Compilers

Start Lecture #7

The DFA for our Main Example

Our main example

    E' → E
    E  → E + T | T
    T  → T * F | F
    F  → ( E ) | id
  

Start constructing the diagram on the board:
Begin with {E' → ·E}, take the closure, and then keep applying GOTO.

Use of the LR(0) Automaton

A state of the automaton is an item set as described previously. The transition function is GOTO. If during a parse we are up to item set I_j (often called state s_j or simply state j) and the next input symbol is b (it of course must be a terminal), and state j has an outgoing transition labeled b, then the parser shifts b to the stack. Actually, although b is removed from the input string, what is pushed on the stack is k, the destination state of the transition labeled b.

If there is no such transition, then the parser performs a reduction; choosing which reduction to use is determined by the items in I_j and the FOLLOW sets. (It is also possible that the parser will now accept the input string or announce that the input string is not in the language).

SLR Parsing Using the Diagram and FOLLOW

Naturally, programs can't use diagrams so we will construct a table, but it is instructive to see how the LR(0) diagram plus the FOLLOW sets we have see before are enough for SLR parsing.

The diagram and FOLLOW sets (plus the production list and the ACTION/GOTO table) can be see here. For now just use the diagram and FOLLOW.

Take a legal input, e.g. id+id*id, and shift whenever the diagram says you can and, when you can't, reduce using the unique completed item having the input symbol in FOLLOW. Don't forget that reducing means backing up the number of grammar symbols in the RHS and then going forward using the arc labeled with the LHS.

4.6.3: The LR-Parsing Algorithm

The LR-parsing algorithm must decide when to shift and when to reduce (and in the latter case, by which production). It does this by consulting two tables, ACTION and GOTO. The basic algorithm is the same for all LR parsers, what changes are the tables ACTION and GOTO.

The LR Parsing Tables

We have already seen GOTO (for SLR).

Remark: Technical point that may, and probably should, be ignored: our GOTO was defined on pairs [item-set,grammar-symbol]. The new GOTO is defined on pairs [state,nonterminal]. A state is simply an item set (so nothing is new here). We will not use the new GOTO on terminals so we just define it on nonterminals.

Given a state i and a terminal a (or the endmarker), ACTION[i,a] can be

1. Shift j. The terminal a is shifted on to the stack and the parser enters state j.
2. Reduce A → α. The parser reduces α on the TOS to A.
3. Accept.
4. Error

So ACTION is the key to deciding shift vs. reduce. We will soon see how this table is computed for SLR.

Since ACTION is defined on [state,terminal] pairs and GOTO is defined on [state,nonterminal] pairs, we can combine these tables into one defined on [state,grammar-symbol] pairs.

LR-Parser Configurations (formalism)

This formalism is useful for stating the actions of the parser precisely, but I believe the parser can be explained without this formalism. The essential idea of the formalism is that the entire state of the parser can be represented by the vector of states on the stack and input symbols not yet processed.

Behavior of the LR Parser

The parser consults the combined ACTION-GOTO table for its current state (TOS) and next input symbol, formally this is ACTION[s_m,a_i], and proceeds based on the value in the table. If the action is a shift, the next state is clear from the DFA We have done this informally just above; here we use the formal treatment).

1. Shift s. The input symbol a is pushed and s becomes the new state. The new configuration is
   (s₀...s_ms,a_i+1...a_n)
2. Reduce A → α. Let r be the number of symbols in the RHS of the production. The parser pops r items off the stack (backing up r states) and enters the state GOTO(s_m-r,A). That is after backing up it goes where A says to go. A real parser would now probably do something, e.g., build a tree node or perform a semantic action. Although we know about this from the chapter 2 overview, we don't officially know about it here. So for now think of it simply printing the production the parser reduced by.
3. Accept.
4. Error.

4.6.4 Constructing SLR-Parsing Tables

The missing piece of the puzzle is finally revealed.

A Simple (but fake) Example

Before defining the ACTION and GOTO tables precisely, I want to do it informally via the simple example on the right. This is a FAKE example, the B'→·B item would be in I₀ and would have only B→·a+b with it.

IMPORTANT: To construct the table, you do need something not in the diagram, namely the FOLLOW sets from top-down parsing.

For convenience number the productions of the grammar to make them easy to reference and assume that the production B → a+b is numbered 2. Also assume FOLLOW(B)={b} and all other follow sets are empty. Again, I am not claiming that there is a grammar with this diagram and these FOLLOW sets.

Show how to calculate every entry in the table using the diagram and the FOLLOW sets. Then consider input a+b and see how the table guides to eventually accepting the a+b string.

The Action-GOTO Table

This table is the SLR equivalent of the predictive-parsing table we constructed for top-down parsing.

