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diff --git a/docs/tutorial/OCamlLangImpl7.rst b/docs/tutorial/OCamlLangImpl7.rst new file mode 100644 index 0000000000..07da3a8ff9 --- /dev/null +++ b/docs/tutorial/OCamlLangImpl7.rst @@ -0,0 +1,1726 @@ +======================================================= +Kaleidoscope: Extending the Language: Mutable Variables +======================================================= + +.. contents:: + :local: + +Written by `Chris Lattner <mailto:sabre@nondot.org>`_ and `Erick +Tryzelaar <mailto:idadesub@users.sourceforge.net>`_ + +Chapter 7 Introduction +====================== + +Welcome to Chapter 7 of the "`Implementing a language with +LLVM <index.html>`_" tutorial. In chapters 1 through 6, we've built a +very respectable, albeit simple, `functional programming +language <http://en.wikipedia.org/wiki/Functional_programming>`_. 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. + +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 `SSA +form <http://en.wikipedia.org/wiki/Static_single_assignment_form>`_. +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. + +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. + +Why is this a hard problem? +=========================== + +To understand why mutable variables cause complexities in SSA +construction, consider this extremely simple C example: + +.. code-block:: c + + int G, H; + int test(_Bool Condition) { + int X; + if (Condition) + X = G; + else + X = H; + return X; + } + +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: + +.. code-block:: llvm + + @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 + } + +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 `online +references <http://en.wikipedia.org/wiki/Static_single_assignment_form>`_. + +The question for this article is "who places the phi nodes when lowering +assignments to mutable variables?". The issue here is that LLVM +*requires* 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. + +Memory in LLVM +============== + +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 `Analysis +Passes <../WritingAnLLVMPass.html>`_ which are computed on demand. + +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. + +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 *space* for an i32 in the global data area, but its +*name* 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 `LLVM alloca +instruction <../LangRef.html#i_alloca>`_: + +.. code-block:: llvm + + 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 + ... + +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: + +.. code-block:: llvm + + @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 + } + +With this, we have discovered a way to handle arbitrary mutable +variables without the need to create Phi nodes at all: + +#. Each mutable variable becomes a stack allocation. +#. Each read of the variable becomes a load from the stack. +#. Each update of the variable becomes a store to the stack. +#. Taking the address of a variable just uses the stack address + directly. + +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: + +.. code-block:: bash + + $ llvm-as < example.ll | opt -mem2reg | llvm-dis + @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 + } + +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: + +#. 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. +#. 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. +#. 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. +#. mem2reg only works on allocas of `first + class <../LangRef.html#t_classifications>`_ 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. + +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: + +- 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. +- 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. +- Needed for debug info generation: `Debug information in + LLVM <../SourceLevelDebugging.html>`_ 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. + +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! + +Mutable Variables in Kaleidoscope +================================= + +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: + +#. The ability to mutate variables with the '=' operator. +#. The ability to define new variables. + +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: + +:: + + # 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) + var a = 1, b = 1, c in + (for i = 3, i < x in + c = a + b : + a = b : + b = c) : + b; + + # Call it. + fibi(10); + +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. + +Adjusting Existing Variables for Mutation +========================================= + +The symbol table in Kaleidoscope is managed at code generation time by +the '``named_values``' 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 +``named_values`` holds the *memory location* 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. + +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. + +To start our transformation of Kaleidoscope, we'll change the +``named_values`` 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: + +**Note:** the ocaml bindings currently model both ``Value*``'s and +``AllocInst*``'s as ``Llvm.llvalue``'s, but this may change in the future +to be more type safe. + +.. code-block:: ocaml + + let named_values:(string, llvalue) Hashtbl.t = Hashtbl.create 10 + +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: + +.. code-block:: ocaml + + (* 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 + +This funny looking code creates an ``Llvm.llbuilder`` 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. + +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: + +.. code-block:: ocaml + + 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 + (* Load the value. *) + build_load v name builder + +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 ``codegen_expr Ast.For ...