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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 Andersonc333e4f2007-10-22 06:35:07 +000022<!-- *********************************************************************** -->
Owen Andersonc333e4f2007-10-22 06:35:07 +000023<div class="doc_section"><a name="intro">A First Function</a></div>
24<!-- *********************************************************************** -->
25
Owen Anderson6f1fd942007-10-20 05:23:06 +000026<div class="doc_text">
27
Chris Lattner729eb142008-02-10 19:11:04 +000028<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 Anderson6f1fd942007-10-20 05:23:06 +000029
30<div class="doc_code">
31<pre>
Owen Anderson7d69c952007-10-20 05:41:39 +000032int mul_add(int x, int y, int z) {
33 return x * y + z;
34}
Owen Anderson6f1fd942007-10-20 05:23:06 +000035</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 Anderson7d69c952007-10-20 05:41:39 +000042define 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 Anderson6f1fd942007-10-20 05:23:06 +000048</pre>
49</div>
50
Owen Anderson0ec16ee2007-10-20 06:12:33 +000051<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 Anderson6f1fd942007-10-20 05:23:06 +000052
Owen Anderson18f09922007-10-25 06:49:29 +000053<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 Anderson89c94f52007-11-06 01:26:56 +000060#include &lt;llvm/CallingConv.h&gt;
Owen Anderson18f09922007-10-25 06:49:29 +000061#include &lt;llvm/Analysis/Verifier.h&gt;
62#include &lt;llvm/Assembly/PrintModulePass.h&gt;
63#include &lt;llvm/Support/LLVMBuilder.h&gt;
64</pre>
65</div>
Owen Anderson6f1fd942007-10-20 05:23:06 +000066
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 Lattner729eb142008-02-10 19:11:04 +000089<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 Anderson6f1fd942007-10-20 05:23:06 +000090
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 Sandse0a34352007-11-05 15:15:50 +000095<p>Now onto the interesting part: creating and populating a module. Here’s the first chunk of our <code>makeLLVMModule()</code>:</p>
Owen Anderson6f1fd942007-10-20 05:23:06 +000096
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 Lattner729eb142008-02-10 19:11:04 +0000109 Constant* c = mod-&gt;getOrInsertFunction("mul_add",
Owen Andersond2ae9a92007-10-22 06:29:31 +0000110 /*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 Anderson6f1fd942007-10-20 05:23:06 +0000115
Owen Anderson2d279f82007-10-20 05:40:47 +0000116 Function* mul_add = cast&lt;Function&gt;(c);
Chris Lattner729eb142008-02-10 19:11:04 +0000117 mul_add-&gt;setCallingConv(CallingConv::C);
Owen Anderson6f1fd942007-10-20 05:23:06 +0000118</pre>
119</div>
120
Chris Lattner729eb142008-02-10 19:11:04 +0000121<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 Anderson6f1fd942007-10-20 05:23:06 +0000122
Chris Lattner729eb142008-02-10 19:11:04 +0000123<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 Anderson2d279f82007-10-20 05:40:47 +0000124
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 Anderson6f1fd942007-10-20 05:23:06 +0000126
127<div class="doc_code">
128<pre>
Chris Lattner729eb142008-02-10 19:11:04 +0000129 Function::arg_iterator args = mul_add-&gt;arg_begin();
Owen Anderson6f1fd942007-10-20 05:23:06 +0000130 Value* x = args++;
Chris Lattner729eb142008-02-10 19:11:04 +0000131 x-&gt;setName("x");
Owen Anderson6f1fd942007-10-20 05:23:06 +0000132 Value* y = args++;
Chris Lattner729eb142008-02-10 19:11:04 +0000133 y-&gt;setName("y");
Owen Anderson6f1fd942007-10-20 05:23:06 +0000134 Value* z = args++;
Chris Lattner729eb142008-02-10 19:11:04 +0000135 z-&gt;setName("z");
Owen Anderson6f1fd942007-10-20 05:23:06 +0000136</pre>
137</div>
138
Chris Lattner729eb142008-02-10 19:11:04 +0000139<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 Anderson6f1fd942007-10-20 05:23:06 +0000140
Chris Lattner729eb142008-02-10 19:11:04 +0000141<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 Anderson6f1fd942007-10-20 05:23:06 +0000142
143<div class="doc_code">
144<pre>
145 BasicBlock* block = new BasicBlock("entry", mul_add);
146 LLVMBuilder builder(block);
147</pre>
148</div>
149
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>LLVMBuilder</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>LLVMBuilder</code> will make your life simpler.</p>
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 Andersond2ae9a92007-10-22 06:29:31 +0000160
161 return mod;
Owen Anderson6f1fd942007-10-20 05:23:06 +0000162}
163</pre>
164</div>
165
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>LLVMBuilder</code> gives us a simple interface for constructing these instructions and appending them to the “entry” block. Each of the calls to <code>LLVMBuilder</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>
167
Chris Lattner729eb142008-02-10 19:11:04 +0000168<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 Anderson6f1fd942007-10-20 05:23:06 +0000169
Owen Anderson18f09922007-10-25 06:49:29 +0000170<div class="doc_code">
171<pre>
Owen Andersonaec96002008-03-24 21:38:01 +0000172# c++ -g tut1.cpp `llvm-config --cxxflags --ldflags --libs core` -o tut1
Chris Lattner729eb142008-02-10 19:11:04 +0000173# ./tut1
Owen Anderson18f09922007-10-25 06:49:29 +0000174</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 Anderson6f1fd942007-10-20 05:23:06 +0000178
Chris Lattner729eb142008-02-10 19:11:04 +0000179<a href="JITTutorial2.html">Next: A More Complicated Function</a>
Owen Anderson6f1fd942007-10-20 05:23:06 +0000180</div>
181
Owen Anderson34ba67a2007-10-25 06:45:01 +0000182<!-- *********************************************************************** -->
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