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diff --git a/docs/tutorial/OCamlLangImpl7.html b/docs/tutorial/OCamlLangImpl7.html deleted file mode 100644 index fd66b10c1a..0000000000 --- a/docs/tutorial/OCamlLangImpl7.html +++ /dev/null @@ -1,1904 +0,0 @@ -<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" - "http://www.w3.org/TR/html4/strict.dtd"> - -<html> -<head> - <title>Kaleidoscope: Extending the Language: Mutable Variables / SSA - construction</title> - <meta http-equiv="Content-Type" content="text/html; charset=utf-8"> - <meta name="author" content="Chris Lattner"> - <meta name="author" content="Erick Tryzelaar"> - <link rel="stylesheet" href="../_static/llvm.css" type="text/css"> -</head> - -<body> - -<h1>Kaleidoscope: Extending the Language: Mutable Variables</h1> - -<ul> -<li><a href="index.html">Up to Tutorial Index</a></li> -<li>Chapter 7 - <ol> - <li><a href="#intro">Chapter 7 Introduction</a></li> - <li><a href="#why">Why is this a hard problem?</a></li> - <li><a href="#memory">Memory in LLVM</a></li> - <li><a href="#kalvars">Mutable Variables in Kaleidoscope</a></li> - <li><a href="#adjustments">Adjusting Existing Variables for - Mutation</a></li> - <li><a href="#assignment">New Assignment Operator</a></li> - <li><a href="#localvars">User-defined Local Variables</a></li> - <li><a href="#code">Full Code Listing</a></li> - </ol> -</li> -<li><a href="OCamlLangImpl8.html">Chapter 8</a>: Conclusion and other useful LLVM - tidbits</li> -</ul> - -<div class="doc_author"> - <p> - Written by <a href="mailto:sabre@nondot.org">Chris Lattner</a> - and <a href="mailto:idadesub@users.sourceforge.net">Erick Tryzelaar</a> - </p> -</div> - -<!-- *********************************************************************** --> -<h2><a name="intro">Chapter 7 Introduction</a></h2> -<!-- *********************************************************************** --> - -<div> - -<p>Welcome to Chapter 7 of the "<a href="index.html">Implementing a language -with LLVM</a>" tutorial. In chapters 1 through 6, we've built a very -respectable, albeit simple, <a -href="http://en.wikipedia.org/wiki/Functional_programming">functional -programming language</a>. In our journey, we learned some parsing techniques, -how to build and represent an AST, how to build LLVM IR, and how to optimize -the resultant code as well as JIT compile it.</p> - -<p>While Kaleidoscope is interesting as a functional language, the fact that it -is functional makes it "too easy" to generate LLVM IR for it. In particular, a -functional language makes it very easy to build LLVM IR directly in <a -href="http://en.wikipedia.org/wiki/Static_single_assignment_form">SSA form</a>. -Since LLVM requires that the input code be in SSA form, this is a very nice -property and it is often unclear to newcomers how to generate code for an -imperative language with mutable variables.</p> - -<p>The short (and happy) summary of this chapter is that there is no need for -your front-end to build SSA form: LLVM provides highly tuned and well tested -support for this, though the way it works is a bit unexpected for some.</p> - -</div> - -<!-- *********************************************************************** --> -<h2><a name="why">Why is this a hard problem?</a></h2> -<!-- *********************************************************************** --> - -<div> - -<p> -To understand why mutable variables cause complexities in SSA construction, -consider this extremely simple C example: -</p> - -<div class="doc_code"> -<pre> -int G, H; -int test(_Bool Condition) { - int X; - if (Condition) - X = G; - else - X = H; - return X; -} -</pre> -</div> - -<p>In this case, we have the variable "X", whose value depends on the path -executed in the program. Because there are two different possible values for X -before the return instruction, a PHI node is inserted to merge the two values. -The LLVM IR that we want for this example looks like this:</p> - -<div class="doc_code"> -<pre> -@G = weak global i32 0 ; type of @G is i32* -@H = weak global i32 0 ; type of @H is i32* - -define i32 @test(i1 %Condition) { -entry: - br i1 %Condition, label %cond_true, label %cond_false - -cond_true: - %X.0 = load i32* @G - br label %cond_next - -cond_false: - %X.1 = load i32* @H - br label %cond_next - -cond_next: - %X.2 = phi i32 [ %X.1, %cond_false ], [ %X.0, %cond_true ] - ret i32 %X.2 -} -</pre> -</div> - -<p>In this example, the loads from the G and H global variables are explicit in -the LLVM IR, and they live in the then/else branches of the if statement -(cond_true/cond_false). In order to merge the incoming values, the X.2 phi node -in the cond_next block selects the right value to use based on where control -flow is coming from: if control flow comes from the cond_false block, X.2 gets -the value of X.1. Alternatively, if control flow comes from cond_true, it gets -the value of X.0. The intent of this chapter is not to explain the details of -SSA form. For more information, see one of the many <a -href="http://en.wikipedia.org/wiki/Static_single_assignment_form">online -references</a>.</p> - -<p>The question for this article is "who places the phi nodes when lowering -assignments to mutable variables?". The issue here is that LLVM -<em>requires</em> that its IR be in SSA form: there is no "non-ssa" mode for it. -However, SSA construction requires non-trivial algorithms and data structures, -so it is inconvenient and wasteful for every front-end to have to reproduce this -logic.</p> - -</div> - -<!-- *********************************************************************** --> -<h2><a name="memory">Memory in LLVM</a></h2> -<!-- *********************************************************************** --> - -<div> - -<p>The 'trick' here is that while LLVM does require all register values to be -in SSA form, it does not require (or permit) memory objects to be in SSA form. -In the example above, note that the loads from G and H are direct accesses to -G and H: they are not renamed or versioned. This differs from some other -compiler systems, which do try to version memory objects. In LLVM, instead of -encoding dataflow analysis of memory into the LLVM IR, it is handled with <a -href="../WritingAnLLVMPass.html">Analysis Passes</a> which are computed on -demand.</p> - -<p> -With this in mind, the high-level idea is that we want to make a stack variable -(which lives in memory, because it is on the stack) for each mutable object in -a function. To take advantage of this trick, we need to talk about how LLVM -represents stack variables. -</p> - -<p>In LLVM, all memory accesses are explicit with load/store instructions, and -it is carefully designed not to have (or need) an "address-of" operator. Notice -how the type of the @G/@H global variables is actually "i32*" even though the -variable is defined as "i32". What this means is that @G defines <em>space</em> -for an i32 in the global data area, but its <em>name</em> actually refers to the -address for that space. Stack variables work the same way, except that instead of -being declared with global variable definitions, they are declared with the -<a href="../LangRef.html#i_alloca">LLVM alloca instruction</a>:</p> - -<div class="doc_code"> -<pre> -define i32 @example() { -entry: - %X = alloca i32 ; type of %X is i32*. - ... - %tmp = load i32* %X ; load the stack value %X from the stack. - %tmp2 = add i32 %tmp, 1 ; increment it - store i32 %tmp2, i32* %X ; store it back - ... -</pre> -</div> - -<p>This code shows an example of how you can declare and manipulate a stack -variable in the LLVM IR. Stack memory allocated with the alloca instruction is -fully general: you can pass the address of the stack slot to functions, you can -store it in other variables, etc. In our example above, we could rewrite the -example to use the alloca technique to avoid using a PHI node:</p> - -<div class="doc_code"> -<pre> -@G = weak global i32 0 ; type of @G is i32* -@H = weak global i32 0 ; type of @H is i32* - -define i32 @test(i1 %Condition) { -entry: - %X = alloca i32 ; type of %X is i32*. - br i1 %Condition, label %cond_true, label %cond_false - -cond_true: - %X.0 = load i32* @G - store i32 %X.0, i32* %X ; Update X - br label %cond_next - -cond_false: - %X.1 = load i32* @H - store i32 %X.1, i32* %X ; Update X - br label %cond_next - -cond_next: - %X.2 = load i32* %X ; Read X - ret i32 %X.2 -} -</pre> -</div> - -<p>With this, we have discovered a way to handle arbitrary mutable variables -without the need to create Phi nodes at all:</p> - -<ol> -<li>Each mutable variable becomes a stack allocation.