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   5<head>
   6  <title>Kaleidoscope: Implementing code generation to LLVM IR</title>
   7  <meta http-equiv="Content-Type" content="text/html; charset=utf-8">
   8  <meta name="author" content="Chris Lattner">
   9  <meta name="author" content="Erick Tryzelaar">
  10  <link rel="stylesheet" href="../_static/llvm.css" type="text/css">
  11</head>
  12
  13<body>
  14
  15<h1>Kaleidoscope: Code generation to LLVM IR</h1>
  16
  17<ul>
  18<li><a href="index.html">Up to Tutorial Index</a></li>
  19<li>Chapter 3
  20  <ol>
  21    <li><a href="#intro">Chapter 3 Introduction</a></li>
  22    <li><a href="#basics">Code Generation Setup</a></li>
  23    <li><a href="#exprs">Expression Code Generation</a></li>
  24    <li><a href="#funcs">Function Code Generation</a></li>
  25    <li><a href="#driver">Driver Changes and Closing Thoughts</a></li>
  26    <li><a href="#code">Full Code Listing</a></li>
  27  </ol>
  28</li>
  29<li><a href="OCamlLangImpl4.html">Chapter 4</a>: Adding JIT and Optimizer
  30Support</li>
  31</ul>
  32
  33<div class="doc_author">
  34	<p>
  35		Written by <a href="mailto:sabre@nondot.org">Chris Lattner</a>
  36		and <a href="mailto:idadesub@users.sourceforge.net">Erick Tryzelaar</a>
  37	</p>
  38</div>
  39
  40<!-- *********************************************************************** -->
  41<h2><a name="intro">Chapter 3 Introduction</a></h2>
  42<!-- *********************************************************************** -->
  43
  44<div>
  45
  46<p>Welcome to Chapter 3 of the "<a href="index.html">Implementing a language
  47with LLVM</a>" tutorial.  This chapter shows you how to transform the <a
  48href="OCamlLangImpl2.html">Abstract Syntax Tree</a>, built in Chapter 2, into
  49LLVM IR.  This will teach you a little bit about how LLVM does things, as well
  50as demonstrate how easy it is to use.  It's much more work to build a lexer and
  51parser than it is to generate LLVM IR code. :)
  52</p>
  53
  54<p><b>Please note</b>: the code in this chapter and later require LLVM 2.3 or
  55LLVM SVN to work.  LLVM 2.2 and before will not work with it.</p>
  56
  57</div>
  58
  59<!-- *********************************************************************** -->
  60<h2><a name="basics">Code Generation Setup</a></h2>
  61<!-- *********************************************************************** -->
  62
  63<div>
  64
  65<p>
  66In order to generate LLVM IR, we want some simple setup to get started.  First
  67we define virtual code generation (codegen) methods in each AST class:</p>
  68
  69<div class="doc_code">
  70<pre>
  71let rec codegen_expr = function
  72  | Ast.Number n -&gt; ...
  73  | Ast.Variable name -&gt; ...
  74</pre>
  75</div>
  76
  77<p>The <tt>Codegen.codegen_expr</tt> function says to emit IR for that AST node
  78along with all the things it depends on, and they all return an LLVM Value
  79object.  "Value" is the class used to represent a "<a
  80href="http://en.wikipedia.org/wiki/Static_single_assignment_form">Static Single
  81Assignment (SSA)</a> register" or "SSA value" in LLVM.  The most distinct aspect
  82of SSA values is that their value is computed as the related instruction
  83executes, and it does not get a new value until (and if) the instruction
  84re-executes.  In other words, there is no way to "change" an SSA value.  For
  85more information, please read up on <a
  86href="http://en.wikipedia.org/wiki/Static_single_assignment_form">Static Single
  87Assignment</a> - the concepts are really quite natural once you grok them.</p>
  88
  89<p>The
  90second thing we want is an "Error" exception like we used for the parser, which
  91will be used to report errors found during code generation (for example, use of
  92an undeclared parameter):</p>
  93
  94<div class="doc_code">
  95<pre>
  96exception Error of string
  97
  98let context = global_context ()
  99let the_module = create_module context "my cool jit"
 100let builder = builder context
 101let named_values:(string, llvalue) Hashtbl.t = Hashtbl.create 10
 102let double_type = double_type context
 103</pre>
 104</div>
 105
 106<p>The static variables will be used during code generation.
