diff] [blame]124
125<p>In addition, we set the calling convention for our new function to be the C calling convention. This isn’t strictly necessary, but it insures that our new function will interoperate properly with C code, which is a good thing.</p>
Owen Andersonb207ff22007-10-20 05:23:06 +0000[diff] [blame]126
127<div class="doc_code">
128<pre>
Chris Lattnera1ad2bf2008-02-10 19:11:04 +0000[diff] [blame]129 Function::arg_iterator args = mul_add-&gt;arg_begin();
Owen Andersonb207ff22007-10-20 05:23:06 +0000[diff] [blame]130 Value* x = args++;
Chris Lattnera1ad2bf2008-02-10 19:11:04 +0000[diff] [blame]131 x-&gt;setName("x");
Owen Andersonb207ff22007-10-20 05:23:06 +0000[diff] [blame]132 Value* y = args++;
Chris Lattnera1ad2bf2008-02-10 19:11:04 +0000[diff] [blame]133 y-&gt;setName("y");
Owen Andersonb207ff22007-10-20 05:23:06 +0000[diff] [blame]134 Value* z = args++;
Chris Lattnera1ad2bf2008-02-10 19:11:04 +0000[diff] [blame]135 z-&gt;setName("z");
Owen Andersonb207ff22007-10-20 05:23:06 +0000[diff] [blame]136</pre>
137</div>
138
Chris Lattnera1ad2bf2008-02-10 19:11:04 +0000[diff] [blame]139<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 Andersonb207ff22007-10-20 05:23:06 +0000[diff] [blame]140
Chris Lattnera1ad2bf2008-02-10 19:11:04 +0000[diff] [blame]141<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 Andersonb207ff22007-10-20 05:23:06 +0000[diff] [blame]142
143<div class="doc_code">
144<pre>
Gabor Greif5b665492008-04-19 22:25:09 +0000[diff] [blame^]145 BasicBlock* block = BasicBlock::Create("entry", mul_add);
Duncan Sandsa07136e2008-04-13 06:22:09 +0000[diff] [blame]146 IRBuilder builder(block);
Owen Andersonb207ff22007-10-20 05:23:06 +0000[diff] [blame]147</pre>
148</div>
149
Duncan Sandsa07136e2008-04-13 06:22:09 +0000[diff] [blame]150<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 Andersonb207ff22007-10-20 05:23:06 +0000[diff] [blame]151
152<div class="doc_code">
153<pre>
154 Value* tmp = builder.CreateBinOp(Instruction::Mul,
155 x, y, "tmp");
156 Value* tmp2 = builder.CreateBinOp(Instruction::Add,
157 tmp, z, "tmp2");
158
159 builder.CreateRet(tmp2);
Owen Andersonbbed0432007-10-22 06:29:31 +0000[diff] [blame]160
161 return mod;
Owen Andersonb207ff22007-10-20 05:23:06 +0000[diff] [blame]162}
163</pre>
164</div>
165
Duncan Sandsa07136e2008-04-13 06:22:09 +0000[diff] [blame]166<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 Andersonb207ff22007-10-20 05:23:06 +0000[diff] [blame]167
Chris Lattnera1ad2bf2008-02-10 19:11:04 +0000[diff] [blame]168<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 Andersonb207ff22007-10-20 05:23:06 +0000[diff] [blame]169
Owen Anderson4c209da2007-10-25 06:49:29 +0000[diff] [blame]170<div class="doc_code">
171<pre>
Owen Andersonce268212008-03-24 21:38:01 +0000[diff] [blame]172# c++ -g tut1.cpp `llvm-config --cxxflags --ldflags --libs core` -o tut1
Chris Lattnera1ad2bf2008-02-10 19:11:04 +0000[diff] [blame]173# ./tut1
Owen Anderson4c209da2007-10-25 06:49:29 +0000[diff] [blame]174</pre>
175</div>
176
177<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 Andersonb207ff22007-10-20 05:23:06 +0000[diff] [blame]178
Chris Lattnera1ad2bf2008-02-10 19:11:04 +0000[diff] [blame]179<a href="JITTutorial2.html">Next: A More Complicated Function</a>
Owen Andersonb207ff22007-10-20 05:23:06 +0000[diff] [blame]180</div>
181
Owen Andersonaf8059c2007-10-25 06:45:01 +0000[diff] [blame]182<!-- *********************************************************************** -->
183<hr>
184<address>
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190 <a href="mailto:owen@apple.com">Owen Anderson</a><br>
191 <a href="http://llvm.org">The LLVM Compiler Infrastructure</a><br>
192 Last modified: $Date: 2007-10-17 11:05:13 -0700 (Wed, 17 Oct 2007) $
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194
Owen Andersonb207ff22007-10-20 05:23:06 +0000[diff] [blame]195</body>
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