| ======================================================= |
| Kaleidoscope: Extending the Language: Mutable Variables |
| ======================================================= |
| |
| .. contents:: |
| :local: |
| |
| 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 |
| *===---------------------------------------------------------------------===*) |
| |
| (* binop_precedence - This holds the precedence for each binary operator that is |
| * defined *) |
| let binop_precedence:(char, int) Hashtbl.t = Hashtbl.create 10 |
| |
| (* precedence - Get the precedence of the pending binary operator token. *) |
| let precedence c = try Hashtbl.find binop_precedence c with Not_found -> -1 |
| |
| (* primary |
| * ::= identifier |
| * ::= numberexpr |
| * ::= parenexpr |
| * ::= ifexpr |
| * ::= forexpr |
| * ::= varexpr *) |
| let rec parse_primary = parser |
| (* numberexpr ::= number *) |
| | [< 'Token.Number n >] -> Ast.Number n |
| |
| (* parenexpr ::= '(' expression ')' *) |
| | [< 'Token.Kwd '('; e=parse_expr; 'Token.Kwd ')' ?? "expected ')'" >] -> e |
| |
| (* identifierexpr |
| * ::= identifier |
| * ::= identifier '(' argumentexpr ')' *) |
| | [< 'Token.Ident id; stream >] -> |
| let rec parse_args accumulator = parser |
| | [< e=parse_expr; stream >] -> |
| begin parser |
| | [< 'Token.Kwd ','; e=parse_args (e :: accumulator) >] -> e |
| | [< >] -> e :: accumulator |
| end stream |
| | [< >] -> accumulator |
| in |
| let rec parse_ident id = parser |
| (* Call. *) |
| | [< 'Token.Kwd '('; |
| args=parse_args []; |
| 'Token.Kwd ')' ?? "expected ')'">] -> |
| Ast.Call (id, Array.of_list (List.rev args)) |
| |
| (* Simple variable ref. *) |
| | [< >] -> Ast.Variable id |
| in |
| parse_ident id stream |
| |
| (* ifexpr ::= 'if' expr 'then' expr 'else' expr *) |
| | [< 'Token.If; c=parse_expr; |
| 'Token.Then ?? "expected 'then'"; t=parse_expr; |
| 'Token.Else ?? "expected 'else'"; e=parse_expr >] -> |
| Ast.If (c, t, e) |
| |
| (* forexpr |
| ::= 'for' identifier '=' expr ',' expr (',' expr)? 'in' expression *) |
| | [< 'Token.For; |
| 'Token.Ident id ?? "expected identifier after for"; |
| 'Token.Kwd '=' ?? "expected '=' after for"; |
| stream >] -> |
| begin parser |
| | [< |
| start=parse_expr; |
| 'Token.Kwd ',' ?? "expected ',' after for"; |
| end_=parse_expr; |
| stream >] -> |
| let step = |
| begin parser |
| | [< 'Token.Kwd ','; step=parse_expr >] -> Some step |
| | [< >] -> None |
| end stream |
| in |
| begin parser |
| | [< 'Token.In; body=parse_expr >] -> |
| Ast.For (id, start, end_, step, body) |
| | [< >] -> |
| raise (Stream.Error "expected 'in' after for") |
| end stream |
| | [< >] -> |
| raise (Stream.Error "expected '=' after for") |
| end stream |
| |
| (* 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) |
| |
| | [< >] -> raise (Stream.Error "unknown token when expecting an expression.") |
| |
| (* unary |
| * ::= primary |
| * ::= '!' unary *) |
| and parse_unary = parser |
| (* If this is a unary operator, read it. *) |
| | [< 'Token.Kwd op when op != '(' && op != ')'; operand=parse_expr >] -> |
| Ast.Unary (op, operand) |
| |
| (* If the current token is not an operator, it must be a primary expr. *) |
| | [< stream >] -> parse_primary stream |
| |
| (* binoprhs |
| * ::= ('+' primary)* *) |
| and parse_bin_rhs expr_prec lhs stream = |
| match Stream.peek stream with |
| (* If this is a binop, find its precedence. *) |
| | Some (Token.Kwd c) when Hashtbl.mem binop_precedence c -> |
| let token_prec = precedence c in |
| |
| (* If this is a binop that binds at least as tightly as the current binop, |
| * consume it, otherwise we are done. *) |
| if token_prec < expr_prec then lhs else begin |
| (* Eat the binop. *) |
| Stream.junk stream; |
| |
| (* Parse the primary expression after the binary operator. *) |
| let rhs = parse_unary stream in |
| |
| (* Okay, we know this is a binop. *) |
| let rhs = |
| match Stream.peek stream with |