 107<tt>Codgen.the_module</tt> is the LLVM construct that contains all of the
 108functions and global variables in a chunk of code.  In many ways, it is the
 109top-level structure that the LLVM IR uses to contain code.</p>
 110
 111<p>The <tt>Codegen.builder</tt> object is a helper object that makes it easy to
 112generate LLVM instructions.  Instances of the <a
 113href="http://llvm.org/doxygen/IRBuilder_8h-source.html"><tt>IRBuilder</tt></a>
 114class keep track of the current place to insert instructions and has methods to
 115create new instructions.</p>
 116
 117<p>The <tt>Codegen.named_values</tt> map keeps track of which values are defined
 118in the current scope and what their LLVM representation is.  (In other words, it
 119is a symbol table for the code).  In this form of Kaleidoscope, the only things
 120that can be referenced are function parameters.  As such, function parameters
 121will be in this map when generating code for their function body.</p>
 122
 123<p>
 124With these basics in place, we can start talking about how to generate code for
 125each expression.  Note that this assumes that the <tt>Codgen.builder</tt> has
 126been set up to generate code <em>into</em> something.  For now, we'll assume
 127that this has already been done, and we'll just use it to emit code.</p>
 128
 129</div>
 130
 131<!-- *********************************************************************** -->
 132<h2><a name="exprs">Expression Code Generation</a></h2>
 133<!-- *********************************************************************** -->
 134
 135<div>
 136
 137<p>Generating LLVM code for expression nodes is very straightforward: less
 138than 30 lines of commented code for all four of our expression nodes.  First
 139we'll do numeric literals:</p>
 140
 141<div class="doc_code">
 142<pre>
 143  | Ast.Number n -&gt; const_float double_type n
 144</pre>
 145</div>
 146
 147<p>In the LLVM IR, numeric constants are represented with the
 148<tt>ConstantFP</tt> class, which holds the numeric value in an <tt>APFloat</tt>
 149internally (<tt>APFloat</tt> has the capability of holding floating point
 150constants of <em>A</em>rbitrary <em>P</em>recision).  This code basically just
 151creates and returns a <tt>ConstantFP</tt>.  Note that in the LLVM IR
 152that constants are all uniqued together and shared.  For this reason, the API
 153uses "the foo::get(..)" idiom instead of "new foo(..)" or "foo::Create(..)".</p>
 154
 155<div class="doc_code">
 156<pre>
 157  | Ast.Variable name -&gt;
 158      (try Hashtbl.find named_values name with
 159        | Not_found -&gt; raise (Error "unknown variable name"))
 160</pre>
 161</div>
 162
 163<p>References to variables are also quite simple using LLVM.  In the simple
 164version of Kaleidoscope, we assume that the variable has already been emitted
 165somewhere and its value is available.  In practice, the only values that can be
 166in the <tt>Codegen.named_values</tt> map are function arguments.  This code
 167simply checks to see that the specified name is in the map (if not, an unknown
 168variable is being referenced) and returns the value for it.  In future chapters,
 169we'll add support for <a href="LangImpl5.html#for">loop induction variables</a>
 170in the symbol table, and for <a href="LangImpl7.html#localvars">local
 171variables</a>.</p>
 172
 173<div class="doc_code">
 174<pre>
 175  | Ast.Binary (op, lhs, rhs) -&gt;
 176      let lhs_val = codegen_expr lhs in
 177      let rhs_val = codegen_expr rhs in
 178      begin
 179        match op with
 180        | '+' -&gt; build_fadd lhs_val rhs_val "addtmp" builder
 181        | '-' -&gt; build_fsub lhs_val rhs_val "subtmp" builder
 182        | '*' -&gt; build_fmul lhs_val rhs_val "multmp" builder
 183        | '&lt;' -&gt;
 184            (* Convert bool 0/1 to double 0.0 or 1.0 *)
 185            let i = build_fcmp Fcmp.Ult lhs_val rhs_val "cmptmp" builder in
 186            build_uitofp i double_type "booltmp" builder
 187        | _ -&gt; raise (Error "invalid binary operator")
 188      end
 189</pre>
 190</div>
 191
 192<p>Binary operators start to get more interesting.  The basic idea here is that
 193we recursively emit code for the left-hand side of the expression, then the
 194right-hand side, then we compute the result of the binary expression.  In this
 195code, we do a simple switch on the opcode to create the right LLVM instruction.
 196</p>
 197
 198<p>In the example above, the LLVM builder class is starting to show its value.
 199IRBuilder knows where to insert the newly created instruction, all you have to
 200do is specify what instruction to create (e.g. with <tt>Llvm.create_add</tt>),
 201which operands to use (<tt>lhs</tt> and <tt>rhs</tt> here) and optionally
 202provide a name for the generated instruction.</p>
 203
 204<p>One nice thing about LLVM is that the name is just a hint.  For instance, if
 205the code above emits multiple "addtmp" variables, LLVM will automatically
 206provide each one with an increasing, unique numeric suffix.  Local value names
 207for instructions are purely optional, but it makes it much easier to read the
 208IR dumps.</p>
 209
 210<p><a href="../LangRef.html#instref">LLVM instructions</a> are constrained by
 211strict rules: for example, the Left and Right operators of
 212an <a href="../LangRef.html#i_add">add instruction</a> must have the same
 213type, and the result type of the add must match the operand types.  Because
 214all values in Kaleidoscope are doubles, this makes for very simple code for add,
 215sub and mul.</p>
 216
 217<p>On the other hand, LLVM specifies that the <a
 218href="../LangRef.html#i_fcmp">fcmp instruction</a> always returns an 'i1' value
 219(a one bit integer).  The problem with this is that Kaleidoscope wants the value to be a 0.0 or 1.0 value.  In order to get these semantics, we combine the fcmp instruction with
 220a <a href="../LangRef.html#i_uitofp">uitofp instruction</a>.  This instruction
 221converts its input integer into a floating point value by treating the input
 222as an unsigned value.  In contrast, if we used the <a
 223href="../LangRef.html#i_sitofp">sitofp instruction</a>, the Kaleidoscope '&lt;'
 224operator would return 0.0 and -1.0, depending on the input value.</p>
 225
 226<div class="doc_code">
 227<pre>
 228  | Ast.Call (callee, args) -&gt;
 229      (* Look up the name in the module table. *)
 230      let callee =
 231        match lookup_function callee the_module with
 232        | Some callee -&gt; callee
 233        | None -&gt; raise (Error "unknown function referenced")
 234      in
 235      let params = params callee in
 236
 237      (* If argument mismatch error. *)
 238      if Array.length params == Array.length args then () else
 239        raise (Error "incorrect # arguments passed");
 240      let args = Array.map codegen_expr args in
 241      build_call callee args "calltmp" builder
 242</pre>
 243</div>
 244
 245<p>Code generation for function calls is quite straightforward with LLVM.  The
 246code above initially does a function name lookup in the LLVM Module's symbol
 247table.  Recall that the LLVM Module is the container that holds all of the
 248functions we are JIT'ing.  By giving each function the same name as what the
 249user specifies, we can use the LLVM symbol table to resolve function names for
 250us.</p>
 251
 252<p>Once we have the function to call, we recursively codegen each argument that
 253is to be passed in, and create an LLVM <a href="../LangRef.html#i_call">call