The action table is defined with states (item sets) as rows and terminals and the $ endmarker as columns. GOTO has the same rows, but has nonterminals as columns. So we construct a combined ACTION-GOTO table, with states as rows and grammar symbols (terminals + nonterminals) plus $ as columns.

1. Each arc in the diagram labeled with a terminal indicates a shift. In the entry with row the state at the tail of the arc and column the labeling terminal place sn, where n is the state at the head of the arc. This indicates that if we are in the given state and the input is the given terminal, we shift to new state n.
2. Each arc in the diagram labeled with a nonterminal informs us what state to enter if we reduce. In the entry with row the state at the tail of the arc and column the labeling nonterminal place n, where n is the state at the head of the arc.
3. Each completed item (dot at the extreme right) indicates a possible reduction. In each entry with row the state containing the completed item and column a terminal in the FOLLOW set of the LHS of the production corresponding to this item, place rn, where n is the number of the production. (In particular, at entry [13,b] place an r2.).
4. In the entry with row (i.e., state) containing S'→S and column $, place accept.
5. If any entry is labelled twice (i.e., a conflict) the grammar is not SLR(1).
6. Any unlabeled entry corresponds to an input error. If the parser accesses this entry, the input sentence is not in the language generated by the grammar.

The Table Construction Algorithm

The diagrams above are constructed for our use; they are not used in constructing a bottom-up parser. Here is the real algorithm, which uses just the augmented grammar (i.e., after adding S' → S) and the FOLLOW sets.

1. Construct the LR(0) automaton with states {I₀,...,I_n} and transition function GOTO.
2. The parsing actions for state i.
   1. If A→α·bβ is in I_i for b a terminal, then ACTION[i,b]=shift j, where GOTO(I_i,b)=I_j.
   2. If A→α· is in I_i, for A≠S', then, for all b in FOLLOW(A), ACTION[i,b]=reduce A→α.
   3. If S'→S· is in I_i, then ACTION[I,$]=accept.
   4. If any conflicts occurred, the grammar is not SLR(1).
3. If GOTO(I_i,A)=I_j, for a nonterminal A, then GOTO[i,A]=j.
4. All entries not yet defined are error.
5. The initial state is the one containing S'→·S.

The Main Example

Our main example, pictured on the right, gives the table shown on the left. The productions and FOLLOW sets are shown as well (the FIRST sets are not used directly, but are needed to calculate FOLLOW). The table entry s5 abbreviates shift and go to state 5. The table entry r2 abbreviates reduce by production number 2, where we have numbered the productions as shown.

The shift actions can be read directly off the DFA. For example I1 with a + goes to I6, I6 with an id goes to I5, and I9 with a * goes to I7.

The reduce actions require FOLLOW, which for this simple grammar is fairly easy to calculate.

Consider I₅={F→id·}. Since the dot is at the end, we are ready to reduce, but we must check if the next symbol can follow the F we are reducing to. Since FOLLOW(F)={+,*,),$}, in row 5 (for I5) we put r6 (for reduce by production 6) in the columns for +, *, ), and $.

The GOTO columns can also be read directly off the DFA. Since there is an E-transition (arc labeled E) from I₀ to I₁, the column labeled E in row 0 contains a 1.

Since the column labeled + is blank for row 7, we see that it would be an error if we arrived in state 7 when the next input character is +.

Finally, if we are in state 1 when the input is exhausted ($ is the next input character), then we have a successfully parsed the input.

Stack	Symbols	Input	Action
0		id*id+id$	shift
05	id	*id+id$	reduce by F→id
03	F	*id+id$	reduct by T→id
02	T	*id+id$	shift
027	T*	id+id$	shift
0275	T*id	+id$	reduce by F→id
027 10	T*F	+id$	reduce by T→T*F
02	T	+id$	reduce by E→T
01	E	+id$	shift
016	E+	id$	shift
0165	E+id	$	reduce by F→id
0163	E+F	$	reduce by T→F
0169	E+T	$	reduce by E→E+T
01	E	$	accept

Example: The diagram on the right shows the actions when SLR is parsing id*id+id. On the blackboard let's do id+id*id and see how the precedence is handled.

Homework: 2 (you already constructed the LR(0) automaton for this example in the previous homework), 3, 4 (this problem refers to 4.2.2(a-g); only use 4.2.2(a-c).

Example: What about ε-productions? Let's do

    A → B D
    B → b B | ε
    D → d
  

Reducing by the ε-production actually adds a state to the stack (it pops ZERO states since there are zero symbols on RHS and pushes one).