| | Some (Token.Kwd c2) -> |
| (* If BinOp binds less tightly with rhs than the operator after |
| * rhs, let the pending operator take rhs as its lhs. *) |
| let next_prec = precedence c2 in |
| if token_prec < next_prec |
| then parse_bin_rhs (token_prec + 1) rhs stream |
| else rhs |
| | _ -> rhs |
| in |
| |
| (* Merge lhs/rhs. *) |
| let lhs = Ast.Binary (c, lhs, rhs) in |
| parse_bin_rhs expr_prec lhs stream |
| end |
| | _ -> lhs |
| |
| 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 |
| |
| (* expression |
| * ::= primary binoprhs *) |
| and parse_expr = parser |
| | [< lhs=parse_unary; stream >] -> parse_bin_rhs 0 lhs stream |
| |
| (* prototype |
| * ::= id '(' id* ')' |
| * ::= binary LETTER number? (id, id) |
| * ::= unary LETTER number? (id) *) |
| let parse_prototype = |
| let rec parse_args accumulator = parser |
| | [< 'Token.Ident id; e=parse_args (id::accumulator) >] -> e |
| | [< >] -> accumulator |
| in |
| let parse_operator = parser |
| | [< 'Token.Unary >] -> "unary", 1 |
| | [< 'Token.Binary >] -> "binary", 2 |
| in |
| let parse_binary_precedence = parser |
| | [< 'Token.Number n >] -> int_of_float n |
| | [< >] -> 30 |
| in |
| parser |
| | [< 'Token.Ident id; |
| 'Token.Kwd '(' ?? "expected '(' in prototype"; |
| args=parse_args []; |
| 'Token.Kwd ')' ?? "expected ')' in prototype" >] -> |
| (* success. *) |
| Ast.Prototype (id, Array.of_list (List.rev args)) |
| | [< (prefix, kind)=parse_operator; |
| 'Token.Kwd op ?? "expected an operator"; |
| (* Read the precedence if present. *) |
| binary_precedence=parse_binary_precedence; |
| 'Token.Kwd '(' ?? "expected '(' in prototype"; |
| args=parse_args []; |
| 'Token.Kwd ')' ?? "expected ')' in prototype" >] -> |
| let name = prefix ^ (String.make 1 op) in |
| let args = Array.of_list (List.rev args) in |
| |
| (* Verify right number of arguments for operator. *) |
| if Array.length args != kind |
| then raise (Stream.Error "invalid number of operands for operator") |
| else |
| if kind == 1 then |
| Ast.Prototype (name, args) |
| else |
| Ast.BinOpPrototype (name, args, binary_precedence) |
| | [< >] -> |
| raise (Stream.Error "expected function name in prototype") |
| |
| (* definition ::= 'def' prototype expression *) |
| let parse_definition = parser |
| | [< 'Token.Def; p=parse_prototype; e=parse_expr >] -> |
| Ast.Function (p, e) |
| |
| (* toplevelexpr ::= expression *) |
| let parse_toplevel = parser |
| | [< e=parse_expr >] -> |
| (* Make an anonymous proto. *) |
| Ast.Function (Ast.Prototype ("", [||]), e) |
| |
| (* external ::= 'extern' prototype *) |
| let parse_extern = parser |
| | [< 'Token.Extern; e=parse_prototype >] -> e |
| |
| codegen.ml: |
| .. code-block:: ocaml |
| |
| (*===----------------------------------------------------------------------=== |
| * Code Generation |
| *===----------------------------------------------------------------------===*) |
| |
| open Llvm |
| |
| exception Error of string |
| |
| let context = global_context () |
| let the_module = create_module context "my cool jit" |
| let builder = builder context |
| let named_values:(string, llvalue) Hashtbl.t = Hashtbl.create 10 |
| let double_type = double_type context |
| |
| (* 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 context (instr_begin (entry_block the_function)) in |
| build_alloca double_type var_name builder |
| |
| let rec codegen_expr = function |
| | Ast.Number n -> const_float double_type n |
| | 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 |
| | Ast.Unary (op, operand) -> |
| let operand = codegen_expr operand in |
| let callee = "unary" ^ (String.make 1 op) in |
| let callee = |
| match lookup_function callee the_module with |
| | Some callee -> callee |
| | None -> raise (Error "unknown unary operator") |
| in |
| build_call callee [|operand|] "unop" builder |
| | Ast.Binary (op, lhs, rhs) -> |
| 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 |