</li> -<li>Each read of the variable becomes a load from the stack.</li> -<li>Each update of the variable becomes a store to the stack.</li> -<li>Taking the address of a variable just uses the stack address directly.</li> -</ol> - -<p>While this solution has solved our immediate problem, it introduced another -one: we have now apparently introduced a lot of stack traffic for very simple -and common operations, a major performance problem. Fortunately for us, the -LLVM optimizer has a highly-tuned optimization pass named "mem2reg" that handles -this case, promoting allocas like this into SSA registers, inserting Phi nodes -as appropriate. If you run this example through the pass, for example, you'll -get:</p> - -<div class="doc_code"> -<pre> -$ <b>llvm-as < example.ll | opt -mem2reg | llvm-dis</b> -@G = weak global i32 0 -@H = weak global i32 0 - -define i32 @test(i1 %Condition) { -entry: - br i1 %Condition, label %cond_true, label %cond_false - -cond_true: - %X.0 = load i32* @G - br label %cond_next - -cond_false: - %X.1 = load i32* @H - br label %cond_next - -cond_next: - %X.01 = phi i32 [ %X.1, %cond_false ], [ %X.0, %cond_true ] - ret i32 %X.01 -} -</pre> -</div> - -<p>The mem2reg pass implements the standard "iterated dominance frontier" -algorithm for constructing SSA form and has a number of optimizations that speed -up (very common) degenerate cases. The mem2reg optimization pass is the answer -to dealing with mutable variables, and we highly recommend that you depend on -it. Note that mem2reg only works on variables in certain circumstances:</p> - -<ol> -<li>mem2reg is alloca-driven: it looks for allocas and if it can handle them, it -promotes them. It does not apply to global variables or heap allocations.</li> - -<li>mem2reg only looks for alloca instructions in the entry block of the -function. Being in the entry block guarantees that the alloca is only executed -once, which makes analysis simpler.</li> - -<li>mem2reg only promotes allocas whose uses are direct loads and stores. If -the address of the stack object is passed to a function, or if any funny pointer -arithmetic is involved, the alloca will not be promoted.</li> - -<li>mem2reg only works on allocas of <a -href="../LangRef.html#t_classifications">first class</a> -values (such as pointers, scalars and vectors), and only if the array size -of the allocation is 1 (or missing in the .ll file). mem2reg is not capable of -promoting structs or arrays to registers. Note that the "scalarrepl" pass is -more powerful and can promote structs, "unions", and arrays in many cases.</li> - -</ol> - -<p> -All of these properties are easy to satisfy for most imperative languages, and -we'll illustrate it below with Kaleidoscope. The final question you may be -asking is: should I bother with this nonsense for my front-end? Wouldn't it be -better if I just did SSA construction directly, avoiding use of the mem2reg -optimization pass? In short, we strongly recommend that you use this technique -for building SSA form, unless there is an extremely good reason not to. Using -this technique is:</p> - -<ul> -<li>Proven and well tested: llvm-gcc and clang both use this technique for local -mutable variables. As such, the most common clients of LLVM are using this to -handle a bulk of their variables. You can be sure that bugs are found fast and -fixed early.</li> - -<li>Extremely Fast: mem2reg has a number of special cases that make it fast in -common cases as well as fully general. For example, it has fast-paths for -variables that are only used in a single block, variables that only have one -assignment point, good heuristics to avoid insertion of unneeded phi nodes, etc. -</li> - -<li>Needed for debug info generation: <a href="../SourceLevelDebugging.html"> -Debug information in LLVM</a> relies on having the address of the variable -exposed so that debug info can be attached to it. This technique dovetails -very naturally with this style of debug info.