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diff --git a/docs/tutorial/LangImpl7.html b/docs/tutorial/LangImpl7.html deleted file mode 100644 index 8fa99b1903..0000000000 --- a/docs/tutorial/LangImpl7.html +++ /dev/null @@ -1,2164 +0,0 @@ -<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" - "http://www.w3.org/TR/html4/strict.dtd"> - -<html> -<head> - <title>Kaleidoscope: Extending the Language: Mutable Variables / SSA - construction</title> - <meta http-equiv="Content-Type" content="text/html; charset=utf-8"> - <meta name="author" content="Chris Lattner"> - <link rel="stylesheet" href="../_static/llvm.css" type="text/css"> -</head> - -<body> - -<h1>Kaleidoscope: Extending the Language: Mutable Variables</h1> - -<ul> -<li><a href="index.html">Up to Tutorial Index</a></li> -<li>Chapter 7 - <ol> - <li><a href="#intro">Chapter 7 Introduction</a></li> - <li><a href="#why">Why is this a hard problem?</a></li> - <li><a href="#memory">Memory in LLVM</a></li> - <li><a href="#kalvars">Mutable Variables in Kaleidoscope</a></li> - <li><a href="#adjustments">Adjusting Existing Variables for - Mutation</a></li> - <li><a href="#assignment">New Assignment Operator</a></li> - <li><a href="#localvars">User-defined Local Variables</a></li> - <li><a href="#code">Full Code Listing</a></li> - </ol> -</li> -<li><a href="LangImpl8.html">Chapter 8</a>: Conclusion and other useful LLVM - tidbits</li> -</ul> - -<div class="doc_author"> - <p>Written by <a href="mailto:sabre@nondot.org">Chris Lattner</a></p> -</div> - -<!-- *********************************************************************** --> -<h2><a name="intro">Chapter 7 Introduction</a></h2> -<!-- *********************************************************************** --> - -<div> - -<p>Welcome to Chapter 7 of the "<a href="index.html">Implementing a language -with LLVM</a>" tutorial. In chapters 1 through 6, we've built a very -respectable, albeit simple, <a -href="http://en.wikipedia.org/wiki/Functional_programming">functional -programming language</a>. In our journey, we learned some parsing techniques, -how to build and represent an AST, how to build LLVM IR, and how to optimize -the resultant code as well as JIT compile it.</p> - -<p>While Kaleidoscope is interesting as a functional language, the fact that it -is functional makes it "too easy" to generate LLVM IR for it. In particular, a -functional language makes it very easy to build LLVM IR directly in <a -href="http://en.wikipedia.org/wiki/Static_single_assignment_form">SSA form</a>. -Since LLVM requires that the input code be in SSA form, this is a very nice -property and it is often unclear to newcomers how to generate code for an -imperative language with mutable variables.</p> - -<p>The short (and happy) summary of this chapter is that there is no need for -your front-end to build SSA form: LLVM provides highly tuned and well tested -support for this, though the way it works is a bit unexpected for some.</p> - -</div> - -<!-- *********************************************************************** --> -<h2><a name="why">Why is this a hard problem?</a></h2> -<!-- *********************************************************************** --> - -<div> - -<p> -To understand why mutable variables cause complexities in SSA construction, -consider this extremely simple C example: -</p> - -<div class="doc_code"> -<pre> -int G, H; -int test(_Bool Condition) { - int X; - if (Condition) - X = G; - else - X = H; - return X; -} -</pre> -</div> - -<p>In this case, we have the variable "X", whose value depends on the path -executed in the program. Because there are two different possible values for X -before the return instruction, a PHI node is inserted to merge the two values. -The LLVM IR that we want for this example looks like this:</p> - -<div class="doc_code"> -<pre> -@G = weak global i32 0 ; type of @G is i32* -@H = weak global i32 0 ; type of @H is i32* - -define i32 @test(i1 %Condition) { -entry: - br i1 %Condition, label %cond_true, label %cond_false - -cond_true: - %X.0 = load i32* @G - br label %cond_next - -cond_false: - %X.1 = load i32* @H - br label %cond_next - -cond_next: - %X.2 = phi i32 [ %X.1, %cond_false ], [ %X.0, %cond_true ] - ret i32 %X.2 -} -</pre> -</div> - -<p>In