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Structure of the Myrddin Compiler Aug 2012 Ori Bernstein TABLE OF CONTENTS: 1. OVERVIEW 1.1. Tree Structure 2. PARSING 2.1. Lexing 2.2. AST Creation 2.3. Type checking 2.4. Generic Specialization 2.5. Serialization 2.6. Usefiles 3. FLATTENING 3.1. Control Flow 3.2. Complex Expressions 4. OPTIMIZATION 4.1. Constant Folding 5. CODE GENERATION 5.1. Instruction Selection 5.2. Register Allocation 6. TUTORIAL: ADDING A STATEMENT 6.1. Stubbing in the node types 6.2. Parsing 6.3. Flattening 6.4. Optimization 6.5. Instruction Selection 1. OVERVIEW: The Myrddin compiler suite consists of a set of binaries, written in C, which translate Myrddin source code to the assembly most appropriate for the target platform, and subsequently invoke the native assembler on it. The linker is not invoked by the compiler, and the final output is an object file for the target platform. The compilers are named with a single character for the target platform, with a single character for the language being compiled. A table of the compilers and their names is below: Compiler Platform ------------------------- 6m x86-64 The compilation is divided into a small number of phases. The first phase is parsing, where the source code is first tokenized, the abstract syntax tree (AST) is generated, and semantically checked. The second phase is the machine-dependent tree flattening. In this phase, the tree is decomposed function by function into simple operations that are relatively close to the machine. Sizes are fixed, and all loops, if statements, etc. are replaced with gotos. The next phase is a machine-independent optimizer, which currenty does nothing other than simply folding trees. In the final phase, the instructions are selected and the registers are allocated. So, to recap, the phases are as follows: parse Tokenize, parse, and analyze the source flatten Rewrite the complex nodes into simpe ones opt Optimize the flattened source trees gen Generate the assembly code 1.1. Tree Structure: File nodes (n->type == Nfile) represent the files being compiled. The current node is held in a global variable called, unsurprisingly, 'file'. The global symbol table, export table, uses, and other compilation-specific information is stored in this node. This implies that the compiler can only process one file at a time. Name nodes (n->type == Nname) are simply names, possibly with a namespace attached. They are left as leaf nodes in the tree, specifying variable names, union tags, and just about anything else with a name. Use nodes (n->type == Nuse) simply tell the compiler that a usefile by the name stored in this node will be loaded. Expression nodes (n->type == Nexpr) represent expressions. They consist of an operator, a type, a few flags, possibly a declaration ID, and a list of arguments. Operators are defined in parse/ops.def, and start with an 'O' by convention; eg: Oadd, Osub, etc. The declaration id (n->expr.did) is only valid on expressions representing a single variable (n->expr.op == Ovar). The DID is a unique identifier representing the declaration node that the variable refers to. This is used for a variety of things, from fast comparisons to allowing us to put the node into a bit set easily. Literal nodes (n->type == Nlit) hold a literal value. The type held is stored in littype, which are defined in parse/lits.def. The various statement nodes (Nloopstmt, Nifstmt, Nmatchstmt, Nblock, Nlbl) are all statements that may appear within a block node (Nblock). Declaration nodes declare a name in a symbol table. TODO: MORE DETAIL. Uelt nodes declare a union element. TODO: MORE DETAIL. Func nodes declare a function. TODO: MORE DETAIL. 2. PARSING: This phase takes in a source file, and outputs a tree that is guaranteed to be valid. The tree nodes are defined in parse/parse.h in struct Node, and have one of the types defined in parse/nodetypes.def. Node types start with 'N' by convention; eg: Nfile, Nifstmt, etc. 2.1. Lexing: Lexing occurs in parse/tok.c. Because we want to use this lexer from within yacc, the entry point to this code is in 'yylex()'. As required by yacc, 'yylex()' returns an integer defining the token type, and sets the 'tok' member of yylval to the token that was taken from the input stream. In addition, to allow for better error messages, the global variable 'curtok' is set to the value of 'yylval.tok'. This allows yyerror to print the last token that was seen. The tokens that are allowable are generated by Yacc from the '%token' definitions in parse/gram.y, and are placed into the file 'parse/gram.h'. The lexer and parser code is the only code that depends on these token constants. The lexer is initalized through 'tokinit(char *file)'. This function will open the file passed in, read all the data from it in one go and set up the internal data for the tokenizer. The tokenizing is then done while the whole file is in memory, which means that this code will work poorly on files that are larger than the address space available to the compiler. If this is a problem, you deserve all the pain that is caused. The file data is stored in the three global variables 'fidx', 'fbuf', and 'fbufsz'. The actual tokenization happens through 'toknext()' and its callees, which operate on these data structures character by character, matching the values read, and shoving them into the 'Tok' data structure. 2.2. AST Creation: The parser used is a traditional Yacc-based parser. It is generated from the source in parse/gram.y. The starting production is 'file', which fills in a global 'file' tree node. This 'file' tree node must be initialized before yyparse() is called. 