</li> -</ul> - -<p>If nothing else, this makes it much easier to get your front-end up and -running, and is very simple to implement. Lets extend Kaleidoscope with mutable -variables now! -</p> - -</div> - -<!-- *********************************************************************** --> -<h2><a name="kalvars">Mutable Variables in Kaleidoscope</a></h2> -<!-- *********************************************************************** --> - -<div> - -<p>Now that we know the sort of problem we want to tackle, lets see what this -looks like in the context of our little Kaleidoscope language. We're going to -add two features:</p> - -<ol> -<li>The ability to mutate variables with the '=' operator.</li> -<li>The ability to define new variables.</li> -</ol> - -<p>While the first item is really what this is about, we only have variables -for incoming arguments as well as for induction variables, and redefining those only -goes so far :). Also, the ability to define new variables is a -useful thing regardless of whether you will be mutating them. Here's a -motivating example that shows how we could use these:</p> - -<div class="doc_code"> -<pre> -# Define ':' for sequencing: as a low-precedence operator that ignores operands -# and just returns the RHS. -def binary : 1 (x y) y; - -# Recursive fib, we could do this before. -def fib(x) - if (x < 3) then - 1 - else - fib(x-1)+fib(x-2); - -# Iterative fib. -def fibi(x) - <b>var a = 1, b = 1, c in</b> - (for i = 3, i < x in - <b>c = a + b</b> : - <b>a = b</b> : - <b>b = c</b>) : - b; - -# Call it. -fibi(10); -</pre> -</div> - -<p> -In order to mutate variables, we have to change our existing variables to use -the "alloca trick". Once we have that, we'll add our new operator, then extend -Kaleidoscope to support new variable definitions. -</p> - -</div> - -<!-- *********************************************************************** --> -<h2><a name="adjustments">Adjusting Existing Variables for Mutation</a></h2> -<!-- *********************************************************************** --> - -<div> - -<p> -The symbol table in Kaleidoscope is managed at code generation time by the -'<tt>named_values</tt>' map. This map currently keeps track of the LLVM -"Value*" that holds the double value for the named variable. In order to -support mutation, we need to change this slightly, so that it -<tt>named_values</tt> holds the <em>memory location</em> of the variable in -question. Note that this change is a refactoring: it changes the structure of -the code, but does not (by itself) change the behavior of the compiler. All of -these changes are isolated in the Kaleidoscope code generator.</p> - -<p> -At this point in Kaleidoscope's development, it only supports variables for two -things: incoming arguments to functions and the induction variable of 'for' -loops. For consistency, we'll allow mutation of these variables in addition to -other user-defined variables. This means that these will both need memory -locations. -</p> - -<p>To start our transformation of Kaleidoscope, we'll change the -<tt>named_values</tt> map so that it maps to AllocaInst* instead of Value*. -Once we do this, the C++ compiler will tell us what parts of the code we need to -update:</p> - -<p><b>Note:</b> the ocaml bindings currently model both <tt>Value*</tt>s and -<tt>AllocInst*</tt>s as <tt>Llvm.llvalue</tt>s, but this may change in the -future to be more type safe.</p> - -<div class="doc_code"> -<pre> -let named_values:(string, llvalue) Hashtbl.t = Hashtbl.create 10 -</pre> -</div> - -<p>Also, since we will need to create these alloca's, we'll use a helper -function that ensures that the allocas are created in the entry block of the -function:</p> - -<div class="doc_code"> -<pre> -(* Create an alloca instruction in the entry block of the function. This - * is used for mutable variables etc. *) -let create_entry_block_alloca the_function var_name = - let builder = builder_at (instr_begin (entry_block the_function)) in - build_alloca double_type var_name builder -</pre> -</div> - -<p>This funny looking code creates an <tt>Llvm.llbuilder</tt> object that is -pointing at the first instruction of the entry block. It then creates an alloca -with the expected name and returns it. Because all values in Kaleidoscope are -doubles, there is no need to pass in a type to use.