this example, the loads from the G and H global variables are explicit in -the LLVM IR, and they live in the then/else branches of the if statement -(cond_true/cond_false). In order to merge the incoming values, the X.2 phi node -in the cond_next block selects the right value to use based on where control -flow is coming from: if control flow comes from the cond_false block, X.2 gets -the value of X.1. Alternatively, if control flow comes from cond_true, it gets -the value of X.0. The intent of this chapter is not to explain the details of -SSA form. For more information, see one of the many <a -href="http://en.wikipedia.org/wiki/Static_single_assignment_form">online -references</a>.</p> - -<p>The question for this article is "who places the phi nodes when lowering -assignments to mutable variables?". The issue here is that LLVM -<em>requires</em> that its IR be in SSA form: there is no "non-ssa" mode for it. -However, SSA construction requires non-trivial algorithms and data structures, -so it is inconvenient and wasteful for every front-end to have to reproduce this -logic.</p> - -</div> - -<!-- *********************************************************************** --> -<h2><a name="memory">Memory in LLVM</a></h2> -<!-- *********************************************************************** --> - -<div> - -<p>The 'trick' here is that while LLVM does require all register values to be -in SSA form, it does not require (or permit) memory objects to be in SSA form. -In the example above, note that the loads from G and H are direct accesses to -G and H: they are not renamed or versioned. This differs from some other -compiler systems, which do try to version memory objects. In LLVM, instead of -encoding dataflow analysis of memory into the LLVM IR, it is handled with <a -href="../WritingAnLLVMPass.html">Analysis Passes</a> which are computed on -demand.</p> - -<p> -With this in mind, the high-level idea is that we want to make a stack variable -(which lives in memory, because it is on the stack) for each mutable object in -a function. To take advantage of this trick, we need to talk about how LLVM -represents stack variables. -</p> - -<p>In LLVM, all memory accesses are explicit with load/store instructions, and -it is carefully designed not to have (or need) an "address-of" operator. Notice -how the type of the @G/@H global variables is actually "i32*" even though the -variable is defined as "i32". What this means is that @G defines <em>space</em> -for an i32 in the global data area, but its <em>name</em> actually refers to the -address for that space. Stack variables work the same way, except that instead of -being declared with global variable definitions, they are declared with the -<a href="../LangRef.html#i_alloca">LLVM alloca instruction</a>:</p> - -<div class="doc_code"> -<pre> -define i32 @example() { -entry: - %X = alloca i32 ; type of %X is i32*. - ... - %tmp = load i32* %X ; load the stack value %X from the stack. - %tmp2 = add i32 %tmp, 1 ; increment it - store i32 %tmp2, i32* %X ; store it back - ... -</pre> -</div> - -<p>This code shows an example of how you can declare and manipulate a stack -variable in the LLVM IR. Stack memory allocated with the alloca instruction is -fully general: you can pass the address of the stack slot to functions, you can -store it in other variables, etc. In our example above, we could rewrite the -example to use the alloca technique to avoid using a PHI node:</p> - -<div class="doc_code"> -<pre> -@G = weak global i32 0 ; type of @G is i32* -@H = weak global i32 0 ; type of @H is i32* - -define i32 @test(i1 %Condition) { -entry: - %X = alloca i32 ; type of %X is i32*. - br i1 %Condition, label %cond_true, label %cond_false - -cond_true: - %X.0 = load i32* @G - store i32 %X.0, i32* %X ; Update X - br label %cond_next - -cond_false: - %X.1 = load i32* @H - store i32 %X.1, i32* %X ; Update X - br label %cond_next - -cond_next: - %X.2 = load i32* %X ; Read X - ret i32 %X.2 -} -</pre> -</div> - -<p>With this, we have discovered a way to handle arbitrary mutable variables -without the need to create Phi nodes at all:</p> - -<ol> -<li>Each mutable variable becomes a stack allocation.</li> -<li>Each read of the variable becomes a load from the stack.