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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 +0000[diff] [blame]22<!-- *********************************************************************** -->
Owen Andersonc333e4f2007-10-22 06:35:07 +0000[diff] [blame]23<div class="doc_section"><a name="intro">A First Function</a></div>
24<!-- *********************************************************************** -->
25
Owen Anderson6f1fd942007-10-20 05:23:06 +0000[diff] [blame]26<div class="doc_text">
27
Chris Lattner729eb142008-02-10 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 Anderson6f1fd942007-10-20 05:23:06 +0000[diff] [blame]29
30<div class="doc_code">
31<pre>
Owen Anderson7d69c952007-10-20 05:41:39 +0000[diff] [blame]32int mul_add(int x, int y, int z) {
33 return x * y + z;
34}
Owen Anderson6f1fd942007-10-20 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 Anderson7d69c952007-10-20 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 Anderson6f1fd942007-10-20 05:23:06 +0000[diff] [blame]48</pre>
49</div>
50
Bill Wendling176e8292008-05-19 00:19:02 +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 Anderson6f1fd942007-10-20 05:23:06 +0000[diff] [blame]52
Owen Anderson18f09922007-10-25 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>
Gabor Greifc540b1f2009-03-11 20:23:40 +0000[diff] [blame^]57#include "llvm/Module.h"
58#include "llvm/Function.h"
59#include "llvm/PassManager.h"
60#include "llvm/CallingConv.h"
61#include "llvm/Analysis/Verifier.h"
62#include "llvm/Assembly/PrintModulePass.h"
63#include "llvm/Support/IRBuilder.h"
Owen Anderson18f09922007-10-25 06:49:29 +0000[diff] [blame]64</pre>
65</div>
Owen Anderson6f1fd942007-10-20 05:23:06 +0000[diff] [blame]66
Bill Wendling176e8292008-05-19 00:19:02 +0000[diff] [blame]67<p>Now, let's get started on our real program. Here's what our basic <code>main()</code> will look like:</p>
Owen Anderson6f1fd942007-10-20 05:23:06 +0000[diff] [blame]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
Bill Wendlingd63729f2008-05-19 00:15:33 +0000[diff] [blame]84 delete Mod;
Owen Anderson6f1fd942007-10-20 05:23:06 +0000[diff] [blame]85 return 0;
86}
87</pre>
88</div>
89
Chris Lattner729eb142008-02-10 19:11:04 +0000[diff] [blame]90<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 +0000[diff] [blame]91
Bill Wendling176e8292008-05-19 00:19:02 +0000[diff] [blame]92<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>
Owen Anderson6f1fd942007-10-20 05:23:06 +0000[diff] [blame]93
Bill Wendlingc8968d92008-05-19 00:05:30 +0000[diff] [blame]94<p>Finally, we instantiate an LLVM <code>PassManager</code> and run
95the <code>PrintModulePass</code> on our module. LLVM uses an explicit pass
96infrastructure to manage optimizations and various other things.
97A <code>PassManager</code>, as should be obvious from its name, manages passes:
98it is responsible for scheduling them, invoking them, and ensuring the proper
99disposal after we’re done with them. For this example, we’re just using a
100trivial pass that prints out our module in textual form.</p>
Owen Anderson6f1fd942007-10-20 05:23:06 +0000[diff] [blame]101
Bill Wendling176e8292008-05-19 00:19:02 +0000[diff] [blame]102<p>Now onto the interesting part: creating and populating a module. Here's the
Bill Wendlingc8968d92008-05-19 00:05:30 +0000[diff] [blame]103first chunk of our <code>makeLLVMModule()</code>:</p>
Owen Anderson6f1fd942007-10-20 05:23:06 +0000[diff] [blame]104
105<div class="doc_code">
106<pre>
107Module* makeLLVMModule() {
108 // Module Construction
109 Module* mod = new Module("test");
110</pre>
111</div>
112
113<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>
114
115<div class="doc_code">
116<pre>
Chris Lattner729eb142008-02-10 19:11:04 +0000[diff] [blame]117 Constant* c = mod-&gt;getOrInsertFunction("mul_add",
Owen Andersond2ae9a92007-10-22 06:29:31 +0000[diff] [blame]118 /*ret type*/ IntegerType::get(32),
119 /*args*/ IntegerType::get(32),
120 IntegerType::get(32),
121 IntegerType::get(32),
122 /*varargs terminated with null*/ NULL);
Owen Anderson6f1fd942007-10-20 05:23:06 +0000[diff] [blame]123
Owen Anderson2d279f82007-10-20 05:40:47 +0000[diff] [blame]124 Function* mul_add = cast&lt;Function&gt;(c);
