| ================================================== |
| Kaleidoscope: Extending the Language: Control Flow |
| ================================================== |
| |
| .. contents:: |
| :local: |
| |
| Chapter 5 Introduction |
| ====================== |
| |
| Welcome to Chapter 5 of the "`Implementing a language with |
| LLVM <index.html>`_" tutorial. Parts 1-4 described the implementation of |
| the simple Kaleidoscope language and included support for generating |
| LLVM IR, followed by optimizations and a JIT compiler. Unfortunately, as |
| presented, Kaleidoscope is mostly useless: it has no control flow other |
| than call and return. This means that you can't have conditional |
| branches in the code, significantly limiting its power. In this episode |
| of "build that compiler", we'll extend Kaleidoscope to have an |
| if/then/else expression plus a simple 'for' loop. |
| |
| If/Then/Else |
| ============ |
| |
| Extending Kaleidoscope to support if/then/else is quite straightforward. |
| It basically requires adding lexer support for this "new" concept to the |
| lexer, parser, AST, and LLVM code emitter. This example is nice, because |
| it shows how easy it is to "grow" a language over time, incrementally |
| extending it as new ideas are discovered. |
| |
| Before we get going on "how" we add this extension, lets talk about |
| "what" we want. The basic idea is that we want to be able to write this |
| sort of thing: |
| |
| :: |
| |
| def fib(x) |
| if x < 3 then |
| 1 |
| else |
| fib(x-1)+fib(x-2); |
| |
| In Kaleidoscope, every construct is an expression: there are no |
| statements. As such, the if/then/else expression needs to return a value |
| like any other. Since we're using a mostly functional form, we'll have |
| it evaluate its conditional, then return the 'then' or 'else' value |
| based on how the condition was resolved. This is very similar to the C |
| "?:" expression. |
| |
| The semantics of the if/then/else expression is that it evaluates the |
| condition to a boolean equality value: 0.0 is considered to be false and |
| everything else is considered to be true. If the condition is true, the |
| first subexpression is evaluated and returned, if the condition is |
| false, the second subexpression is evaluated and returned. Since |
| Kaleidoscope allows side-effects, this behavior is important to nail |
| down. |
| |
| Now that we know what we "want", lets break this down into its |
| constituent pieces. |
| |
| Lexer Extensions for If/Then/Else |
| --------------------------------- |
| |
| The lexer extensions are straightforward. First we add new variants for |
| the relevant tokens: |
| |
| .. code-block:: ocaml |
| |
| (* control *) |
| | If | Then | Else | For | In |
| |
| Once we have that, we recognize the new keywords in the lexer. This is |
| pretty simple stuff: |
| |
| .. code-block:: ocaml |
| |
| ... |
| 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 >] |
| | id -> [< 'Token.Ident id; stream >] |
| |
| AST Extensions for If/Then/Else |
| ------------------------------- |
| |
| To represent the new expression we add a new AST variant for it: |
| |
| .. code-block:: ocaml |
| |
| type expr = |
| ... |
| (* variant for if/then/else. *) |
| | If of expr * expr * expr |
| |
| The AST variant just has pointers to the various subexpressions. |
| |
| Parser Extensions for If/Then/Else |
| ---------------------------------- |
| |
| Now that we have the relevant tokens coming from the lexer and we have |
| the AST node to build, our parsing logic is relatively straightforward. |
| Next we add a new case for parsing a if-expression as a primary expression: |
| |
| .. code-block:: ocaml |
| |
| let rec parse_primary = parser |
| ... |
| (* 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) |
| |
| LLVM IR for If/Then/Else |
| ------------------------ |
| |
| Now that we have it parsing and building the AST, the final piece is |
| adding LLVM code generation support. This is the most interesting part |
| of the if/then/else example, because this is where it starts to |
| introduce new concepts. All of the code above has been thoroughly |
| described in previous chapters. |
| |
| To motivate the code we want to produce, lets take a look at a simple |
| example. Consider: |
| |
| :: |
| |
| extern foo(); |
| extern bar(); |
| def baz(x) if x then foo() else bar(); |