</li> -<li>Each update of the variable becomes a store to the stack.</li> -<li>Taking the address of a variable just uses the stack address directly.</li> -</ol> - -<p>While this solution has solved our immediate problem, it introduced another -one: we have now apparently introduced a lot of stack traffic for very simple -and common operations, a major performance problem. Fortunately for us, the -LLVM optimizer has a highly-tuned optimization pass named "mem2reg" that handles -this case, promoting allocas like this into SSA registers, inserting Phi nodes -as appropriate. If you run this example through the pass, for example, you'll -get:</p> - -<div class="doc_code"> -<pre> -$ <b>llvm-as < example.ll | opt -mem2reg | llvm-dis</b> -@G = weak global i32 0 -@H = weak global i32 0 - -define i32 @test(i1 %Condition) { -entry: - br i1 %Condition, label %cond_true, label %cond_false - -cond_true: - %X.0 = load i32* @G - br label %cond_next - -cond_false: - %X.1 = load i32* @H - br label %cond_next - -cond_next: - %X.01 = phi i32 [ %X.1, %cond_false ], [ %X.0, %cond_true ] - ret i32 %X.01 -} -</pre> -</div> - -<p>The mem2reg pass implements the standard "iterated dominance frontier" -algorithm for constructing SSA form and has a number of optimizations that speed -up (very common) degenerate cases. The mem2reg optimization pass is the answer to dealing -with mutable variables, and we highly recommend that you depend on it. Note that -mem2reg only works on variables in certain circumstances:</p> - -<ol> -<li>mem2reg is alloca-driven: it looks for allocas and if it can handle them, it -promotes them. It does not apply to global variables or heap allocations.</li> - -<li>mem2reg only looks for alloca instructions in the entry block of the -function. Being in the entry block guarantees that the alloca is only executed -once, which makes analysis simpler.</li> - -<li>mem2reg only promotes allocas whose uses are direct loads and stores. If -the address of the stack object is passed to a function, or if any funny pointer -arithmetic is involved, the alloca will not be promoted.</li> - -<li>mem2reg only works on allocas of <a -href="../LangRef.html#t_classifications">first class</a> -values (such as pointers, scalars and vectors), and only if the array size -of the allocation is 1 (or missing in the .ll file). mem2reg is not capable of -promoting structs or arrays to registers. Note that the "scalarrepl" pass is -more powerful and can promote structs, "unions", and arrays in many cases.</li> - -</ol> - -<p> -All of these properties are easy to satisfy for most imperative languages, and -we'll illustrate it below with Kaleidoscope. The final question you may be -asking is: should I bother with this nonsense for my front-end? Wouldn't it be -better if I just did SSA construction directly, avoiding use of the mem2reg -optimization pass? In short, we strongly recommend that you use this technique -for building SSA form, unless there is an extremely good reason not to. Using -this technique is:</p> - -<ul> -<li>Proven and well tested: llvm-gcc and clang both use this technique for local -mutable variables. As such, the most common clients of LLVM are using this to -handle a bulk of their variables. You can be sure that bugs are found fast and -fixed early.</li> - -<li>Extremely Fast: mem2reg has a number of special cases that make it fast in -common cases as well as fully general. For example, it has fast-paths for -variables that are only used in a single block, variables that only have one -assignment point, good heuristics to avoid insertion of unneeded phi nodes, etc. -</li> - -<li>Needed for debug info generation: <a href="../SourceLevelDebugging.html"> -Debug information in LLVM</a> relies on having the address of the variable -exposed so that debug info can be attached to it. This technique dovetails -very naturally with this style of debug info.</li> -</ul> - -<p>If nothing else, this makes it much easier to get your front-end up and -running, and is very simple to implement. Lets extend Kaleidoscope with mutable -variables now! -</p> - -</div> - -<!-- *********************************************************************** --> -<h2><a name="kalvars">Mutable Variables in Kaleidoscope</a></h2> -<!