</p> - -<p>With this in place, the first functionality change we want to make is to -variable references. In our new scheme, variables live on the stack, so code -generating a reference to them actually needs to produce a load from the stack -slot:</p> - -<div class="doc_code"> -<pre> -let rec codegen_expr = function - ... - | Ast.Variable name -> - let v = try Hashtbl.find named_values name with - | Not_found -> raise (Error "unknown variable name") - in - <b>(* Load the value. *) - build_load v name builder</b> -</pre> -</div> - -<p>As you can see, this is pretty straightforward. Now we need to update the -things that define the variables to set up the alloca. We'll start with -<tt>codegen_expr Ast.For ...</tt> (see the <a href="#code">full code listing</a> -for the unabridged code):</p> - -<div class="doc_code"> -<pre> - | Ast.For (var_name, start, end_, step, body) -> - let the_function = block_parent (insertion_block builder) in - - (* Create an alloca for the variable in the entry block. *) - <b>let alloca = create_entry_block_alloca the_function var_name in</b> - - (* Emit the start code first, without 'variable' in scope. *) - let start_val = codegen_expr start in - - <b>(* Store the value into the alloca. *) - ignore(build_store start_val alloca builder);</b> - - ... - - (* Within the loop, the variable is defined equal to the PHI node. If it - * shadows an existing variable, we have to restore it, so save it - * now. *) - let old_val = - try Some (Hashtbl.find named_values var_name) with Not_found -> None - in - <b>Hashtbl.add named_values var_name alloca;</b> - - ... - - (* Compute the end condition. *) - let end_cond = codegen_expr end_ in - - <b>(* Reload, increment, and restore the alloca. This handles the case where - * the body of the loop mutates the variable. *) - let cur_var = build_load alloca var_name builder in - let next_var = build_add cur_var step_val "nextvar" builder in - ignore(build_store next_var alloca builder);</b> - ... -</pre> -</div> - -<p>This code is virtually identical to the code <a -href="OCamlLangImpl5.html#forcodegen">before we allowed mutable variables</a>. -The big difference is that we no longer have to construct a PHI node, and we use -load/store to access the variable as needed.</p> - -<p>To support mutable argument variables, we need to also make allocas for them. -The code for this is also pretty simple:</p> - -<div class="doc_code"> -<pre> -(* Create an alloca for each argument and register the argument in the symbol - * table so that references to it will succeed. *) -let create_argument_allocas the_function proto = - let args = match proto with - | Ast.Prototype (_, args) | Ast.BinOpPrototype (_, args, _) -> args - in - Array.iteri (fun i ai -> - let var_name = args.(i) in - (* Create an alloca for this variable. *) - let alloca = create_entry_block_alloca the_function var_name in - - (* Store the initial value into the alloca. *) - ignore(build_store ai alloca builder); - - (* Add arguments to variable symbol table. *) - Hashtbl.add named_values var_name alloca; - ) (params the_function) -</pre> -</div> - -<p>For each argument, we make an alloca, store the input value to the function -into the alloca, and register the alloca as the memory location for the -argument. This method gets invoked by <tt>Codegen.codegen_func</tt> right after -it sets up the entry block for the function.</p> - -<p>The final missing piece is adding the mem2reg pass, which allows us to get -good codegen once again:</p> - -<div class="doc_code"> -<pre> -let main () = - ... - let the_fpm = PassManager.create_function Codegen.the_module in - - (* Set up the optimizer pipeline. Start with registering info about how the - * target lays out data structures. *) - DataLayout.add (ExecutionEngine.target_data the_execution_engine) the_fpm; - - <b>(* Promote allocas to registers. *) - add_memory_to_register_promotion the_fpm;</b> - - (* Do simple "peephole" optimizations and bit-twiddling optzn. *) - add_instruction_combining the_fpm; - - (* reassociate expressions. *) - add_reassociation the_fpm; -</pre> -</div> - -<p>It is interesting to see what the code looks like before and after the -mem2reg