| |
| If you disable optimizations, the code you'll (soon) get from |
| Kaleidoscope looks like this: |
| |
| .. code-block:: llvm |
| |
| declare double @foo() |
| |
| declare double @bar() |
| |
| define double @baz(double %x) { |
| entry: |
| %ifcond = fcmp one double %x, 0.000000e+00 |
| br i1 %ifcond, label %then, label %else |
| |
| then: ; preds = %entry |
| %calltmp = call double @foo() |
| br label %ifcont |
| |
| else: ; preds = %entry |
| %calltmp1 = call double @bar() |
| br label %ifcont |
| |
| ifcont: ; preds = %else, %then |
| %iftmp = phi double [ %calltmp, %then ], [ %calltmp1, %else ] |
| ret double %iftmp |
| } |
| |
| To visualize the control flow graph, you can use a nifty feature of the |
| LLVM '`opt <http://llvm.org/cmds/opt.html>`_' tool. If you put this LLVM |
| IR into "t.ll" and run "``llvm-as < t.ll | opt -analyze -view-cfg``", `a |
| window will pop up <../ProgrammersManual.html#viewing-graphs-while-debugging-code>`_ and you'll |
| see this graph: |
| |
| .. figure:: LangImpl05-cfg.png |
| :align: center |
| :alt: Example CFG |
| |
| Example CFG |
| |
| Another way to get this is to call |
| "``Llvm_analysis.view_function_cfg f``" or |
| "``Llvm_analysis.view_function_cfg_only f``" (where ``f`` is a |
| "``Function``") either by inserting actual calls into the code and |
| recompiling or by calling these in the debugger. LLVM has many nice |
| features for visualizing various graphs. |
| |
| Getting back to the generated code, it is fairly simple: the entry block |
| evaluates the conditional expression ("x" in our case here) and compares |
| the result to 0.0 with the "``fcmp one``" instruction ('one' is "Ordered |
| and Not Equal"). Based on the result of this expression, the code jumps |
| to either the "then" or "else" blocks, which contain the expressions for |
| the true/false cases. |
| |
| Once the then/else blocks are finished executing, they both branch back |
| to the 'ifcont' block to execute the code that happens after the |
| if/then/else. In this case the only thing left to do is to return to the |
| caller of the function. The question then becomes: how does the code |
| know which expression to return? |
| |
| The answer to this question involves an important SSA operation: the |
| `Phi |
| operation <http://en.wikipedia.org/wiki/Static_single_assignment_form>`_. |
| If you're not familiar with SSA, `the wikipedia |
| article <http://en.wikipedia.org/wiki/Static_single_assignment_form>`_ |
| is a good introduction and there are various other introductions to it |
| available on your favorite search engine. The short version is that |
| "execution" of the Phi operation requires "remembering" which block |
| control came from. The Phi operation takes on the value corresponding to |
| the input control block. In this case, if control comes in from the |
| "then" block, it gets the value of "calltmp". If control comes from the |
| "else" block, it gets the value of "calltmp1". |
| |
| At this point, you are probably starting to think "Oh no! This means my |
| simple and elegant front-end will have to start generating SSA form in |
| order to use LLVM!". Fortunately, this is not the case, and we strongly |
| advise *not* implementing an SSA construction algorithm in your |
| front-end unless there is an amazingly good reason to do so. In |
| practice, there are two sorts of values that float around in code |
| written for your average imperative programming language that might need |
| Phi nodes: |
| |
| #. Code that involves user variables: ``x = 1; x = x + 1;`` |
| #. Values that are implicit in the structure of your AST, such as the |
| Phi node in this case. |
| |
| In `Chapter 7 <OCamlLangImpl7.html>`_ of this tutorial ("mutable |
| variables"), we'll talk about #1 in depth. For now, just believe me that |
| you don't need SSA construction to handle this case. For #2, you have |
| the choice of using the techniques that we will describe for #1, or you |
| can insert Phi nodes directly, if convenient. In this case, it is really |
| really easy to generate the Phi node, so we choose to do it directly. |
| |
| Okay, enough of the motivation and overview, lets generate code! |
| |
| Code Generation for If/Then/Else |
| -------------------------------- |
| |
| In order to generate code for this, we implement the ``Codegen`` method |