2.3. Type Checking: Type checking is done through unification of types. It's implemented in parse/infer.c. It proceeds through a simple unification algorithm, which is documented in lang.txt. As a result, only the internal details of this algorithm will be discussed here. The first step done is loading and resolving use files. This is deferred to the type checking phase for two reasons. First, we do not want to force tools to have all dependencies compiled if they use this parser, even though type full type checking is impossible until all usefiles are loaded. And second, this is when the information is actually needed. Next, the types declared in the package section are merged with the exported types, allowing us to start off with our type information as complete as possible, and making sure that the types of globals actually match up with the exported types. The next step is the actual type inference. We do a bottom-up walk of the tree, unifying types as we go. There are subtleties with the member operator, however. Because the '.' operator is used for both member lookups and namespace lookups, before we descend into a node that has operator Omemb, we need to check if it's a namespaced name, or an actual member reference. If it is a namespaced name, we replace the expression with an Ovar expression. This check happens in the 'checkns()' function. Second, because we need to know the LHS of a member expression before we can check if the RHS is valid, and we are not guaranteed to know this at the first time that we see it, the expression is assumed to be valid, and this asumption is verified in a post-processing pass. Casts are validated in a deferred manner similarly. Generic variables are added to a list of generic callsites to specialize when they are seen in as a leaf of an Ovar node. The type inference, to this point, has only built up a mapping of types. So, for example, if we were to have the inferred types for the following set of statements: var a var b var c a = b c = b + 1 We would have the mappings: $t0 -> $t1 $t1 -> $t2 $t2 -> int So, in the 'typesub()' function, we iterate over the entire tree, replacing every instance of a non-concrete type with the final mapped type. If a type does not map to a fully concrete type, this is where we flag an error. FIXME: DESCRIBE HOW YOU FIXED GENERICS ONCE YOU FIX GENERICS. 2.4. Generic Specialization: After type inference (well, technially, as the final step of it), we walk through the list of callsites that need instantiations of generics, and create a specialized generic instance for each of them. This specialization is done, unsurprisingly, in specialize.c, by the simple algorithm of cloning the entire tree that needs to be specialized, and walking over all nodes substituting the types that are replacing the type parameters. 2.5. Serialization: Trees of all sorts can be serialized and deserialized from files, as long as they are fully typed. Trees containing type variables (ie, uninferred types) cannot be serialized, as type variables cannot be deserialized meaningfully. The format for this is only documented in the source, and is a straighforward dump of the trees to memory. It is constantly shifting, and will not reliably work between compiler versions. 2.6. Usefiles: Usefiles are more or less files that consist of a single character tag that tells us what type of tree to deserialize. Because serialized trees are compiler version dependent, so are usefiles. 3. FLATTENING: This phase is invoked repeatedly on each top-level declaration that we want to generate code for. There is a good chance that this flattening phase should be made machine-independent, and passed as a parameter a machine description describing known integer and pointer sizes, among other machine attributes. However, for now, it is machine-dependent, and lives in 6/simp.c. The goal of flattening a tree is to take semantically involved constructs such as looping, and simplify things into something that is easy to generate code for, as well as something that is easier to analyze for optimization. 3.1. Control Flow: All if statements, loops, and other complex constructs are simplified to jumps and conditional jumps. Loops are generally simplified from something that would look like this: loop init cond inc body To something that would look like this: init jmp cond .loop: body inc .cond: cjmp cond .loop .end .end: Boolean expressions are simplified to a location to jump to, as described in section 8.4 of the Dragon book[1]. 3.2. Complex Expressions: Complex expressions such as copying types larger than a single machine word, pulling members out of structures, emulating multiplication and division for larger integers sizes, and similar operations are reduced to trees that are expressible in terms of simple machine operations. By the end of the simplification pass, the following operators should not be present in the trees: Obad Oret Opreinc Opostinc Opredec Opostdec Olor Oland Oaddeq Osubeq Omuleq Odiveq Omodeq Oboreq Obandeq Obxoreq Obsleq Obsreq Omemb Oslice Oidx Osize Numops Oucon Ouget Otup Oarr Oslbase Osllen Ocast 4. OPTIMIZATION: Currently, there is virtually no optimization done on the trees after flattening. The only optimization that is done is constant folding. 4.1. Constant Folding: Expressions with constant values are simplified algebraically. For example, the expression 'x*1' is simplified to 'x', '0/n' is simplified to '0', and so on. 5. CODE GENERATION: 5.1. Instruction Selection: Instruction selection is done via a simple handwritten bottom-up pass over the tree. Common patterns such as scaled or offset indexing are recognized by the patterns, but no attempts are made at finding an optimal tiling. 5.2. Register Allocation: Register allocation is done via the algorithm described in "Iterated Regster Coalescing" by Appel and George. As of the time of this writing, the register allocator does not yet implement overlapping register classes. This will be done as described in "A generalized algorithm for graph-coloring register allocation" by Smith, Ramsey, and Holloway. 6: TUTORIAL: ADDING A STATEMENT: 6.1. Stubbing in the node types: 6.2. Parsing: 6.3. Flattening: 6.4. Optimization: 6.5. Instruction Selection: [1] Aho, Sethi, Ullman: Compilers: Principles, Techniques, and Tools, 1988. ISBN 0-201-10088-6