-- *********************************************************************** --> - -<div> - -<p>Now that we know the sort of problem we want to tackle, lets see what this -looks like in the context of our little Kaleidoscope language. We're going to -add two features:</p> - -<ol> -<li>The ability to mutate variables with the '=' operator.</li> -<li>The ability to define new variables.</li> -</ol> - -<p>While the first item is really what this is about, we only have variables -for incoming arguments as well as for induction variables, and redefining those only -goes so far :). Also, the ability to define new variables is a -useful thing regardless of whether you will be mutating them. Here's a -motivating example that shows how we could use these:</p> - -<div class="doc_code"> -<pre> -# Define ':' for sequencing: as a low-precedence operator that ignores operands -# and just returns the RHS. -def binary : 1 (x y) y; - -# Recursive fib, we could do this before. -def fib(x) - if (x < 3) then - 1 - else - fib(x-1)+fib(x-2); - -# Iterative fib. -def fibi(x) - <b>var a = 1, b = 1, c in</b> - (for i = 3, i < x in - <b>c = a + b</b> : - <b>a = b</b> : - <b>b = c</b>) : - b; - -# Call it. -fibi(10); -</pre> -</div> - -<p> -In order to mutate variables, we have to change our existing variables to use -the "alloca trick". Once we have that, we'll add our new operator, then extend -Kaleidoscope to support new variable definitions. -</p> - -</div> - -<!-- *********************************************************************** --> -<h2><a name="adjustments">Adjusting Existing Variables for Mutation</a></h2> -<!-- *********************************************************************** --> - -<div> - -<p> -The symbol table in Kaleidoscope is managed at code generation time by the -'<tt>NamedValues</tt>' map. This map currently keeps track of the LLVM "Value*" -that holds the double value for the named variable. In order to support -mutation, we need to change this slightly, so that it <tt>NamedValues</tt> holds -the <em>memory location</em> of the variable in question. Note that this -change is a refactoring: it changes the structure of the code, but does not -(by itself) change the behavior of the compiler. All of these changes are -isolated in the Kaleidoscope code generator.</p> - -<p> -At this point in Kaleidoscope's development, it only supports variables for two -things: incoming arguments to functions and the induction variable of 'for' -loops. For consistency, we'll allow mutation of these variables in addition to -other user-defined variables. This means that these will both need memory -locations. -</p> - -<p>To start our transformation of Kaleidoscope, we'll change the NamedValues -map so that it maps to AllocaInst* instead of Value*. Once we do this, the C++ -compiler will tell us what parts of the code we need to update:</p> - -<div class="doc_code"> -<pre> -static std::map<std::string, AllocaInst*> NamedValues; -</pre> -</div> - -<p>Also, since we will need to create these alloca's, we'll use a helper -function that ensures that the allocas are created in the entry block of the -function:</p> - -<div class="doc_code"> -<pre> -/// CreateEntryBlockAlloca - Create an alloca instruction in the entry block of -/// the function. This is used for mutable variables etc. -static AllocaInst *CreateEntryBlockAlloca(Function *TheFunction, - const std::string &VarName) { - IRBuilder<> TmpB(&TheFunction->getEntryBlock(), - TheFunction->getEntryBlock().begin()); - return TmpB.CreateAlloca(Type::getDoubleTy(getGlobalContext()), 0, - VarName.c_str()); -} -</pre> -</div> - -<p>This funny looking code creates an IRBuilder object that is pointing at -the first instruction (.begin()) of the entry block. It then creates an alloca -with the expected name and returns it. Because all values in Kaleidoscope are -doubles, there is no need to pass in a type to use.