optimization runs. For example, this is the before/after code for our -recursive fib function. Before the optimization:</p> - -<div class="doc_code"> -<pre> -define double @fib(double %x) { -entry: - <b>%x1 = alloca double - store double %x, double* %x1 - %x2 = load double* %x1</b> - %cmptmp = fcmp ult double %x2, 3.000000e+00 - %booltmp = uitofp i1 %cmptmp to double - %ifcond = fcmp one double %booltmp, 0.000000e+00 - br i1 %ifcond, label %then, label %else - -then: ; preds = %entry - br label %ifcont - -else: ; preds = %entry - <b>%x3 = load double* %x1</b> - %subtmp = fsub double %x3, 1.000000e+00 - %calltmp = call double @fib(double %subtmp) - <b>%x4 = load double* %x1</b> - %subtmp5 = fsub double %x4, 2.000000e+00 - %calltmp6 = call double @fib(double %subtmp5) - %addtmp = fadd double %calltmp, %calltmp6 - br label %ifcont - -ifcont: ; preds = %else, %then - %iftmp = phi double [ 1.000000e+00, %then ], [ %addtmp, %else ] - ret double %iftmp -} -</pre> -</div> - -<p>Here there is only one variable (x, the input argument) but you can still -see the extremely simple-minded code generation strategy we are using. In the -entry block, an alloca is created, and the initial input value is stored into -it. Each reference to the variable does a reload from the stack. Also, note -that we didn't modify the if/then/else expression, so it still inserts a PHI -node. While we could make an alloca for it, it is actually easier to create a -PHI node for it, so we still just make the PHI.</p> - -<p>Here is the code after the mem2reg pass runs:</p> - -<div class="doc_code"> -<pre> -define double @fib(double %x) { -entry: - %cmptmp = fcmp ult double <b>%x</b>, 3.000000e+00 - %booltmp = uitofp i1 %cmptmp to double - %ifcond = fcmp one double %booltmp, 0.000000e+00 - br i1 %ifcond, label %then, label %else - -then: - br label %ifcont - -else: - %subtmp = fsub double <b>%x</b>, 1.000000e+00 - %calltmp = call double @fib(double %subtmp) - %subtmp5 = fsub double <b>%x</b>, 2.000000e+00 - %calltmp6 = call double @fib(double %subtmp5) - %addtmp = fadd double %calltmp, %calltmp6 - br label %ifcont - -ifcont: ; preds = %else, %then - %iftmp = phi double [ 1.000000e+00, %then ], [ %addtmp, %else ] - ret double %iftmp -} -</pre> -</div> - -<p>This is a trivial case for mem2reg, since there are no redefinitions of the -variable. The point of showing this is to calm your tension about inserting -such blatent inefficiencies :).</p> - -<p>After the rest of the optimizers run, we get:</p> - -<div class="doc_code"> -<pre> -define double @fib(double %x) { -entry: - %cmptmp = fcmp ult double %x, 3.000000e+00 - %booltmp = uitofp i1 %cmptmp to double - %ifcond = fcmp ueq double %booltmp, 0.000000e+00 - br i1 %ifcond, label %else, label %ifcont - -else: - %subtmp = fsub double %x, 1.000000e+00 - %calltmp = call double @fib(double %subtmp) - %subtmp5 = fsub double %x, 2.000000e+00 - %calltmp6 = call double @fib(double %subtmp5) - %addtmp = fadd double %calltmp, %calltmp6 - ret double %addtmp - -ifcont: - ret double 1.000000e+00 -} -</pre> -</div> - -<p>Here we see that the simplifycfg pass decided to clone the return instruction -into the end of the 'else' block. This allowed it to eliminate some branches -and the PHI node.</p> - -<p>Now that all symbol table references are updated to use stack variables, -we'll add the assignment operator.</p> - -</div> - -<!-- *********************************************************************** --> -<h2><a name="assignment">New Assignment Operator</a></h2> -<!-- *********************************************************************** --> - -<div> - -<p>With our current framework, adding a new assignment operator is really -simple. We will parse it just like any other binary operator, but handle it -internally (instead of allowing the user to define it). The first step is to -set a precedence:</p> - -<div class="doc_code"> -<pre> -let main () = - (* Install standard binary operators. - * 1 is the lowest precedence. *) - <b>Hashtbl.add Parser.binop_precedence '=' 2;</b> - Hashtbl.add Parser.binop_precedence '<' 10; - Hashtbl.add Parser.binop_precedence '+' 20; - Hashtbl.add Parser.binop_precedence '-' 20; - ... -</pre> -</div> - -<p>Now that