 254instruction</a>.  Note that LLVM uses the native C calling conventions by
 255default, allowing these calls to also call into standard library functions like
 256"sin" and "cos", with no additional effort.</p>
 257
 258<p>This wraps up our handling of the four basic expressions that we have so far
 259in Kaleidoscope.  Feel free to go in and add some more.  For example, by
 260browsing the <a href="../LangRef.html">LLVM language reference</a> you'll find
 261several other interesting instructions that are really easy to plug into our
 262basic framework.</p>
 263
 264</div>
 265
 266<!-- *********************************************************************** -->
 267<h2><a name="funcs">Function Code Generation</a></h2>
 268<!-- *********************************************************************** -->
 269
 270<div>
 271
 272<p>Code generation for prototypes and functions must handle a number of
 273details, which make their code less beautiful than expression code
 274generation, but allows us to illustrate some important points.  First, lets
 275talk about code generation for prototypes: they are used both for function
 276bodies and external function declarations.  The code starts with:</p>
 277
 278<div class="doc_code">
 279<pre>
 280let codegen_proto = function
 281  | Ast.Prototype (name, args) -&gt;
 282      (* Make the function type: double(double,double) etc. *)
 283      let doubles = Array.make (Array.length args) double_type in
 284      let ft = function_type double_type doubles in
 285      let f =
 286        match lookup_function name the_module with
 287</pre>
 288</div>
 289
 290<p>This code packs a lot of power into a few lines.  Note first that this
 291function returns a "Function*" instead of a "Value*" (although at the moment
 292they both are modeled by <tt>llvalue</tt> in ocaml).  Because a "prototype"
 293really talks about the external interface for a function (not the value computed
 294by an expression), it makes sense for it to return the LLVM Function it
 295corresponds to when codegen'd.</p>
 296
 297<p>The call to <tt>Llvm.function_type</tt> creates the <tt>Llvm.llvalue</tt>
 298that should be used for a given Prototype.  Since all function arguments in
 299Kaleidoscope are of type double, the first line creates a vector of "N" LLVM
 300double types.  It then uses the <tt>Llvm.function_type</tt> method to create a
 301function type that takes "N" doubles as arguments, returns one double as a
 302result, and that is not vararg (that uses the function
 303<tt>Llvm.var_arg_function_type</tt>).  Note that Types in LLVM are uniqued just
 304like <tt>Constant</tt>s are, so you don't "new" a type, you "get" it.</p>
 305
 306<p>The final line above checks if the function has already been defined in
 307<tt>Codegen.the_module</tt>. If not, we will create it.</p>
 308
 309<div class="doc_code">
 310<pre>
 311        | None -&gt; declare_function name ft the_module
 312</pre>
 313</div>
 314
 315<p>This indicates the type and name to use, as well as which module to insert
 316into.  By default we assume a function has
 317<tt>Llvm.Linkage.ExternalLinkage</tt>.  "<a href="LangRef.html#linkage">external
 318linkage</a>" means that the function may be defined outside the current module
 319and/or that it is callable by functions outside the module.  The "<tt>name</tt>"
 320passed in is the name the user specified: this name is registered in
 321"<tt>Codegen.the_module</tt>"s symbol table, which is used by the function call
 322code above.</p>
 323
 324<p>In Kaleidoscope, I choose to allow redefinitions of functions in two cases:
 325first, we want to allow 'extern'ing a function more than once, as long as the
 326prototypes for the externs match (since all arguments have the same type, we
 327just have to check that the number of arguments match).  Second, we want to
 328allow 'extern'ing a function and then defining a body for it.  This is useful
 329when defining mutually recursive functions.</p>
 330
 331<div class="doc_code">
 332<pre>
 333        (* If 'f' conflicted, there was already something named 'name'. If it
 334         * has a body, don't allow redefinition or reextern. *)
 335        | Some f -&gt;
 336            (* If 'f' already has a body, reject this. *)
 337            if Array.length (basic_blocks f) == 0 then () else
 338              raise (Error "redefinition of function");
 339
 340            (* If 'f' took a different number of arguments, reject. *)
 341            if Array.length (params f) == Array.length args then () else
 342              raise (Error "redefinition of function with different # args");
 343            f
 344      in
 345</pre>
 346</div>
 347
 348<p>In order to verify the logic above, we first check to see if the pre-existing
 349function is "empty".  In this case, empty means that it has no basic blocks in
 350it, which means it has no body.  If it has no body, it is a forward
 351declaration.  Since we don't allow anything after a full definition of the
 352function, the code rejects this case.  If the previous reference to a function
 353was an 'extern', we simply verify that the number of arguments for that
 354definition and this one match up.  If not, we emit an error.</p>
 355
 356<div class="doc_code">
 357<pre>
 358      (* Set names for all arguments. *)
 359      Array.iteri (fun i a -&gt;
 360        let n = args.(i) in
 361        set_value_name n a;
 362        Hashtbl.add named_values n a;
 363      ) (params f);
 364      f
 365</pre>
 366</div>
 367
 368<p>The last bit of code for prototypes loops over all of the arguments in the
 369function, setting the name of the LLVM Argument objects to match, and registering
 370the arguments in the <tt>Codegen.named_values</tt> map for future use by the
 371<tt>Ast.Variable</tt> variant.  Once this is set up, it returns the Function
 372object to the caller.  Note that we don't check for conflicting
 373argument names here (e.g. "extern foo(a b a)").  Doing so would be very
 374straight-forward with the mechanics we have already used above.</p>
 375
 376<div class="doc_code">
 377<pre>
 378let codegen_func = function
 379  | Ast.Function (proto, body) -&gt;
 380      Hashtbl.clear named_values;
 381      let the_function = codegen_proto proto in
 382</pre>
 383</div>
 384
 385<p>Code generation for function definitions starts out simply enough: we just
 386codegen the prototype (Proto) and verify that it is ok.  We then clear out the
 387<tt>Codegen.named_values</tt> map to make sure that there isn't anything in it
 388from the last function we compiled.  Code generation of the prototype ensures
 389that there is an LLVM Function object that is ready to go for us.</p>
 390
 391<div class="doc_code">
 392<pre>
 393      (* Create a new basic block to start insertion into. *)
 394      let bb = append_block context "entry" the_function in
 395      position_at_end bb builder;
 396
 397      try
 398        let ret_val = codegen_expr body in
 399</pre>
 400</div>
 401
 402<p>Now we get to the point where the <tt>Codegen.builder</tt> is set up.  The
 403first line creates a new
 404<a href="http://en.wikipedia.org/wiki/Basic_block">basic block</a> (named
 405"entry"), which is inserted into <tt>the_function</tt>.  The second line then
 406tells the builder that new instructions should be inserted into the end of the
 407new basic block.  Basic blocks in LLVM are an important part of functions that
 408define the <a
 409href="http://en.wikipedia.org/wiki/Control_flow_graph">Control Flow Graph</a>.