</p> - -<p>With this in place, the first functionality change we want to make is to -variable references. In our new scheme, variables live on the stack, so code -generating a reference to them actually needs to produce a load from the stack -slot:</p> - -<div class="doc_code"> -<pre> -Value *VariableExprAST::Codegen() { - // Look this variable up in the function. - Value *V = NamedValues[Name]; - if (V == 0) return ErrorV("Unknown variable name"); - - <b>// Load the value. - return Builder.CreateLoad(V, Name.c_str());</b> -} -</pre> -</div> - -<p>As you can see, this is pretty straightforward. Now we need to update the -things that define the variables to set up the alloca. We'll start with -<tt>ForExprAST::Codegen</tt> (see the <a href="#code">full code listing</a> for -the unabridged code):</p> - -<div class="doc_code"> -<pre> - Function *TheFunction = Builder.GetInsertBlock()->getParent(); - - <b>// Create an alloca for the variable in the entry block. - AllocaInst *Alloca = CreateEntryBlockAlloca(TheFunction, VarName);</b> - - // Emit the start code first, without 'variable' in scope. - Value *StartVal = Start->Codegen(); - if (StartVal == 0) return 0; - - <b>// Store the value into the alloca. - Builder.CreateStore(StartVal, Alloca);</b> - ... - - // Compute the end condition. - Value *EndCond = End->Codegen(); - if (EndCond == 0) return EndCond; - - <b>// Reload, increment, and restore the alloca. This handles the case where - // the body of the loop mutates the variable. - Value *CurVar = Builder.CreateLoad(Alloca); - Value *NextVar = Builder.CreateFAdd(CurVar, StepVal, "nextvar"); - Builder.CreateStore(NextVar, Alloca);</b> - ... -</pre> -</div> - -<p>This code is virtually identical to the code <a -href="LangImpl5.html#forcodegen">before we allowed mutable variables</a>. The -big difference is that we no longer have to construct a PHI node, and we use -load/store to access the variable as needed.</p> - -<p>To support mutable argument variables, we need to also make allocas for them. -The code for this is also pretty simple:</p> - -<div class="doc_code"> -<pre> -/// CreateArgumentAllocas - Create an alloca for each argument and register the -/// argument in the symbol table so that references to it will succeed. -void PrototypeAST::CreateArgumentAllocas(Function *F) { - Function::arg_iterator AI = F->arg_begin(); - for (unsigned Idx = 0, e = Args.size(); Idx != e; ++Idx, ++AI) { - // Create an alloca for this variable. - AllocaInst *Alloca = CreateEntryBlockAlloca(F, Args[Idx]); - - // Store the initial value into the alloca. - Builder.CreateStore(AI, Alloca); - - // Add arguments to variable symbol table. - NamedValues[Args[Idx]] = Alloca; - } -} -</pre> -</div> - -<p>For each argument, we make an alloca, store the input value to the function -into the alloca, and register the alloca as the memory location for the -argument. This method gets invoked by <tt>FunctionAST::Codegen</tt> right after -it sets up the entry block for the function.</p> - -<p>The final missing piece is adding the mem2reg pass, which allows us to get -good codegen once again:</p> - -<div class="doc_code"> -<pre> - // Set up the optimizer pipeline. Start with registering info about how the - // target lays out data structures. - OurFPM.add(new DataLayout(*TheExecutionEngine->getDataLayout())); - <b>// Promote allocas to registers. - OurFPM.add(createPromoteMemoryToRegisterPass());</b> - // Do simple "peephole" optimizations and bit-twiddling optzns. - OurFPM.add(createInstructionCombiningPass()); - // Reassociate expressions. - OurFPM.add(createReassociatePass()); -</pre> -</div> - -<p>It is interesting to see what the code looks like before and after the -mem2reg optimization runs. For example, this is the before/after code for our -recursive fib function. Before the optimization:</p> - -<div class="doc_code"> -<pre> -define double @fib(double %x) { -entry: - <b>%x1 = alloca double - store double %x, double* %x1 - %x2 = load double* %x1</b> - %cmptmp = fcmp ult double %x2, 3.000000e+00 - %booltmp = uitofp i1 %cmptmp to double - %ifcond = fcmp one double %booltmp, 0.000000e+00 - br i1 %ifcond, label %then, label %else - -then: ; preds = %entry - br label %ifcont - -else: ; preds = %entry - <b>%x3 = load double* %x1</b> - %subtmp = fsub double %x3, 1.000000e+00 - %calltmp = call double @fib(double %subtmp) - <b>%x4 = load double* %x1</b> - %subtmp5 = fsub double %x4, 2.000000e+00 - %calltmp6 = call double @fib(double %subtmp5) - %addtmp = fadd double %calltmp, %calltmp6 - br label %ifcont - -ifcont: ; preds = %else, %then - %iftmp = phi double [ 1.000000e+00, %then ], [ %addtmp, %else ] - ret double %iftmp -} -</pre> -</div> - -<p>Here there is only one variable (x, the input argument) but you can still -see the extremely simple-minded code generation strategy we are using. In the -entry block, an alloca is created, and the initial input value is stored into -it. Each reference to the variable does a reload from the stack. Also, note -that we didn't modify the if/then/else expression, so it still inserts a PHI -node. While we could make an alloca for it, it is actually easier to create a -PHI node for it, so we still just make the PHI.