`` (see the `full code listing <#code>`_ +for the unabridged code): + +.. code-block:: ocaml + + | 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. *) + let alloca = create_entry_block_alloca the_function var_name in + + (* Emit the start code first, without 'variable' in scope. *) + let start_val = codegen_expr start in + + (* Store the value into the alloca. *) + ignore(build_store start_val alloca builder); + + ... + + (* 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 + Hashtbl.add named_values var_name alloca; + + ... + + (* Compute the end condition. *) + let end_cond = codegen_expr end_ in + + (* 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); + ... + +This code is virtually identical to the code `before we allowed mutable +variables <OCamlLangImpl5.html#forcodegen>`_. 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. + +To support mutable argument variables, we need to also make allocas for +them. The code for this is also pretty simple: + +.. code-block:: ocaml + + (* 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) + +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 ``Codegen.codegen_func`` +right after it sets up the entry block for the function. + +The final missing piece is adding the mem2reg pass, which allows us to +get good codegen once again: + +.. code-block:: ocaml + + 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; + + (* Promote allocas to registers. *) + add_memory_to_register_promotion the_fpm; + + (* Do simple "peephole" optimizations and bit-twiddling optzn. *) + add_instruction_combining the_fpm; + + (* reassociate expressions. *) + add_reassociation the_fpm; + +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: + +.. code-block:: llvm + + define double @fib(double %x) { + entry: + %x1 = alloca double + store double %x, double* %x1 + %x2 = load double* %x1 + %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 + %x3 = load double* %x1 + %subtmp = fsub double %x3, 1.000000e+00 + %calltmp = call double @fib(double %subtmp) + %x4 = load double* %x1 + %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 + } + +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. + +Here is the code after the mem2reg pass runs: + +.. code-block:: llvm + + define double @fib(double %x) { + entry: + %cmptmp = fcmp ult double %x, 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 %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 + br label %ifcont + + ifcont: ; preds = %else, %then + %iftmp = phi double [ 1.000000e+00, %then ], [ %addtmp, %else ] + ret double %iftmp + } + +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 :). + +After the rest of the optimizers run, we get: + +.. code-block:: llvm + + 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 + } + +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. + +Now that all symbol table references are updated to use stack variables, +we'll add the assignment operator. + +New Assignment Operator +======================= + +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: + +.. code-block:: ocaml + + let main () = + (* Install standard binary operators. + * 1 is the lowest precedence. *) + Hashtbl.add Parser.binop_precedence '=' 2; + Hashtbl.add Parser.binop_precedence '<' 10; + Hashtbl.add Parser.binop_precedence '+' 20; + Hashtbl.add Parser.binop_precedence '-' 20; + ... + +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: + +.. code-block:: ocaml + + 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 + +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. + +.. code-block:: ocaml + + (* 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_ + | _ -> + ... + +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)". + +Now that we have an assignment operator, we can mutate loop variables +and arguments. For example, we can now run code like this: + +:: + + # 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); + +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! + +User-defined Local Variables +============================ + +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: + +.. code-block:: ocaml + + type token = + ... + (* var definition *) + | Var + + ... + + and lex_ident buffer = parser + ... + | "in" -> [< 'Token.In; stream >] + | "binary" -> [< 'Token.Binary; stream >] + | "unary" -> [< 'Token.Unary; stream >] + | "var" -> [< 'Token.Var; stream >] + ... + +The next step is to define the AST node that we will construct. For +var/in, it looks like this: + +.. code-block:: ocaml + + type expr = + ... + (* variant for var/in. *) + | Var of (string * expr option) array * expr + ... + +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. + +With this in place, we can define the parser pieces. The first thing we +do is add it as a primary expression: + +.. code-block:: ocaml + + (* primary + * ::= identifier + * ::= numberexpr + * ::= parenexpr + * ::= ifexpr + * ::= forexpr + * ::= varexpr *) + let rec parse_primary = parser + ... + (* 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) + + ... + + 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 + +Now that we can parse and represent the code, we need to support +emission of LLVM IR for