 410Since we don't have any control flow, our functions will only contain one
 411block at this point.  We'll fix this in <a href="OCamlLangImpl5.html">Chapter
 4125</a> :).</p>
 413
 414<div class="doc_code">
 415<pre>
 416        let ret_val = codegen_expr body in
 417
 418        (* Finish off the function. *)
 419        let _ = build_ret ret_val builder in
 420
 421        (* Validate the generated code, checking for consistency. *)
 422        Llvm_analysis.assert_valid_function the_function;
 423
 424        the_function
 425</pre>
 426</div>
 427
 428<p>Once the insertion point is set up, we call the <tt>Codegen.codegen_func</tt>
 429method for the root expression of the function.  If no error happens, this emits
 430code to compute the expression into the entry block and returns the value that
 431was computed.  Assuming no error, we then create an LLVM <a
 432href="../LangRef.html#i_ret">ret instruction</a>, which completes the function.
 433Once the function is built, we call
 434<tt>Llvm_analysis.assert_valid_function</tt>, which is provided by LLVM.  This
 435function does a variety of consistency checks on the generated code, to
 436determine if our compiler is doing everything right.  Using this is important:
 437it can catch a lot of bugs.  Once the function is finished and validated, we
 438return it.</p>
 439
 440<div class="doc_code">
 441<pre>
 442      with e -&gt;
 443        delete_function the_function;
 444        raise e
 445</pre>
 446</div>
 447
 448<p>The only piece left here is handling of the error case.  For simplicity, we
 449handle this by merely deleting the function we produced with the
 450<tt>Llvm.delete_function</tt> method.  This allows the user to redefine a
 451function that they incorrectly typed in before: if we didn't delete it, it
 452would live in the symbol table, with a body, preventing future redefinition.</p>
 453
 454<p>This code does have a bug, though.  Since the <tt>Codegen.codegen_proto</tt>
 455can return a previously defined forward declaration, our code can actually delete
 456a forward declaration.  There are a number of ways to fix this bug, see what you
 457can come up with!  Here is a testcase:</p>
 458
 459<div class="doc_code">
 460<pre>
 461extern foo(a b);     # ok, defines foo.
 462def foo(a b) c;      # error, 'c' is invalid.
 463def bar() foo(1, 2); # error, unknown function "foo"
 464</pre>
 465</div>
 466
 467</div>
 468
 469<!-- *********************************************************************** -->
 470<h2><a name="driver">Driver Changes and Closing Thoughts</a></h2>
 471<!-- *********************************************************************** -->
 472
 473<div>
 474
 475<p>
 476For now, code generation to LLVM doesn't really get us much, except that we can
 477look at the pretty IR calls.  The sample code inserts calls to Codegen into the
 478"<tt>Toplevel.main_loop</tt>", and then dumps out the LLVM IR.  This gives a
 479nice way to look at the LLVM IR for simple functions.  For example:
 480</p>
 481
 482<div class="doc_code">
 483<pre>
 484ready&gt; <b>4+5</b>;
 485Read top-level expression:
 486define double @""() {
 487entry:
 488        %addtmp = fadd double 4.000000e+00, 5.000000e+00
 489        ret double %addtmp
 490}
 491</pre>
 492</div>
 493
 494<p>Note how the parser turns the top-level expression into anonymous functions
 495for us.  This will be handy when we add <a href="OCamlLangImpl4.html#jit">JIT
 496support</a> in the next chapter.  Also note that the code is very literally
 497transcribed, no optimizations are being performed.  We will
 498<a href="OCamlLangImpl4.html#trivialconstfold">add optimizations</a> explicitly
 499in the next chapter.</p>
 500
 501<div class="doc_code">
 502<pre>
 503ready&gt; <b>def foo(a b) a*a + 2*a*b + b*b;</b>
 504Read function definition:
 505define double @foo(double %a, double %b) {
 506entry:
 507        %multmp = fmul double %a, %a
 508        %multmp1 = fmul double 2.000000e+00, %a
 509        %multmp2 = fmul double %multmp1, %b
 510        %addtmp = fadd double %multmp, %multmp2
 511        %multmp3 = fmul double %b, %b
 512        %addtmp4 = fadd double %addtmp, %multmp3
 513        ret double %addtmp4
 514}
 515</pre>
 516</div>
 517
 518<p>This shows some simple arithmetic. Notice the striking similarity to the
 519LLVM builder calls that we use to create the instructions.</p>
 520
 521<div class="doc_code">
 522<pre>
 523ready&gt; <b>def bar(a) foo(a, 4.0) + bar(31337);</b>
 524Read function definition:
 525define double @bar(double %a) {
 526entry:
 527        %calltmp = call double @foo(double %a, double 4.000000e+00)