| for ``IfExprAST``: |
| |
| .. code-block:: ocaml |
| |
| let rec codegen_expr = function |
| ... |
| | 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 |
| |
| This code is straightforward and similar to what we saw before. We emit |
| the expression for the condition, then compare that value to zero to get |
| a truth value as a 1-bit (bool) value. |
| |
| .. code-block:: ocaml |
| |
| (* 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 |
| position_at_end then_bb builder; |
| |
| As opposed to the `C++ tutorial <LangImpl05.html>`_, we have to build our |
| basic blocks bottom up since we can't have dangling BasicBlocks. We |
| start off by saving a pointer to the first block (which might not be the |
| entry block), which we'll need to build a conditional branch later. We |
| do this by asking the ``builder`` for the current BasicBlock. The fourth |
| line gets the current Function object that is being built. It gets this |
| by the ``start_bb`` for its "parent" (the function it is currently |
| embedded into). |
| |
| Once it has that, it creates one block. It is automatically appended |
| into the function's list of blocks. |
| |
| .. code-block:: ocaml |
| |
| (* 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 |
| |
| We move the builder to start inserting into the "then" block. Strictly |
| speaking, this call moves the insertion point to be at the end of the |
| specified block. However, since the "then" block is empty, it also |
| starts out by inserting at the beginning of the block. :) |
| |
| Once the insertion point is set, we recursively codegen the "then" |
| expression from the AST. |
| |
| The final line here is quite subtle, but is very important. The basic |
| issue is that when we create the Phi node in the merge block, we need to |
| set up the block/value pairs that indicate how the Phi will work. |
| Importantly, the Phi node expects to have an entry for each predecessor |
| of the block in the CFG. Why then, are we getting the current block when |
| we just set it to ThenBB 5 lines above? The problem is that the "Then" |
| expression may actually itself change the block that the Builder is |
| emitting into if, for example, it contains a nested "if/then/else" |
| expression. Because calling Codegen recursively could arbitrarily change |
| the notion of the current block, we are required to get an up-to-date |
| value for code that will set up the Phi node. |
| |
| .. code-block:: ocaml |
| |
| (* 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 |
| |
| Code generation for the 'else' block is basically identical to codegen |
| for the 'then' block. |
| |
| .. code-block:: ocaml |
| |
| (* 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 |
| |
| The first two lines here are now familiar: the first adds the "merge" |
| block to the Function object. The second changes the insertion |
| point so that newly created code will go into the "merge" block. Once |
| that is done, we need to create the PHI node and set up the block/value |
| pairs for the PHI. |
| |
| .. code-block:: ocaml |
| |
| (* 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); |
| |
| Once the blocks are created, we can emit the conditional branch that |
| chooses between them. Note that creating new blocks does not implicitly |
| affect the IRBuilder, so it is still inserting into the block that the |
| condition went into. This is why we needed to save the "start" block. |
| |
| .. code-block:: ocaml |
| |
| (* 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 |
| |
| To finish off the blocks, we create an unconditional branch to the merge |
| block. One interesting (and very important) aspect of the LLVM IR is |
| that it `requires all basic blocks to be |
| "terminated" <../LangRef.html#functionstructure>`_ with a `control flow |
| instruction <../LangRef.html#terminators>`_ such as return or branch. |
| This means that all control flow, *including fall throughs* must be made |
| explicit in the LLVM IR. If you violate this rule, the verifier will |
| emit an error. |
| |
| Finally, the CodeGen function returns the phi node as the value computed |
| by the if/then/else expression. In our example above, this returned |
| value will feed into the code for the top-level function, which will |
| create the return instruction. |
| |
| Overall, we now have the ability to execute conditional code in |