</p> - -<p>Here is the code after the mem2reg pass runs:</p> - -<div class="doc_code"> -<pre> -define double @fib(double %x) { -entry: - %cmptmp = fcmp ult double <b>%x</b>, 3.000000e+00 - %booltmp = uitofp i1 %cmptmp to double - %ifcond = fcmp one double %booltmp, 0.000000e+00 - br i1 %ifcond, label %then, label %else - -then: - br label %ifcont - -else: - %subtmp = fsub double <b>%x</b>, 1.000000e+00 - %calltmp = call double @fib(double %subtmp) - %subtmp5 = fsub double <b>%x</b>, 2.000000e+00 - %calltmp6 = call double @fib(double %subtmp5) - %addtmp = fadd double %calltmp, %calltmp6 - br label %ifcont - -ifcont: ; preds = %else, %then - %iftmp = phi double [ 1.000000e+00, %then ], [ %addtmp, %else ] - ret double %iftmp -} -</pre> -</div> - -<p>This is a trivial case for mem2reg, since there are no redefinitions of the -variable. The point of showing this is to calm your tension about inserting -such blatent inefficiencies :).</p> - -<p>After the rest of the optimizers run, we get:</p> - -<div class="doc_code"> -<pre> -define double @fib(double %x) { -entry: - %cmptmp = fcmp ult double %x, 3.000000e+00 - %booltmp = uitofp i1 %cmptmp to double - %ifcond = fcmp ueq double %booltmp, 0.000000e+00 - br i1 %ifcond, label %else, label %ifcont - -else: - %subtmp = fsub double %x, 1.000000e+00 - %calltmp = call double @fib(double %subtmp) - %subtmp5 = fsub double %x, 2.000000e+00 - %calltmp6 = call double @fib(double %subtmp5) - %addtmp = fadd double %calltmp, %calltmp6 - ret double %addtmp - -ifcont: - ret double 1.000000e+00 -} -</pre> -</div> - -<p>Here we see that the simplifycfg pass decided to clone the return instruction -into the end of the 'else' block. This allowed it to eliminate some branches -and the PHI node.</p> - -<p>Now that all symbol table references are updated to use stack variables, -we'll add the assignment operator.</p> - -</div> - -<!-- *********************************************************************** --> -<h2><a name="assignment">New Assignment Operator</a></h2> -<!-- *********************************************************************** --> - -<div> - -<p>With our current framework, adding a new assignment operator is really -simple. We will parse it just like any other binary operator, but handle it -internally (instead of allowing the user to define it). The first step is to -set a precedence:</p> - -<div class="doc_code"> -<pre> - int main() { - // Install standard binary operators. - // 1 is lowest precedence. - <b>BinopPrecedence['='] = 2;</b> - BinopPrecedence['<'] = 10; - BinopPrecedence['+'] = 20; - BinopPrecedence['-'] = 20; -</pre> -</div> - -<p>Now that the parser knows the precedence of the binary operator, it takes -care of all the parsing and AST generation. We just need to implement codegen -for the assignment operator. This looks like:</p> - -<div class="doc_code"> -<pre> -Value *BinaryExprAST::Codegen() { - // Special case '=' because we don't want to emit the LHS as an expression. - if (Op == '=') { - // Assignment requires the LHS to be an identifier. - VariableExprAST *LHSE = dynamic_cast<VariableExprAST*>(LHS); - if (!LHSE) - return ErrorV("destination of '=' must be a variable"); -</pre> -</div> - -<p>Unlike the rest of the binary operators, our assignment operator doesn't -follow the "emit LHS, emit RHS, do computation" model. As such, it is handled -as a special case before the other binary operators are handled. The other -strange thing is that it requires the LHS to be a variable. It is invalid to -have "(x+1) = expr" - only things like "x = expr" are allowed. -</p> - -<div class="doc_code"> -<pre> - // Codegen the RHS. - Value *Val = RHS->Codegen(); - if (Val == 0) return 0; - - // Look up the name. - Value *Variable = NamedValues[LHSE->getName()]; - if (Variable == 0) return ErrorV("Unknown variable name"); - - Builder.CreateStore(Val, Variable); - return Val; - } - ... -</pre> -</div> - -<p>Once we have the variable, codegen'ing the assignment is straightforward: -we emit the RHS of the assignment, create a store, and return the computed -value. Returning a value allows for chained assignments like "X = (Y = Z)".