 528        %calltmp1 = call double @bar(double 3.133700e+04)
 529        %addtmp = fadd double %calltmp, %calltmp1
 530        ret double %addtmp
 531}
 532</pre>
 533</div>
 534
 535<p>This shows some function calls.  Note that this function will take a long
 536time to execute if you call it.  In the future we'll add conditional control
 537flow to actually make recursion useful :).</p>
 538
 539<div class="doc_code">
 540<pre>
 541ready&gt; <b>extern cos(x);</b>
 542Read extern:
 543declare double @cos(double)
 544
 545ready&gt; <b>cos(1.234);</b>
 546Read top-level expression:
 547define double @""() {
 548entry:
 549        %calltmp = call double @cos(double 1.234000e+00)
 550        ret double %calltmp
 551}
 552</pre>
 553</div>
 554
 555<p>This shows an extern for the libm "cos" function, and a call to it.</p>
 556
 557
 558<div class="doc_code">
 559<pre>
 560ready&gt; <b>^D</b>
 561; ModuleID = 'my cool jit'
 562
 563define double @""() {
 564entry:
 565        %addtmp = fadd double 4.000000e+00, 5.000000e+00
 566        ret double %addtmp
 567}
 568
 569define double @foo(double %a, double %b) {
 570entry:
 571        %multmp = fmul double %a, %a
 572        %multmp1 = fmul double 2.000000e+00, %a
 573        %multmp2 = fmul double %multmp1, %b
 574        %addtmp = fadd double %multmp, %multmp2
 575        %multmp3 = fmul double %b, %b
 576        %addtmp4 = fadd double %addtmp, %multmp3
 577        ret double %addtmp4
 578}
 579
 580define double @bar(double %a) {
 581entry:
 582        %calltmp = call double @foo(double %a, double 4.000000e+00)
 583        %calltmp1 = call double @bar(double 3.133700e+04)
 584        %addtmp = fadd double %calltmp, %calltmp1
 585        ret double %addtmp
 586}
 587
 588declare double @cos(double)
 589
 590define double @""() {
 591entry:
 592        %calltmp = call double @cos(double 1.234000e+00)
 593        ret double %calltmp
 594}
 595</pre>
 596</div>
 597
 598<p>When you quit the current demo, it dumps out the IR for the entire module
 599generated.  Here you can see the big picture with all the functions referencing
 600each other.</p>
 601
 602<p>This wraps up the third chapter of the Kaleidoscope tutorial.  Up next, we'll
 603describe how to <a href="OCamlLangImpl4.html">add JIT codegen and optimizer
 604support</a> to this so we can actually start running code!</p>
 605
 606</div>
 607
 608
 609<!-- *********************************************************************** -->
 610<h2><a name="code">Full Code Listing</a></h2>
 611<!-- *********************************************************************** -->
 612
 613<div>
 614
 615<p>
 616Here is the complete code listing for our running example, enhanced with the
 617LLVM code generator.    Because this uses the LLVM libraries, we need to link
 618them in.  To do this, we use the <a
 619href="http://llvm.org/cmds/llvm-config.html">llvm-config</a> tool to inform
 620our makefile/command line about which options to use:</p>
 621
 622<div class="doc_code">
 623<pre>
 624# Compile
 625ocamlbuild toy.byte
 626# Run
 627./toy.byte
 628</pre>
 629</div>
 630
 631<p>Here is the code:</p>
 632
 633<dl>
 634<dt>_tags:</dt>
 635<dd class="doc_code">
 636<pre>
 637&lt;{lexer,parser}.ml&gt;: use_camlp4, pp(camlp4of)
 638&lt;*.{byte,native}&gt;: g++, use_llvm, use_llvm_analysis
 639</pre>
 640</dd>
 641
 642<dt>myocamlbuild.ml:</dt>
 643<dd class="doc_code">
 644<pre>
 645open Ocamlbuild_plugin;;
 646
 647ocaml_lib ~extern:true "llvm";;
 648ocaml_lib ~extern:true "llvm_analysis";;
 649
 650flag ["link"; "ocaml"; "g++"] (S[A"-cc"; A"g++"]);;
 651</pre>
 652</dd>
 653
 654<dt>token.ml:</dt>
 655<dd class="doc_code">
 656<pre>
 657(*===----------------------------------------------------------------------===
 658 * Lexer Tokens
 659 *===----------------------------------------------------------------------===*)
 660
 661(* The lexer returns these 'Kwd' if it is an unknown character, otherwise one of
 662 * these others for known things. *)
 663type token =
 664  (* commands *)
 665  | Def | Extern
 666
 667  (* primary *)
 668  | Ident of string | Number of float
 669
 670  (* unknown *)
 671  | Kwd of char
 672</pre>
 673</dd>
 674
 675<dt>lexer.ml:</dt>
 676<dd class="doc_code">
 677<pre>
 678(*===----------------------------------------------------------------------===
 679 * Lexer
 680 *===----------------------------------------------------------------------===*)
 681
 682let rec lex = parser
 683  (* Skip any whitespace. *)
 684  | [&lt; ' (' ' | '\n' | '\r' | '\t'); stream &gt;] -&gt; lex stream
 685
 686  (* identifier: [a-zA-Z][a-zA-Z0-9] *)