Chris Lattner729eb142008-02-10 19:11:04 +0000[diff] [blame]125 mul_add-&gt;setCallingConv(CallingConv::C);
Owen Anderson6f1fd942007-10-20 05:23:06 +0000[diff] [blame]126</pre>
127</div>
128
Chris Lattner729eb142008-02-10 19:11:04 +0000[diff] [blame]129<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 +0000[diff] [blame]130
Chris Lattner729eb142008-02-10 19:11:04 +0000[diff] [blame]131<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 +0000[diff] [blame]132
Bill Wendlingc8968d92008-05-19 00:05:30 +0000[diff] [blame]133<p>In addition, we set the calling convention for our new function to be the C
134calling convention. This isn’t strictly necessary, but it ensures that our new
135function will interoperate properly with C code, which is a good thing.</p>
Owen Anderson6f1fd942007-10-20 05:23:06 +0000[diff] [blame]136
137<div class="doc_code">
138<pre>
Chris Lattner729eb142008-02-10 19:11:04 +0000[diff] [blame]139 Function::arg_iterator args = mul_add-&gt;arg_begin();
Owen Anderson6f1fd942007-10-20 05:23:06 +0000[diff] [blame]140 Value* x = args++;
Chris Lattner729eb142008-02-10 19:11:04 +0000[diff] [blame]141 x-&gt;setName("x");
Owen Anderson6f1fd942007-10-20 05:23:06 +0000[diff] [blame]142 Value* y = args++;
Chris Lattner729eb142008-02-10 19:11:04 +0000[diff] [blame]143 y-&gt;setName("y");
Owen Anderson6f1fd942007-10-20 05:23:06 +0000[diff] [blame]144 Value* z = args++;
Chris Lattner729eb142008-02-10 19:11:04 +0000[diff] [blame]145 z-&gt;setName("z");
Owen Anderson6f1fd942007-10-20 05:23:06 +0000[diff] [blame]146</pre>
147</div>
148
Bill Wendling176e8292008-05-19 00:19:02 +0000[diff] [blame]149<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 +0000[diff] [blame]150
Chris Lattner729eb142008-02-10 19:11:04 +0000[diff] [blame]151<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 +0000[diff] [blame]152
153<div class="doc_code">
154<pre>
Gabor Greifdf7d2b42008-04-19 22:25:09 +0000[diff] [blame]155 BasicBlock* block = BasicBlock::Create("entry", mul_add);
Gabor Greifd6c1ed02009-03-11 19:51:07 +0000[diff] [blame]156 IRBuilder&lt;&gt; builder(block);
Owen Anderson6f1fd942007-10-20 05:23:06 +0000[diff] [blame]157</pre>
158</div>
159
Duncan Sands89f6d882008-04-13 06:22:09 +0000[diff] [blame]160<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 Anderson6f1fd942007-10-20 05:23:06 +0000[diff] [blame]161
162<div class="doc_code">
163<pre>
164 Value* tmp = builder.CreateBinOp(Instruction::Mul,
Bill Wendlingd63729f2008-05-19 00:15:33 +0000[diff] [blame]165 x, y, "tmp");
Owen Anderson6f1fd942007-10-20 05:23:06 +0000[diff] [blame]166 Value* tmp2 = builder.CreateBinOp(Instruction::Add,
167 tmp, z, "tmp2");
168
169 builder.CreateRet(tmp2);
Owen Andersond2ae9a92007-10-22 06:29:31 +0000[diff] [blame]170
171 return mod;
Owen Anderson6f1fd942007-10-20 05:23:06 +0000[diff] [blame]172}
173</pre>
174</div>
175
Bill Wendling176e8292008-05-19 00:19:02 +0000[diff] [blame]176<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 Anderson6f1fd942007-10-20 05:23:06 +0000[diff] [blame]177
Bill Wendling176e8292008-05-19 00:19:02 +0000[diff] [blame]178<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 +0000[diff] [blame]179
Owen Anderson18f09922007-10-25 06:49:29 +0000[diff] [blame]180<div class="doc_code">
181<pre>
Owen Andersonaec96002008-03-24 21:38:01 +0000[diff] [blame]182# c++ -g tut1.cpp `llvm-config --cxxflags --ldflags --libs core` -o tut1
Chris Lattner729eb142008-02-10 19:11:04 +0000[diff] [blame]183# ./tut1
Owen Anderson18f09922007-10-25 06:49:29 +0000[diff] [blame]184</pre>
185</div>
186
187<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 +0000[diff] [blame]188
Chris Lattner729eb142008-02-10 19:11:04 +0000[diff] [blame]189<a href="JITTutorial2.html">Next: A More Complicated Function</a>
Owen Anderson6f1fd942007-10-20 05:23:06 +0000[diff] [blame]190</div>
191
Owen Anderson34ba67a2007-10-25 06:45:01 +0000[diff] [blame]192<!-- *********************************************************************** -->
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200 <a href="mailto:owen@apple.com">Owen Anderson</a><br>
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Owen Anderson6f1fd942007-10-20 05:23:06 +0000[diff] [blame]205</body>
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