the parser knows the precedence of the binary operator, it takes -care of all the parsing and AST generation. We just need to implement codegen -for the assignment operator. This looks like:</p> - -<div class="doc_code"> -<pre> -let rec codegen_expr = function - begin match op with - | '=' -> - (* Special case '=' because we don't want to emit the LHS as an - * expression. *) - let name = - match lhs with - | Ast.Variable name -> name - | _ -> raise (Error "destination of '=' must be a variable") - in -</pre> -</div> - -<p>Unlike the rest of the binary operators, our assignment operator doesn't -follow the "emit LHS, emit RHS, do computation" model. As such, it is handled -as a special case before the other binary operators are handled. The other -strange thing is that it requires the LHS to be a variable. It is invalid to -have "(x+1) = expr" - only things like "x = expr" are allowed. -</p> - - -<div class="doc_code"> -<pre> - (* Codegen the rhs. *) - let val_ = codegen_expr rhs in - - (* Lookup the name. *) - let variable = try Hashtbl.find named_values name with - | Not_found -> raise (Error "unknown variable name") - in - ignore(build_store val_ variable builder); - val_ - | _ -> - ... -</pre> -</div> - -<p>Once we have the variable, codegen'ing the assignment is straightforward: -we emit the RHS of the assignment, create a store, and return the computed -value. Returning a value allows for chained assignments like "X = (Y = Z)".</p> - -<p>Now that we have an assignment operator, we can mutate loop variables and -arguments. For example, we can now run code like this:</p> - -<div class="doc_code"> -<pre> -# Function to print a double. -extern printd(x); - -# Define ':' for sequencing: as a low-precedence operator that ignores operands -# and just returns the RHS. -def binary : 1 (x y) y; - -def test(x) - printd(x) : - x = 4 : - printd(x); - -test(123); -</pre> -</div> - -<p>When run, this example prints "123" and then "4", showing that we did -actually mutate the value! Okay, we have now officially implemented our goal: -getting this to work requires SSA construction in the general case. However, -to be really useful, we want the ability to define our own local variables, lets -add this next! -</p> - -</div> - -<!-- *********************************************************************** --> -<h2><a name="localvars">User-defined Local Variables</a></h2> -<!-- *********************************************************************** --> - -<div> - -<p>Adding var/in is just like any other other extensions we made to -Kaleidoscope: we extend the lexer, the parser, the AST and the code generator. -The first step for adding our new 'var/in' construct is to extend the lexer. -As before, this is pretty trivial, the code looks like this:</p> - -<div class="doc_code"> -<pre> -type token = - ... - <b>(* var definition *) - | Var</b> - -... - -and lex_ident buffer = parser - ... - | "in" -> [< 'Token.In; stream >] - | "binary" -> [< 'Token.Binary; stream >] - | "unary" -> [< 'Token.Unary; stream >] - <b>| "var" -> [< 'Token.Var; stream >]</b> - ... -</pre> -</div> - -<p>The next step is to define the AST node that we will construct. For var/in, -it looks like this:</p> - -<div class="doc_code"> -<pre> -type expr = - ... - (* variant for var/in. *) - | Var of (string * expr option) array * expr - ... -</pre> -</div> - -<p>var/in allows a list of names to be defined all at once, and each name can -optionally have an initializer value. As such, we capture this information in -the VarNames vector. Also, var/in has a body, this body is allowed to access -the variables defined by the var/in.</p> - -<p>With this in place, we can define the parser pieces. The first thing we do -is add it as a primary expression:</p> - -<div class="doc_code"> -<pre> -(* primary - * ::= identifier - * ::= numberexpr - * ::= parenexpr - * ::= ifexpr - * ::= forexpr - <b>* ::= varexpr</b> *) -let rec parse_primary = parser - ... - <b>(* varexpr - * ::= 'var' identifier ('=' expression? - * (',' identifier ('=' expression)?)