| |
| (* 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_ |
| | _ -> |
| let lhs_val = codegen_expr lhs in |
| let rhs_val = codegen_expr rhs in |
| begin |
| match op with |
| | '+' -> build_add lhs_val rhs_val "addtmp" builder |
| | '-' -> build_sub lhs_val rhs_val "subtmp" builder |
| | '*' -> build_mul lhs_val rhs_val "multmp" builder |
| | '<' -> |
| (* Convert bool 0/1 to double 0.0 or 1.0 *) |
| let i = build_fcmp Fcmp.Ult lhs_val rhs_val "cmptmp" builder in |
| build_uitofp i double_type "booltmp" builder |
| | _ -> |
| (* If it wasn't a builtin binary operator, it must be a user defined |
| * one. Emit a call to it. *) |
| let callee = "binary" ^ (String.make 1 op) in |
| let callee = |
| match lookup_function callee the_module with |
| | Some callee -> callee |
| | None -> raise (Error "binary operator not found!") |
| in |
| build_call callee [|lhs_val; rhs_val|] "binop" builder |
| end |
| end |
| | Ast.Call (callee, args) -> |
| (* Look up the name in the module table. *) |
| let callee = |
| match lookup_function callee the_module with |
| | Some callee -> callee |
| | None -> raise (Error "unknown function referenced") |
| in |
| let params = params callee in |
| |
| (* If argument mismatch error. *) |
| if Array.length params == Array.length args then () else |
| raise (Error "incorrect # arguments passed"); |
| let args = Array.map codegen_expr args in |
| build_call callee args "calltmp" builder |
| | Ast.If (cond, then_, else_) -> |
| let cond = codegen_expr cond in |
| |
| (* Convert condition to a bool by comparing equal to 0.0 *) |
| let zero = const_float double_type 0.0 in |
| let cond_val = build_fcmp Fcmp.One cond zero "ifcond" builder in |
| |
| (* Grab the first block so that we might later add the conditional branch |
| * to it at the end of the function. *) |
| let start_bb = insertion_block builder in |
| let the_function = block_parent start_bb in |
| |
| let then_bb = append_block context "then" the_function in |
| |
| (* Emit 'then' value. *) |
| position_at_end then_bb builder; |
| let then_val = codegen_expr then_ in |
| |
| (* Codegen of 'then' can change the current block, update then_bb for the |
| * phi. We create a new name because one is used for the phi node, and the |
| * other is used for the conditional branch. *) |
| let new_then_bb = insertion_block builder in |
| |
| (* Emit 'else' value. *) |
| let else_bb = append_block context "else" the_function in |
| position_at_end else_bb builder; |
| let else_val = codegen_expr else_ in |
| |
| (* Codegen of 'else' can change the current block, update else_bb for the |
| * phi. *) |
| let new_else_bb = insertion_block builder in |
| |
| (* Emit merge block. *) |
| let merge_bb = append_block context "ifcont" the_function in |
| position_at_end merge_bb builder; |
| let incoming = [(then_val, new_then_bb); (else_val, new_else_bb)] in |
| let phi = build_phi incoming "iftmp" builder in |
| |
| (* Return to the start block to add the conditional branch. *) |
| position_at_end start_bb builder; |
| ignore (build_cond_br cond_val then_bb else_bb builder); |
| |
| (* Set a unconditional branch at the end of the 'then' block and the |
| * 'else' block to the 'merge' block. *) |
| position_at_end new_then_bb builder; ignore (build_br merge_bb builder); |
| position_at_end new_else_bb builder; ignore (build_br merge_bb builder); |
| |
| (* Finally, set the builder to the end of the merge block. *) |
| position_at_end merge_bb builder; |
| |
| phi |
| | Ast.For (var_name, start, end_, step, body) -> |
| (* Output this as: |
| * var = alloca double |
| * ... |
| * start = startexpr |
| * store start -> var |
| * goto loop |
| * loop: |
| * ... |
| * bodyexpr |
| * ... |
| * loopend: |
| * step = stepexpr |
| * endcond = endexpr |
| * |
| * curvar = load var |
| * nextvar = curvar + step |
| * store nextvar -> var |
| * br endcond, loop, endloop |
| * outloop: *) |