</p> - -<p>Now that we have an assignment operator, we can mutate loop variables and -arguments. For example, we can now run code like this:</p> - -<div class="doc_code"> -<pre> -# Function to print a double. -extern printd(x); - -# Define ':' for sequencing: as a low-precedence operator that ignores operands -# and just returns the RHS. -def binary : 1 (x y) y; - -def test(x) - printd(x) : - x = 4 : - printd(x); - -test(123); -</pre> -</div> - -<p>When run, this example prints "123" and then "4", showing that we did -actually mutate the value! Okay, we have now officially implemented our goal: -getting this to work requires SSA construction in the general case. However, -to be really useful, we want the ability to define our own local variables, lets -add this next! -</p> - -</div> - -<!-- *********************************************************************** --> -<h2><a name="localvars">User-defined Local Variables</a></h2> -<!-- *********************************************************************** --> - -<div> - -<p>Adding var/in is just like any other other extensions we made to -Kaleidoscope: we extend the lexer, the parser, the AST and the code generator. -The first step for adding our new 'var/in' construct is to extend the lexer. -As before, this is pretty trivial, the code looks like this:</p> - -<div class="doc_code"> -<pre> -enum Token { - ... - <b>// var definition - tok_var = -13</b> -... -} -... -static int gettok() { -... - if (IdentifierStr == "in") return tok_in; - if (IdentifierStr == "binary") return tok_binary; - if (IdentifierStr == "unary") return tok_unary; - <b>if (IdentifierStr == "var") return tok_var;</b> - return tok_identifier; -... -</pre> -</div> - -<p>The next step is to define the AST node that we will construct. For var/in, -it looks like this:</p> - -<div class="doc_code"> -<pre> -/// VarExprAST - Expression class for var/in -class VarExprAST : public ExprAST { - std::vector<std::pair<std::string, ExprAST*> > VarNames; - ExprAST *Body; -public: - VarExprAST(const std::vector<std::pair<std::string, ExprAST*> > &varnames, - ExprAST *body) - : VarNames(varnames), Body(body) {} - - virtual Value *Codegen(); -}; -</pre> -</div> - -<p>var/in allows a list of names to be defined all at once, and each name can -optionally have an initializer value. As such, we capture this information in -the VarNames vector. Also, var/in has a body, this body is allowed to access -the variables defined by the var/in.</p> - -<p>With this in place, we can define the parser pieces. The first thing we do is add -it as a primary expression:</p> - -<div class="doc_code"> -<pre> -/// primary -/// ::= identifierexpr -/// ::= numberexpr -/// ::= parenexpr -/// ::= ifexpr -/// ::= forexpr -<b>/// ::= varexpr</b> -static ExprAST *ParsePrimary() { - switch (CurTok) { - default: return Error("unknown token when expecting an expression"); - case tok_identifier: return ParseIdentifierExpr(); - case tok_number: return ParseNumberExpr(); - case '(': return ParseParenExpr(); - case tok_if: return ParseIfExpr(); - case tok_for: return ParseForExpr(); - <b>case tok_var: return ParseVarExpr();</b> - } -} -</pre> -</div> - -<p>Next we define ParseVarExpr:</p> - -<div class="doc_code"> -<pre> -/// varexpr ::= 'var' identifier ('=' expression)? -// (',' identifier ('=' expression)?)