 687  | [&lt; ' ('A' .. 'Z' | 'a' .. 'z' as c); stream &gt;] -&gt;
 688      let buffer = Buffer.create 1 in
 689      Buffer.add_char buffer c;
 690      lex_ident buffer stream
 691
 692  (* number: [0-9.]+ *)
 693  | [&lt; ' ('0' .. '9' as c); stream &gt;] -&gt;
 694      let buffer = Buffer.create 1 in
 695      Buffer.add_char buffer c;
 696      lex_number buffer stream
 697
 698  (* Comment until end of line. *)
 699  | [&lt; ' ('#'); stream &gt;] -&gt;
 700      lex_comment stream
 701
 702  (* Otherwise, just return the character as its ascii value. *)
 703  | [&lt; 'c; stream &gt;] -&gt;
 704      [&lt; 'Token.Kwd c; lex stream &gt;]
 705
 706  (* end of stream. *)
 707  | [&lt; &gt;] -&gt; [&lt; &gt;]
 708
 709and lex_number buffer = parser
 710  | [&lt; ' ('0' .. '9' | '.' as c); stream &gt;] -&gt;
 711      Buffer.add_char buffer c;
 712      lex_number buffer stream
 713  | [&lt; stream=lex &gt;] -&gt;
 714      [&lt; 'Token.Number (float_of_string (Buffer.contents buffer)); stream &gt;]
 715
 716and lex_ident buffer = parser
 717  | [&lt; ' ('A' .. 'Z' | 'a' .. 'z' | '0' .. '9' as c); stream &gt;] -&gt;
 718      Buffer.add_char buffer c;
 719      lex_ident buffer stream
 720  | [&lt; stream=lex &gt;] -&gt;
 721      match Buffer.contents buffer with
 722      | "def" -&gt; [&lt; 'Token.Def; stream &gt;]
 723      | "extern" -&gt; [&lt; 'Token.Extern; stream &gt;]
 724      | id -&gt; [&lt; 'Token.Ident id; stream &gt;]
 725
 726and lex_comment = parser
 727  | [&lt; ' ('\n'); stream=lex &gt;] -&gt; stream
 728  | [&lt; 'c; e=lex_comment &gt;] -&gt; e
 729  | [&lt; &gt;] -&gt; [&lt; &gt;]
 730</pre>
 731</dd>
 732
 733<dt>ast.ml:</dt>
 734<dd class="doc_code">
 735<pre>
 736(*===----------------------------------------------------------------------===
 737 * Abstract Syntax Tree (aka Parse Tree)
 738 *===----------------------------------------------------------------------===*)
 739
 740(* expr - Base type for all expression nodes. *)
 741type expr =
 742  (* variant for numeric literals like "1.0". *)
 743  | Number of float
 744
 745  (* variant for referencing a variable, like "a". *)
 746  | Variable of string
 747
 748  (* variant for a binary operator. *)
 749  | Binary of char * expr * expr
 750
 751  (* variant for function calls. *)
 752  | Call of string * expr array
 753
 754(* proto - This type represents the "prototype" for a function, which captures
 755 * its name, and its argument names (thus implicitly the number of arguments the
 756 * function takes). *)
 757type proto = Prototype of string * string array
 758
 759(* func - This type represents a function definition itself. *)
 760type func = Function of proto * expr
 761</pre>
 762</dd>
 763
 764<dt>parser.ml:</dt>
 765<dd class="doc_code">
 766<pre>
 767(*===---------------------------------------------------------------------===
 768 * Parser
 769 *===---------------------------------------------------------------------===*)
 770
 771(* binop_precedence - This holds the precedence for each binary operator that is
 772 * defined *)
 773let binop_precedence:(char, int) Hashtbl.t = Hashtbl.create 10
 774
 775(* precedence - Get the precedence of the pending binary operator token. *)
 776let precedence c = try Hashtbl.find binop_precedence c with Not_found -&gt; -1
 777
 778(* primary
 779 *   ::= identifier
 780 *   ::= numberexpr
 781 *   ::= parenexpr *)
 782let rec parse_primary = parser
 783  (* numberexpr ::= number *)
 784  | [&lt; 'Token.Number n &gt;] -&gt; Ast.Number n
 785
 786  (* parenexpr ::= '(' expression ')' *)
 787  | [&lt; 'Token.Kwd '('; e=parse_expr; 'Token.Kwd ')' ?? "expected ')'" &gt;] -&gt; e
 788
 789  (* identifierexpr
 790   *   ::= identifier
 791   *   ::= identifier '(' argumentexpr ')' *)
 792  | [&lt; 'Token.Ident id; stream &gt;] -&gt;
 793      let rec parse_args accumulator = parser
 794        | [&lt; e=parse_expr; stream &gt;] -&gt;
 795            begin parser
 796              | [&lt; 'Token.Kwd ','; e=parse_args (e :: accumulator) &gt;] -&gt; e
 797              | [&lt; &gt;] -&gt; e :: accumulator
 798            end stream
 799        | [&lt; &gt;] -&gt; accumulator
 800      in
 801      let rec parse_ident id = parser
 802        (* Call. *)
 803        | [&lt; 'Token.Kwd '(';
 804             args=parse_args [];
 805             'Token.Kwd ')' ?? "expected ')'"&gt;] -&gt;
 806            Ast.Call (id, Array.of_list (List.rev args))
 807
 808        (* Simple variable ref. *)
 809        | [&lt; &gt;] -&gt; Ast.Variable id
 810      in
 811      parse_ident id stream
 812
 813  | [&lt; &gt;] -&gt; raise (Stream.Error "unknown token when expecting an expression.")