| |
| 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); |
| |
| (* Make the new basic block for the loop header, inserting after current |
| * block. *) |
| let loop_bb = append_block context "loop" the_function in |
| |
| (* Insert an explicit fall through from the current block to the |
| * loop_bb. *) |
| ignore (build_br loop_bb builder); |
| |
| (* Start insertion in loop_bb. *) |
| position_at_end loop_bb 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; |
| |
| (* Emit the body of the loop. This, like any other expr, can change the |
| * current BB. Note that we ignore the value computed by the body, but |
| * don't allow an error *) |
| ignore (codegen_expr body); |
| |
| (* Emit the step value. *) |
| let step_val = |
| match step with |
| | Some step -> codegen_expr step |
| (* If not specified, use 1.0. *) |
| | None -> const_float double_type 1.0 |
| in |
| |
| (* 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); |
| |
| (* Convert condition to a bool by comparing equal to 0.0. *) |
| let zero = const_float double_type 0.0 in |
| let end_cond = build_fcmp Fcmp.One end_cond zero "loopcond" builder in |
| |
| (* Create the "after loop" block and insert it. *) |
| let after_bb = append_block context "afterloop" the_function in |
| |
| (* Insert the conditional branch into the end of loop_end_bb. *) |
| ignore (build_cond_br end_cond loop_bb after_bb builder); |
| |
| (* Any new code will be inserted in after_bb. *) |
| position_at_end after_bb builder; |
| |
| (* Restore the unshadowed variable. *) |
| begin match old_val with |
| | Some old_val -> Hashtbl.add named_values var_name old_val |
| | None -> () |
| end; |
| |
| (* for expr always returns 0.0. *) |
| const_null double_type |
| | 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) -> |
| (* 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; |
| |
| (* Codegen the body, now that all vars are in scope. *) |
| let body_val = codegen_expr body in |
| |
| (* 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 |
| |
| let codegen_proto = function |
| | Ast.Prototype (name, args) | Ast.BinOpPrototype (name, args, _) -> |
| (* Make the function type: double(double,double) etc. *) |
| let doubles = Array.make (Array.length args) double_type in |
| let ft = function_type double_type doubles in |
| let f = |
| match lookup_function name the_module with |
| | None -> declare_function name ft the_module |
| |
| (* If 'f' conflicted, there was already something named 'name'. If it |
| * has a body, don't allow redefinition or reextern. *) |
| | Some f -> |
| (* If 'f' already has a body, reject this. *) |
| if block_begin f <> At_end f then |
| raise (Error "redefinition of function"); |
| |
| (* If 'f' took a different number of arguments, reject. *) |
| if element_type (type_of f) <> ft then |
| raise (Error "redefinition of function with different # args"); |
| f |
| in |
| |
| (* Set names for all arguments. *) |
| Array.iteri (fun i a -> |
| let n = args.(i) in |
| set_value_name n a; |
| Hashtbl.add named_values n a; |
| ) (params f); |
| f |
| |
| (* 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) |
| |
| let codegen_func the_fpm = function |
| | Ast.Function (proto, body) -> |
| Hashtbl.clear named_values; |
| let the_function = codegen_proto proto in |
| |
| (* If this is an operator, install it. *) |
| begin match proto with |
| | Ast.BinOpPrototype (name, args, prec) -> |
| let op = name.[String.length name - 1] in |
| Hashtbl.add Parser.binop_precedence op prec; |
| | _ -> () |
| end; |
| |
| (* Create a new basic block to start insertion into. *) |
| let bb = append_block context "entry" the_function in |
| position_at_end bb builder; |
| |
| try |
| (* Add all arguments to the symbol table and create their allocas. *) |
| create_argument_allocas the_function proto; |
| |
| let ret_val = codegen_expr body in |
| |
| (* Finish off the function. *) |