* 'in' expression *) - | [< 'Token.Var; - (* At least one variable name is required. *) - 'Token.Ident id ?? "expected identifier after var"; - init=parse_var_init; - var_names=parse_var_names [(id, init)]; - (* At this point, we have to have 'in'. *) - 'Token.In ?? "expected 'in' keyword after 'var'"; - body=parse_expr >] -> - Ast.Var (Array.of_list (List.rev var_names), body)</b> - -... - -and parse_var_init = parser - (* read in the optional initializer. *) - | [< 'Token.Kwd '='; e=parse_expr >] -> Some e - | [< >] -> None - -and parse_var_names accumulator = parser - | [< 'Token.Kwd ','; - 'Token.Ident id ?? "expected identifier list after var"; - init=parse_var_init; - e=parse_var_names ((id, init) :: accumulator) >] -> e - | [< >] -> accumulator -</pre> -</div> - -<p>Now that we can parse and represent the code, we need to support emission of -LLVM IR for it. This code starts out with:</p> - -<div class="doc_code"> -<pre> -let rec codegen_expr = function - ... - | Ast.Var (var_names, body) - let old_bindings = ref [] in - - let the_function = block_parent (insertion_block builder) in - - (* Register all variables and emit their initializer. *) - Array.iter (fun (var_name, init) -> -</pre> -</div> - -<p>Basically it loops over all the variables, installing them one at a time. -For each variable we put into the symbol table, we remember the previous value -that we replace in OldBindings.</p> - -<div class="doc_code"> -<pre> - (* Emit the initializer before adding the variable to scope, this - * prevents the initializer from referencing the variable itself, and - * permits stuff like this: - * var a = 1 in - * var a = a in ... # refers to outer 'a'. *) - let init_val = - match init with - | Some init -> codegen_expr init - (* If not specified, use 0.0. *) - | None -> const_float double_type 0.0 - in - - let alloca = create_entry_block_alloca the_function var_name in - ignore(build_store init_val alloca builder); - - (* Remember the old variable binding so that we can restore the binding - * when we unrecurse. *) - - begin - try - let old_value = Hashtbl.find named_values var_name in - old_bindings := (var_name, old_value) :: !old_bindings; - with Not_found > () - end; - - (* Remember this binding. *) - Hashtbl.add named_values var_name alloca; - ) var_names; -</pre> -</div> - -<p>There are more comments here than code. The basic idea is that we emit the -initializer, create the alloca, then update the symbol table to point to it. -Once all the variables are installed in the symbol table, we evaluate the body -of the var/in expression:</p> - -<div class="doc_code"> -<pre> - (* Codegen the body, now that all vars are in scope. *) - let body_val = codegen_expr body in -</pre> -</div> - -<p>Finally, before returning, we restore the previous variable bindings:</p> - -<div class="doc_code"> -<pre> - (* Pop all our variables from scope. *) - List.iter (fun (var_name, old_value) -> - Hashtbl.add named_values var_name old_value - ) !old_bindings; - - (* Return the body computation. *) - body_val -</pre> -</div> - -<p>The end result of all of this is that we get properly scoped variable -definitions, and we even (trivially) allow mutation of them :).</p> - -<p>With this, we completed what we set out to do. Our nice iterative fib -example from the intro compiles and runs just fine. The mem2reg pass optimizes -all of our stack variables into SSA registers, inserting PHI nodes where needed, -and our front-end remains simple: no "iterated dominance frontier" computation -anywhere in sight.</p> - -</div> - -<!-- *********************************************************************** --> -<h2><a name="code">Full Code Listing</a></h2> -<!-- *********************************************************************** --> - -<div> - -<p> -Here is the complete code listing for our running example, enhanced with mutable -variables and var/in support. To build this example, use: -</p> - -<div class="doc_code"> -<pre> -# Compile -ocamlbuild toy.byte -# Run -./toy.byte -</pre> -</div> - -<p>Here is the code:</p> - -<dl> -<dt>_tags:</dt> -<dd class="doc_code"> -<pre> -<{lexer,parser}.ml>: use_camlp4, pp(camlp4of) -<*.{byte,native}>: g++, use_llvm, use_llvm_analysis -<*.{byte,native}>: use_llvm_executionengine, use_llvm_target -<*.{byte,native}>: use_llvm_scalar_opts, use_bindings -</pre> -</dd> - -<dt>myocamlbuild.ml:</dt> -<dd c |