 814
 815(* binoprhs
 816 *   ::= ('+' primary)* *)
 817and parse_bin_rhs expr_prec lhs stream =
 818  match Stream.peek stream with
 819  (* If this is a binop, find its precedence. *)
 820  | Some (Token.Kwd c) when Hashtbl.mem binop_precedence c -&gt;
 821      let token_prec = precedence c in
 822
 823      (* If this is a binop that binds at least as tightly as the current binop,
 824       * consume it, otherwise we are done. *)
 825      if token_prec &lt; expr_prec then lhs else begin
 826        (* Eat the binop. *)
 827        Stream.junk stream;
 828
 829        (* Parse the primary expression after the binary operator. *)
 830        let rhs = parse_primary stream in
 831
 832        (* Okay, we know this is a binop. *)
 833        let rhs =
 834          match Stream.peek stream with
 835          | Some (Token.Kwd c2) -&gt;
 836              (* If BinOp binds less tightly with rhs than the operator after
 837               * rhs, let the pending operator take rhs as its lhs. *)
 838              let next_prec = precedence c2 in
 839              if token_prec &lt; next_prec
 840              then parse_bin_rhs (token_prec + 1) rhs stream
 841              else rhs
 842          | _ -&gt; rhs
 843        in
 844
 845        (* Merge lhs/rhs. *)
 846        let lhs = Ast.Binary (c, lhs, rhs) in
 847        parse_bin_rhs expr_prec lhs stream
 848      end
 849  | _ -&gt; lhs
 850
 851(* expression
 852 *   ::= primary binoprhs *)
 853and parse_expr = parser
 854  | [&lt; lhs=parse_primary; stream &gt;] -&gt; parse_bin_rhs 0 lhs stream
 855
 856(* prototype
 857 *   ::= id '(' id* ')' *)
 858let parse_prototype =
 859  let rec parse_args accumulator = parser
 860    | [&lt; 'Token.Ident id; e=parse_args (id::accumulator) &gt;] -&gt; e
 861    | [&lt; &gt;] -&gt; accumulator
 862  in
 863
 864  parser
 865  | [&lt; 'Token.Ident id;
 866       'Token.Kwd '(' ?? "expected '(' in prototype";
 867       args=parse_args [];
 868       'Token.Kwd ')' ?? "expected ')' in prototype" &gt;] -&gt;
 869      (* success. *)
 870      Ast.Prototype (id, Array.of_list (List.rev args))
 871
 872  | [&lt; &gt;] -&gt;
 873      raise (Stream.Error "expected function name in prototype")
 874
 875(* definition ::= 'def' prototype expression *)
 876let parse_definition = parser
 877  | [&lt; 'Token.Def; p=parse_prototype; e=parse_expr &gt;] -&gt;
 878      Ast.Function (p, e)
 879
 880(* toplevelexpr ::= expression *)
 881let parse_toplevel = parser
 882  | [&lt; e=parse_expr &gt;] -&gt;
 883      (* Make an anonymous proto. *)
 884      Ast.Function (Ast.Prototype ("", [||]), e)
 885
 886(*  external ::= 'extern' prototype *)
 887let parse_extern = parser
 888  | [&lt; 'Token.Extern; e=parse_prototype &gt;] -&gt; e
 889</pre>
 890</dd>
 891
 892<dt>codegen.ml:</dt>
 893<dd class="doc_code">
 894<pre>
 895(*===----------------------------------------------------------------------===
 896 * Code Generation
 897 *===----------------------------------------------------------------------===*)
 898
 899open Llvm
 900
 901exception Error of string
 902
 903let context = global_context ()
 904let the_module = create_module context "my cool jit"
 905let builder = builder context
 906let named_values:(string, llvalue) Hashtbl.t = Hashtbl.create 10
 907let double_type = double_type context
 908
 909let rec codegen_expr = function
 910  | Ast.Number n -&gt; const_float double_type n
 911  | Ast.Variable name -&gt;
 912      (try Hashtbl.find named_values name with
 913        | Not_found -&gt; raise (Error "unknown variable name"))
 914  | Ast.Binary (op, lhs, rhs) -&gt;
 915      let lhs_val = codegen_expr lhs in
 916      let rhs_val = codegen_expr rhs in
 917      begin
 918        match op with
 919        | '+' -&gt; build_add lhs_val rhs_val "addtmp" builder
 920        | '-' -&gt; build_sub lhs_val rhs_val "subtmp" builder
 921        | '*' -&gt; build_mul lhs_val rhs_val "multmp" builder
 922        | '&lt;' -&gt;
 923            (* Convert bool 0/1 to double 0.0 or 1.0 *)
 924            let i = build_fcmp Fcmp.Ult lhs_val rhs_val "cmptmp" builder in
 925            build_uitofp i double_type "booltmp" builder
 926        | _ -&gt; raise (Error "invalid binary operator")
 927      end
 928  | Ast.Call (callee, args) -&gt;
 929      (* Look up the name in the module table. *)
 930      let callee =
 931        match lookup_function callee the_module with
 932        | Some callee -&gt; callee
 933        | None -&gt; raise (Error "unknown function referenced")
 934      in
 935      let params = params callee in
 936
 937      (* If argument mismatch error. *)
 938      if Array.length params == Array.length args then () else
 939        raise (Error "incorrect # arguments passed");
 940      let args = Array.map codegen_expr args in
 941      build_call callee args "calltmp" builder
 942
 943let codegen_proto = function
 944  | Ast.Prototype (name, args) -&gt;
 945      (* Make the function type: double(double,double) etc. *)
 946      let doubles = Array.make (Array.length args) double_type in
 947      let ft = function_type double_type doubles in
 948      let f =