* 'in' expression -static ExprAST *ParseVarExpr() { - getNextToken(); // eat the var. - - std::vector<std::pair<std::string, ExprAST*> > VarNames; - - // At least one variable name is required. - if (CurTok != tok_identifier) - return Error("expected identifier after var"); -</pre> -</div> - -<p>The first part of this code parses the list of identifier/expr pairs into the -local <tt>VarNames</tt> vector. - -<div class="doc_code"> -<pre> - while (1) { - std::string Name = IdentifierStr; - getNextToken(); // eat identifier. - - // Read the optional initializer. - ExprAST *Init = 0; - if (CurTok == '=') { - getNextToken(); // eat the '='. - - Init = ParseExpression(); - if (Init == 0) return 0; - } - - VarNames.push_back(std::make_pair(Name, Init)); - - // End of var list, exit loop. - if (CurTok != ',') break; - getNextToken(); // eat the ','. - - if (CurTok != tok_identifier) - return Error("expected identifier list after var"); - } -</pre> -</div> - -<p>Once all the variables are parsed, we then parse the body and create the -AST node:</p> - -<div class="doc_code"> -<pre> - // At this point, we have to have 'in'. - if (CurTok != tok_in) - return Error("expected 'in' keyword after 'var'"); - getNextToken(); // eat 'in'. - - ExprAST *Body = ParseExpression(); - if (Body == 0) return 0; - - return new VarExprAST(VarNames, Body); -} -</pre> -</div> - -<p>Now that we can parse and represent the code, we need to support emission of -LLVM IR for it. This code starts out with:</p> - -<div class="doc_code"> -<pre> -Value *VarExprAST::Codegen() { - std::vector<AllocaInst *> OldBindings; - - Function *TheFunction = Builder.GetInsertBlock()->getParent(); - - // Register all variables and emit their initializer. - for (unsigned i = 0, e = VarNames.size(); i != e; ++i) { - const std::string &VarName = VarNames[i].first; - ExprAST *Init = VarNames[i].second; -</pre> -</div> - -<p>Basically it loops over all the variables, installing them one at a time. -For each variable we put into the symbol table, we remember the previous value -that we replace in OldBindings.</p> - -<div class="doc_code"> -<pre> - // Emit the initializer before adding the variable to scope, this prevents - // the initializer from referencing the variable itself, and permits stuff - // like this: - // var a = 1 in - // var a = a in ... # refers to outer 'a'. - Value *InitVal; - if (Init) { - InitVal = Init->Codegen(); - if (InitVal == 0) return 0; - } else { // If not specified, use 0.0. - InitVal = ConstantFP::get(getGlobalContext(), APFloat(0.0)); - } - - AllocaInst *Alloca = CreateEntryBlockAlloca(TheFunction, VarName); - Builder.CreateStore(InitVal, Alloca); - - // Remember the old variable binding so that we can restore the binding when - // we unrecurse. - OldBindings.push_back(NamedValues[VarName]); - - // Remember this binding. - NamedValues[VarName] = Alloca; - } -</pre> -</div> - -<p>There are more comments here than code. The basic idea is that we emit the -initializer, create the alloca, then update the symbol table to point to it. -Once all the variables are installed in the symbol table, we evaluate the body -of the var/in expression:</p> - -<div class="doc_code"> -<pre> - // Codegen the body, now that all vars are in scope. - Value *BodyVal = Body->Codegen(); - if (BodyVal == 0) return 0; -</pre> -</div> - -<p>Finally, before returning, we restore the previous variable bindings:</p> - -<div class="doc_code"> -<pre> - // Pop all our variables from scope. - for (unsigned i = 0, e = VarNames.size(); i != e; ++i) - NamedValues[VarNames[i].first] = OldBindings[i]; - - // Return the body computation. - return BodyVal; -} -</pre> -</div> - -<p>The end result of all of this is that we get properly scoped variable -definitions, and we even (trivially) allow mutation of them :).</p> - -<p>With this, we completed what we set out to do. Our nice iterative fib -example from the intro compiles and runs just fine. The mem2reg pass optimizes -all of our stack variables into SSA registers, inserting PHI nodes where needed, -and our front-end remains simple: no "iterated dominance frontier" computation -anywhere in sight.</p> - -</div> - -<!-- *********************************************************************** --> -<h2><a name="code">Full Code Listing</a></h2> -<!-- *********************************************************************** --> - -<div> - -<p> -Here is the complete code listing for our running example, enhanced with mutable -variables and var/in support. To build this example, use: -</p> - -<div class="doc_code"> -<pre> -# Compile -clang++ -g toy.cpp `llvm-config --cppflags --ldflags --libs core jit native` -O3 -o toy -# Run -./toy -</pre> -< |