| let _ = build_ret ret_val builder in |
| |
| (* Validate the generated code, checking for consistency. *) |
| Llvm_analysis.assert_valid_function the_function; |
| |
| (* Optimize the function. *) |
| let _ = PassManager.run_function the_function the_fpm in |
| |
| the_function |
| with e -> |
| delete_function the_function; |
| raise e |
| |
| toplevel.ml: |
| .. code-block:: ocaml |
| |
| (*===----------------------------------------------------------------------=== |
| * Top-Level parsing and JIT Driver |
| *===----------------------------------------------------------------------===*) |
| |
| open Llvm |
| open Llvm_executionengine |
| |
| (* top ::= definition | external | expression | ';' *) |
| let rec main_loop the_fpm the_execution_engine stream = |
| match Stream.peek stream with |
| | None -> () |
| |
| (* ignore top-level semicolons. *) |
| | Some (Token.Kwd ';') -> |
| Stream.junk stream; |
| main_loop the_fpm the_execution_engine stream |
| |
| | Some token -> |
| begin |
| try match token with |
| | Token.Def -> |
| let e = Parser.parse_definition stream in |
| print_endline "parsed a function definition."; |
| dump_value (Codegen.codegen_func the_fpm e); |
| | Token.Extern -> |
| let e = Parser.parse_extern stream in |
| print_endline "parsed an extern."; |
| dump_value (Codegen.codegen_proto e); |
| | _ -> |
| (* Evaluate a top-level expression into an anonymous function. *) |
| let e = Parser.parse_toplevel stream in |
| print_endline "parsed a top-level expr"; |
| let the_function = Codegen.codegen_func the_fpm e in |
| dump_value the_function; |
| |
| (* JIT the function, returning a function pointer. *) |
| let result = ExecutionEngine.run_function the_function [||] |
| the_execution_engine in |
| |
| print_string "Evaluated to "; |
| print_float (GenericValue.as_float Codegen.double_type result); |
| print_newline (); |
| with Stream.Error s | Codegen.Error s -> |
| (* Skip token for error recovery. *) |
| Stream.junk stream; |
| print_endline s; |
| end; |
| print_string "ready> "; flush stdout; |
| main_loop the_fpm the_execution_engine stream |
| |
| toy.ml: |
| .. code-block:: ocaml |
| |
| (*===----------------------------------------------------------------------=== |
| * Main driver code. |
| *===----------------------------------------------------------------------===*) |
| |
| open Llvm |
| open Llvm_executionengine |
| open Llvm_target |
| open Llvm_scalar_opts |
| |
| let main () = |
| ignore (initialize_native_target ()); |
| |
| (* 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; |
| Hashtbl.add Parser.binop_precedence '*' 40; (* highest. *) |
| |
| (* Prime the first token. *) |
| print_string "ready> "; flush stdout; |
| let stream = Lexer.lex (Stream.of_channel stdin) in |
| |
| (* Create the JIT. *) |
| let the_execution_engine = ExecutionEngine.create Codegen.the_module in |
| 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_combination the_fpm; |
| |
| (* reassociate expressions. *) |
| add_reassociation the_fpm; |
| |
| (* Eliminate Common SubExpressions. *) |
| add_gvn the_fpm; |
| |
| (* Simplify the control flow graph (deleting unreachable blocks, etc). *) |
| add_cfg_simplification the_fpm; |
| |
| ignore (PassManager.initialize the_fpm); |
| |
| (* Run the main "interpreter loop" now. *) |
| Toplevel.main_loop the_fpm the_execution_engine stream; |
| |
| (* Print out all the generated code. *) |
| dump_module Codegen.the_module |
| ;; |
| |
| main () |
| |
| bindings.c |
| .. code-block:: c |
| |
| #include <stdio.h> |
| |
| /* putchard - putchar that takes a double and returns 0. */ |
| extern double putchard(double X) { |
| putchar((char)X); |
| return 0; |
| } |
| |
| /* printd - printf that takes a double prints it as "%f\n", returning 0. */ |
| extern double printd(double X) { |
| printf("%f\n", X); |
| return 0; |
| } |
| |
| `Next: Conclusion and other useful LLVM tidbits <OCamlLangImpl8.html>`_ |
| |