| Kaleidoscope. With this extension, Kaleidoscope is a fairly complete |
| language that can calculate a wide variety of numeric functions. Next up |
| we'll add another useful expression that is familiar from non-functional |
| languages... |
| |
| 'for' Loop Expression |
| ===================== |
| |
| Now that we know how to add basic control flow constructs to the |
| language, we have the tools to add more powerful things. Lets add |
| something more aggressive, a 'for' expression: |
| |
| :: |
| |
| extern putchard(char); |
| def printstar(n) |
| for i = 1, i < n, 1.0 in |
| putchard(42); # ascii 42 = '*' |
| |
| # print 100 '*' characters |
| printstar(100); |
| |
| This expression defines a new variable ("i" in this case) which iterates |
| from a starting value, while the condition ("i < n" in this case) is |
| true, incrementing by an optional step value ("1.0" in this case). If |
| the step value is omitted, it defaults to 1.0. While the loop is true, |
| it executes its body expression. Because we don't have anything better |
| to return, we'll just define the loop as always returning 0.0. In the |
| future when we have mutable variables, it will get more useful. |
| |
| As before, lets talk about the changes that we need to Kaleidoscope to |
| support this. |
| |
| Lexer Extensions for the 'for' Loop |
| ----------------------------------- |
| |
| The lexer extensions are the same sort of thing as for if/then/else: |
| |
| .. code-block:: ocaml |
| |
| ... in Token.token ... |
| (* control *) |
| | If | Then | Else |
| | For | In |
| |
| ... in Lexer.lex_ident... |
| 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 >] |
| | id -> [< 'Token.Ident id; stream >] |
| |
| AST Extensions for the 'for' Loop |
| --------------------------------- |
| |
| The AST variant is just as simple. It basically boils down to capturing |
| the variable name and the constituent expressions in the node. |
| |
| .. code-block:: ocaml |
| |
| type expr = |
| ... |
| (* variant for for/in. *) |
| | For of string * expr * expr * expr option * expr |
| |
| Parser Extensions for the 'for' Loop |
| ------------------------------------ |
| |
| The parser code is also fairly standard. The only interesting thing here |
| is handling of the optional step value. The parser code handles it by |
| checking to see if the second comma is present. If not, it sets the step |
| value to null in the AST node: |
| |
| .. code-block:: ocaml |
| |
| let rec parse_primary = parser |
| ... |
| (* 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 |
| |
| LLVM IR for the 'for' Loop |
| -------------------------- |
| |
| Now we get to the good part: the LLVM IR we want to generate for this |
| thing. With the simple example above, we get this LLVM IR (note that |
| this dump is generated with optimizations disabled for clarity): |
| |
| .. code-block:: llvm |
| |
| declare double @putchard(double) |
| |
| define double @printstar(double %n) { |
| entry: |
| ; initial value = 1.0 (inlined into phi) |
| br label %loop |
| |
| loop: ; preds = %loop, %entry |
| %i = phi double [ 1.000000e+00, %entry ], [ %nextvar, %loop ] |
| ; body |
| %calltmp = call double @putchard(double 4.200000e+01) |
| ; increment |
| %nextvar = fadd double %i, 1.000000e+00 |
| |
| ; termination test |
| %cmptmp = fcmp ult double %i, %n |
| %booltmp = uitofp i1 %cmptmp to double |
| %loopcond = fcmp one double %booltmp, 0.000000e+00 |
| br i1 %loopcond, label %loop, label %afterloop |
| |
| afterloop: ; preds = %loop |
| ; loop always returns 0.0 |
| ret double 0.000000e+00 |
| } |
| |
| This loop contains all the same constructs we saw before: a phi node, |
| several expressions, and some basic blocks. Lets see how this fits |
| together. |
| |
| Code Generation for the 'for' Loop |
| ---------------------------------- |
| |
| The first part of Codegen is very simple: we just output the start |
| expression for the loop value: |
| |
| .. code-block:: ocaml |
| |
| let rec codegen_expr = function |
| ... |
| | Ast.For (var_name, start, end_, step, body) -> |
| (* Emit the start code first, without 'variable' in scope. *) |
| let start_val = codegen_expr start in |
| |
| With this out of the way, the next step is to set up the LLVM basic |
| block for the start of the loop body. In the case above, the whole loop |