 949        match lookup_function name the_module with
 950        | None -&gt; declare_function name ft the_module
 951
 952        (* If 'f' conflicted, there was already something named 'name'. If it
 953         * has a body, don't allow redefinition or reextern. *)
 954        | Some f -&gt;
 955            (* If 'f' already has a body, reject this. *)
 956            if block_begin f &lt;&gt; At_end f then
 957              raise (Error "redefinition of function");
 958
 959            (* If 'f' took a different number of arguments, reject. *)
 960            if element_type (type_of f) &lt;&gt; ft then
 961              raise (Error "redefinition of function with different # args");
 962            f
 963      in
 964
 965      (* Set names for all arguments. *)
 966      Array.iteri (fun i a -&gt;
 967        let n = args.(i) in
 968        set_value_name n a;
 969        Hashtbl.add named_values n a;
 970      ) (params f);
 971      f
 972
 973let codegen_func = function
 974  | Ast.Function (proto, body) -&gt;
 975      Hashtbl.clear named_values;
 976      let the_function = codegen_proto proto in
 977
 978      (* Create a new basic block to start insertion into. *)
 979      let bb = append_block context "entry" the_function in
 980      position_at_end bb builder;
 981
 982      try
 983        let ret_val = codegen_expr body in
 984
 985        (* Finish off the function. *)
 986        let _ = build_ret ret_val builder in
 987
 988        (* Validate the generated code, checking for consistency. *)
 989        Llvm_analysis.assert_valid_function the_function;
 990
 991        the_function
 992      with e -&gt;
 993        delete_function the_function;
 994        raise e
 995</pre>
 996</dd>
 997
 998<dt>toplevel.ml:</dt>
 999<dd class="doc_code">
1000<pre>
1001(*===----------------------------------------------------------------------===
1002 * Top-Level parsing and JIT Driver
1003 *===----------------------------------------------------------------------===*)
1004
1005open Llvm
1006
1007(* top ::= definition | external | expression | ';' *)
1008let rec main_loop stream =
1009  match Stream.peek stream with
1010  | None -&gt; ()
1011
1012  (* ignore top-level semicolons. *)
1013  | Some (Token.Kwd ';') -&gt;
1014      Stream.junk stream;
1015      main_loop stream
1016
1017  | Some token -&gt;
1018      begin
1019        try match token with
1020        | Token.Def -&gt;
1021            let e = Parser.parse_definition stream in
1022            print_endline "parsed a function definition.";
1023            dump_value (Codegen.codegen_func e);
1024        | Token.Extern -&gt;
1025            let e = Parser.parse_extern stream in
1026            print_endline "parsed an extern.";
1027            dump_value (Codegen.codegen_proto e);
1028        | _ -&gt;
1029            (* Evaluate a top-level expression into an anonymous function. *)
1030            let e = Parser.parse_toplevel stream in
1031            print_endline "parsed a top-level expr";
1032            dump_value (Codegen.codegen_func e);
1033        with Stream.Error s | Codegen.Error s -&gt;
1034          (* Skip token for error recovery. *)
1035          Stream.junk stream;
1036          print_endline s;
1037      end;
1038      print_string "ready&gt; "; flush stdout;
1039      main_loop stream
1040</pre>
1041</dd>
1042
1043<dt>toy.ml:</dt>
1044<dd class="doc_code">
1045<pre>
1046(*===----------------------------------------------------------------------===
1047 * Main driver code.
1048 *===----------------------------------------------------------------------===*)
1049
1050open Llvm
1051
1052let main () =
1053  (* Install standard binary operators.
1054   * 1 is the lowest precedence. *)
1055  Hashtbl.add Parser.binop_precedence '&lt;' 10;
1056  Hashtbl.add Parser.binop_precedence '+' 20;
1057  Hashtbl.add Parser.binop_precedence '-' 20;
1058  Hashtbl.add Parser.binop_precedence '*' 40;    (* highest. *)
1059
1060  (* Prime the first token. *)
1061  print_string "ready&gt; "; flush stdout;
1062  let stream = Lexer.lex (Stream.of_channel stdin) in
1063
1064  (* Run the main "interpreter loop" now. *)
1065  Toplevel.main_loop stream;
1066
1067  (* Print out all the generated code. *)
1068  dump_module Codegen.the_module
1069;;
1070
1071main ()
1072</pre>
1073</dd>
1074</dl>
1075
1076<a href="OCamlLangImpl4.html">Next: Adding JIT and Optimizer Support</a>
1077</div>
1078
1079<!-- *********************************************************************** -->
1080<hr>
1081<address>
1082  <a href="http://jigsaw.w3.org/css-validator/check/referer"><img
1083  src="http://jigsaw.w3.org/css-validator/images/vcss" alt="Valid CSS!"></a>
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1085  src="http://www.w3.org/Icons/valid-html401" alt="Valid HTML 4.01!"></a>
1086
1087  <a href="mailto:sabre@nondot.org">Chris Lattner</a><br>
1088  <a href="mailto:idadesub@users.sourceforge.net">Erick Tryzelaar</a><br>
1089  <a href="http://llvm.org/">The LLVM Compiler Infrastructure</a><br>
1090  Last modified: $Date: 2012-05-03 06:46:36 +0800 (周四, 03 δΊ”ζœˆ 2012) $
1091</address>
1092</body>
1093</html>