it. This code starts out with: + +.. code-block:: ocaml + + 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) -> + +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. + +.. code-block:: ocaml + + (* 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; + +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: + +.. code-block:: ocaml + + (* Codegen the body, now that all vars are in scope. *) + let body_val = codegen_expr body in + +Finally, before returning, we restore the previous variable bindings: + +.. code-block:: ocaml + + (* 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 + +The end result of all of this is that we get properly scoped variable +definitions, and we even (trivially) allow mutation of them :). + +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. + +Full Code Listing +================= + +Here is the complete code listing for our running example, enhanced with +mutable variables and var/in support. To build this example, use: + +.. code-block:: bash + + # Compile + ocamlbuild toy.byte + # Run + ./toy.byte + +Here is the code: + +\_tags: + :: + + <{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 + +myocamlbuild.ml: + .. code-block:: ocaml + + open Ocamlbuild_plugin;; + + ocaml_lib ~extern:true "llvm";; + ocaml_lib ~extern:true "llvm_analysis";; + ocaml_lib ~extern:true "llvm_executionengine";; + ocaml_lib ~extern:true "llvm_target";; + ocaml_lib ~extern:true "llvm_scalar_opts";; + + flag ["link"; "ocaml"; "g++"] (S[A"-cc"; A"g++"; A"-cclib"; A"-rdynamic"]);; + dep ["link"; "ocaml"; "use_bindings"] ["bindings.o"];; + +token.ml: + .. code-block:: ocaml + + (*===----------------------------------------------------------------------=== + * Lexer Tokens + *===----------------------------------------------------------------------===*) + + (* The lexer returns these 'Kwd' if it is an unknown character, otherwise one of + * these others for known things. *) + type token = + (* commands *) + | Def | Extern + + (* primary *) + | Ident of string | Number of float + + (* unknown *) + | Kwd of char + + (* control *) + | If | Then | Else + | For | In + + (* operators *) + | Binary | Unary + + (* var definition *) + | Var + +lexer.ml: + .. code-block:: ocaml + + (*===----------------------------------------------------------------------=== + * Lexer + *===----------------------------------------------------------------------===*) + + let rec lex = parser + (* Skip any whitespace. *) + | [< ' (' ' | '\n' | '\r' | '\t'); stream >] -> lex stream + + (* identifier: [a-zA-Z][a-zA-Z0-9] *) + | [< ' ('A' .. 'Z' | 'a' .. 'z' as c); stream >] -> + let buffer = Buffer.create 1 in + Buffer.add_char buffer c; + lex_ident buffer stream + + (* number: [0-9.]+ *) + | [< ' ('0' .. '9' as c); stream >] -> + let buffer = Buffer.create 1 in + Buffer.add_char buffer c; + lex_number buffer stream + + (* Comment until end of line. *) + | [< ' ('#'); stream >] -> + lex_comment stream + + (* Otherwise, just return the character as its ascii value. *) + | [< 'c; stream >] -> + [< 'Token.Kwd c; lex stream >] + + (* end of stream. *) + | [< >] -> [< >] + + and lex_number buffer = parser + | [< ' ('0' .. '9' | '.' as c); stream >] -> + Buffer.add_char buffer c; + lex_number buffer stream + | [< stream=lex >] -> + [< 'Token.Number (float_of_string (Buffer.contents buffer)); stream >] + + and lex_ident buffer = parser + | [< ' ('A' .. 'Z' | 'a' .. 'z' | '0' .. '9' as c); stream >] -> + Buffer.add_char buffer c; + lex_ident buffer stream + | [< stream=lex >] -> + match Buffer.contents buffer with + | "def" -> [< 'Token.Def; stream >] + | "extern" -> [< 'Token.Extern; stream >] + | "if" -> [< 'Token.If; stream >] + | "then" -> [< 'Token.Then; stream >] + | "else" -> [< 'Token.Else; stream >] + | "for" -> [< 'Token.For; stream >] + | "in" -> [< 'Token.In; stream >] + | "binary" -> [< 'Token.Binary; stream >] + | "unary" -> [< 'Token.Unary; stream >] + | "var" -> [< 'Token.Var; stream >] + | id -> [< 'Token.Ident id; stream >] + + and lex_comment = parser + | [< ' ('\n'); stream=lex >] -> stream + | [< 'c; e=lex_comment >] -> e + | [< >] -> [< >] + +ast.ml: + .. code-block:: ocaml + + (*===----------------------------------------------------------------------=== + * Abstract Syntax Tree (aka Parse Tree) + *===----------------------------------------------------------------------===*) + + (* expr - Base type for all expression nodes. *) + type expr = + (* variant for numeric literals like "1.0". *) + | Number of float + + (* variant for referencing a variable, like "a". *) + | Variable of string + + (* variant for a unary operator. *) + | Unary of char * expr + + (* variant for a binary operator. *) + | Binary of char * expr * expr + + (* variant for function calls. *) + | Call of string * expr array + + (* variant for if/then/else. *) + | If of expr * expr * expr + + (* variant for for/in. *) + | For of string * expr * expr * expr option * expr + + (* variant for var/in. *) + | Var of (string * expr option) array * expr + + (* proto - This type represents the "prototype" for a function, which captures + * its name, and its argument names (thus implicitly the number of arguments the + * function takes). *) + type proto = + | Prototype of string * string array + | BinOpPrototype of string * string array * int + + (* func - This type represents a function definition itself. *) + type func = Function of proto * expr + +parser.ml: + .. code-block:: ocaml + + (*===---------------------------------------------------------------------=== + * Parser + |