| body is one block, but remember that the body code itself could consist |
| of multiple blocks (e.g. if it contains an if/then/else or a for/in |
| expression). |
| |
| .. code-block:: ocaml |
| |
| (* Make the new basic block for the loop header, inserting after current |
| * block. *) |
| let preheader_bb = insertion_block builder in |
| let the_function = block_parent preheader_bb in |
| 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); |
| |
| This code is similar to what we saw for if/then/else. Because we will |
| need it to create the Phi node, we remember the block that falls through |
| into the loop. Once we have that, we create the actual block that starts |
| the loop and create an unconditional branch for the fall-through between |
| the two blocks. |
| |
| .. code-block:: ocaml |
| |
| (* Start insertion in loop_bb. *) |
| position_at_end loop_bb builder; |
| |
| (* Start the PHI node with an entry for start. *) |
| let variable = build_phi [(start_val, preheader_bb)] var_name builder in |
| |
| Now that the "preheader" for the loop is set up, we switch to emitting |
| code for the loop body. To begin with, we move the insertion point and |
| create the PHI node for the loop induction variable. Since we already |
| know the incoming value for the starting value, we add it to the Phi |
| node. Note that the Phi will eventually get a second value for the |
| backedge, but we can't set it up yet (because it doesn't exist!). |
| |
| .. code-block:: ocaml |
| |
| (* 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 variable; |
| |
| (* 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); |
| |
| Now the code starts to get more interesting. Our 'for' loop introduces a |
| new variable to the symbol table. This means that our symbol table can |
| now contain either function arguments or loop variables. To handle this, |
| before we codegen the body of the loop, we add the loop variable as the |
| current value for its name. Note that it is possible that there is a |
| variable of the same name in the outer scope. It would be easy to make |
| this an error (emit an error and return null if there is already an |
| entry for VarName) but we choose to allow shadowing of variables. In |
| order to handle this correctly, we remember the Value that we are |
| potentially shadowing in ``old_val`` (which will be None if there is no |
| shadowed variable). |
| |
| Once the loop variable is set into the symbol table, the code |
| recursively codegen's the body. This allows the body to use the loop |
| variable: any references to it will naturally find it in the symbol |
| table. |
| |
| .. code-block:: ocaml |
| |
| (* 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 |
| |
| let next_var = build_add variable step_val "nextvar" builder in |
| |
| Now that the body is emitted, we compute the next value of the iteration |
| variable by adding the step value, or 1.0 if it isn't present. |
| '``next_var``' will be the value of the loop variable on the next |
| iteration of the loop. |
| |
| .. code-block:: ocaml |
| |
| (* Compute the end condition. *) |
| let end_cond = codegen_expr end_ in |
| |
| (* 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 |
| |
| Finally, we evaluate the exit value of the loop, to determine whether |
| the loop should exit. This mirrors the condition evaluation for the |
| if/then/else statement. |
| |
| .. code-block:: ocaml |
| |
| (* Create the "after loop" block and insert it. *) |
| let loop_end_bb = insertion_block builder in |
| 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; |
| |
| With the code for the body of the loop complete, we just need to finish |
| up the control flow for it. This code remembers the end block (for the |
| phi node), then creates the block for the loop exit ("afterloop"). Based |
| on the value of the exit condition, it creates a conditional branch that |
| chooses between executing the loop again and exiting the loop. Any |
| future code is emitted in the "afterloop" block, so it sets the |
| insertion position to it. |
| |
| .. code-block:: ocaml |
| |
| (* Add a new entry to the PHI node for the backedge. *) |
| add_incoming (next_var, loop_end_bb) variable; |
| |
| (* 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 |
| |
| The final code handles various cleanups: now that we have the |
