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| author | Erick Tryzelaar <idadesub@users.sourceforge.net> | 2008-03-31 08:44:50 +0000 |
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| committer | Erick Tryzelaar <idadesub@users.sourceforge.net> | 2008-03-31 08:44:50 +0000 |
| commit | 35295ffd500b39781e3ed015293e31c665a8b5bc (patch) | |
| tree | 6c4d914d54f96a7df264747a60381499cd4c35bb /docs/tutorial/OCamlLangImpl7.html | |
| parent | 9d15abe8385d17aa86c8144c8bbbac958fb91f17 (diff) | |
Chapter 5, 6, and 7 of the ocaml/kaleidoscope tutorial
and fix some tabs in chapter 3 and 4.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@48978 91177308-0d34-0410-b5e6-96231b3b80d8
Diffstat (limited to 'docs/tutorial/OCamlLangImpl7.html')
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diff --git a/docs/tutorial/OCamlLangImpl7.html b/docs/tutorial/OCamlLangImpl7.html new file mode 100644 index 0000000000..abda44011c --- /dev/null +++ b/docs/tutorial/OCamlLangImpl7.html @@ -0,0 +1,1902 @@ +<!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="../llvm.css" type="text/css"> +</head> + +<body> + +<div class="doc_title">Kaleidoscope: Extending the Language: Mutable Variables</div> + +<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="LangImpl8.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> + +<!-- *********************************************************************** --> +<div class="doc_section"><a name="intro">Chapter 7 Introduction</a></div> +<!-- *********************************************************************** --> + +<div class="doc_text"> + +<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> + +<!-- *********************************************************************** --> +<div class="doc_section"><a name="why">Why is this a hard problem?</a></div> +<!-- *********************************************************************** --> + +<div class="doc_text"> + +<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> + +<!-- *********************************************************************** --> +<div class="doc_section"><a name="memory">Memory in LLVM</a></div> +<!-- *********************************************************************** --> + +<div class="doc_text"> + +<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> + +<!-- *********************************************************************** --> +<div class="doc_section"><a name="kalvars">Mutable Variables in +Kaleidoscope</a></div> +<!-- *********************************************************************** --> + +<div class="doc_text"> + +<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> + +<!-- *********************************************************************** --> +<div class="doc_section"><a name="adjustments">Adjusting Existing Variables for +Mutation</a></div> +<!-- *********************************************************************** --> + +<div class="doc_text"> + +<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 the_module_provider in + + (* Set up the optimizer pipeline. Start with registering info about how the + * target lays out data structures. *) + TargetData.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 = sub double %x3, 1.000000e+00 + %calltmp = call double @fib( double %subtmp ) + <b>%x4 = load double* %x1</b> + %subtmp5 = sub double %x4, 2.000000e+00 + %calltmp6 = call double @fib( double %subtmp5 ) + %addtmp = add 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 = sub double <b>%x</b>, 1.000000e+00 + %calltmp = call double @fib( double %subtmp ) + %subtmp5 = sub double <b>%x</b>, 2.000000e+00 + %calltmp6 = call double @fib( double %subtmp5 ) + %addtmp = add 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 = sub double %x, 1.000000e+00 + %calltmp = call double @fib( double %subtmp ) + %subtmp5 = sub double %x, 2.000000e+00 + %calltmp6 = call double @fib( double %subtmp5 ) + %addtmp = add 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> + +<!-- *********************************************************************** --> +<div class="doc_section"><a name="assignment">New Assignment Operator</a></div> +<!-- *********************************************************************** --> + +<div class="doc_text"> + +<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> + +<!-- *********************************************************************** --> +<div class="doc_section"><a name="localvars">User-defined Local +Variables</a></div> +<!-- *********************************************************************** --> + +<div class="doc_text"> + +<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> + |