| "``next_var``" value, we can add the incoming value to the loop PHI |
| node. After that, we remove the loop variable from the symbol table, so |
| that it isn't in scope after the for loop. Finally, code generation of |
| the for loop always returns 0.0, so that is what we return from |
| ``Codegen.codegen_expr``. |
| |
| With this, we conclude the "adding control flow to Kaleidoscope" chapter |
| of the tutorial. In this chapter we added two control flow constructs, |
| and used them to motivate a couple of aspects of the LLVM IR that are |
| important for front-end implementors to know. In the next chapter of our |
| saga, we will get a bit crazier and add `user-defined |
| operators <OCamlLangImpl6.html>`_ to our poor innocent language. |
| |
| Full Code Listing |
| ================= |
| |
| Here is the complete code listing for our running example, enhanced with |
| the if/then/else and for expressions.. 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++"]);; |
| 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 |
| |
| 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 >] |
| | 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 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 |
| |
| (* 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 |
| |
| (* 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 *) |
| 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 |
| |
| | [< >] -> raise (Stream.Error "unknown token when expecting an expression.") |
| |
| (* 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_primary 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 |
| |
| (* expression |
| * ::= primary binoprhs *) |
| and parse_expr = parser |
| | [< lhs=parse_primary; stream >] -> parse_bin_rhs 0 lhs stream |
| |
| (* prototype |
| * ::= id '(' id* ')' *) |
| let parse_prototype = |
| let rec parse_args accumulator = parser |
| | [< 'Token.Ident id; e=parse_args (id::accumulator) >] -> e |
| | [< >] -> accumulator |
| 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)) |
| |
| | [< >] -> |
| 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 |
| |
| let rec codegen_expr = function |
| | Ast.Number n -> const_float double_type n |
| | Ast.Variable name -> |
| (try Hashtbl.find named_values name with |
| | Not_found -> raise (Error "unknown variable name")) |
| | Ast.Binary (op, lhs, rhs) -> |
| 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 |
| | _ -> raise (Error "invalid binary operator") |
| 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) -> |
| (* Emit the start code first, without 'variable' in scope. *) |
| let start_val = codegen_expr start in |
| |
| (* Make the new basic block for the loop header, inserting after current |
| * block. *) |
| let preheader_bb = insertion_block builder in |
| let the_function = block_parent preheader_bb in |
| 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; |
| |
| (* Start the PHI node with an entry for start. *) |
| let variable = build_phi [(start_val, preheader_bb)] var_name builder in |
| |
| (* 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 variable; |
| |
| (* 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 |
| |
| let next_var = build_add variable step_val "nextvar" builder in |
| |
| (* Compute the end condition. *) |
| let end_cond = codegen_expr end_ in |
| |
| (* 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 loop_end_bb = insertion_block builder in |
| 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; |
| |
| (* Add a new entry to the PHI node for the backedge. *) |
| add_incoming (next_var, loop_end_bb) variable; |
| |
| (* 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 |
| |
| let codegen_proto = function |
| | Ast.Prototype (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 |
| |
| let codegen_func the_fpm = function |
| | Ast.Function (proto, body) -> |
| Hashtbl.clear named_values; |
| let the_function = codegen_proto proto in |
| |
| (* Create a new basic block to start insertion into. *) |
| let bb = append_block context "entry" the_function in |
| position_at_end bb builder; |
| |
| try |
| 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 '<' 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; |
| |
| (* 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; |
| } |
| |
| `Next: Extending the language: user-defined |
| operators <OCamlLangImpl6.html>`_ |
| |