Homework: Do the following extension

    A → B D
    B → b B | ε
    D → d D | ε
  

4.7: More Powerful LR Parsers

We consider very briefly two alternatives to SLR, canonical-LR or LR, and lookahead-LR or LALR.

4.7.1: Canonical LR(1) Items

SLR used the LR(0) items, that is the items used were productions with an embedded dot, but contained no other (lookahead) information. The LR(1) items contain the same productions with embedded dots, but add a second component, which is a terminal (or $). This second component becomes important only when the dot is at the extreme right (indicating that a reduction can be made if the input symbol is in the appropriate FOLLOW set). For LR(1) we do that reduction only if the input symbol is exactly the second component of the item. This finer control of when to perform reductions, enables the parsing of a larger class of languages.

4.7.4: Constructing LALR Parsing Tables

For LALR we merge various LR(1) item sets together, obtaining nearly the LR(0) item sets we used in SLR. LR(1) items have two components, the first, called the core, is a production with a dot; the second a terminal. For LALR we merge all the item sets that have the same set of cores by combining the 2nd components of items with like cores (thus permitting reductions when any of these terminals is the next input symbol). Thus we obtain the same number of states (item sets) as in SLR since only the cores distinguish item sets.

Unlike SLR, we limit reductions to occurring only for certain specified input symbols. LR(1) gives finer control; it is possible for the LALR merger to have reduce-reduce conflicts when the LR(1) items on which it is based is conflict free.

Although these conflicts are possible, they are rare and the size reduction from LR(1) to LALR is quite large. LALR is the current method of choice for bottom-up, shift-reduce parsing.

4.9: Parser Generators

4.9.1: The Parser Generator Yacc

The tool corresponding to Lex for parsing is yacc, which (at least originally) stood for yet another compiler compiler. This name is cute but somewhat misleading since yacc (like the previous compiler compilers) does not produce a complete compiler, just a parser.

The structure of the yacc user input is similar to that for lex, but instead of regular definitions, one includes productions with semantic actions.

There are ways to specify associativity and precedence of operators. It is not done with multiple grammar symbols as in a pure parser, but more like declarations.

Use of Yacc requires a serious session with its manual.

Chapter 5: Syntax-Directed Translation

Homework: Read Chapter 5.

Again we are redoing, more formally and completely, things we briefly discussed when breezing over chapter 2.

Recall that a syntax-directed definition (SDD) adds semantic rules to the productions of a grammar. For example to the production T → T / F we might add the rule

    T.code = T1.code || F.code || '/'
  

Rather than constantly copying ever larger strings to finally output at the root of the tree after a depth first traversal, we can perform the output incrementally by embedding semantic actions within the productions themselves. The above example becomes

    T → T1 / F { print '/' }
  

Since we are generating postfix, the action comes at the end (after we have generated the subtrees for T₁ and F, and hence performed their actions). In general the actions occur within the production, not necessarily after the last symbol.

For SDD's we conceptually need to have the entire tree available after the parse so that we can run the postorder traversal. (It was postorder for the infix→postfix translator since we had a simple S-attributed SDD. We will traverse the parse tree in other orders when the SDD is not S-attributed, and will see situations when no traversal order is possible.)

In some situations semantic actions can be performed during the parse, without saving the tree. However, this does not occur for large commercial compilers since they make several passes over the tree for optimizations. We will not make use of this possibility either; your lab 3 (parser) will produce a tree and your lab 4 (semantic analyzer / intermediate code generator) will read this tree and perform the semantic rules specified by the SDD.

5.1: Syntax-Directed Definitions (SDDs)

Formally, attributes are values (of any type) that are associated with grammar symbols. Write X.a for the attribute a of symbol X. You can think of attributes as fields in a record/struct/object.

Semantic rules (rules for short) are associated with productions and give formulas to calculate attributes of nonterminals in the production.

5.1.1: Inherited and Synthesized Attributes

Terminals can have synthesized attributes (not yet officially defined); these are given to it by the lexer (not the parser). For example the token id might well have the attribute lexeme, where id.lexeme is the lexeme that the lexer converted into this instance of id. There are no rules in an SDD giving attribute values to terminals. Terminals do not have inherited attributes (to be defined shortly).

A nonterminal B can have both inherited and synthesized attributes. The difference is how they are computed. In either case the computation is specified by a rule associated with the production at a node N of the parse tree.

Definition: A synthesized attribute of a nonterminal B is defined at a node N where B is the LHS of the production associated with N. The attribute can depend on only (synthesized or inherited) attribute values at the children of N (the RHS of the production) and on inherited attribute values at N itself.

The arithmetic division example above was synthesized.

Production	Semantic Rules
L → E $	L.val = E.val
E → E1 + T	E.val = E1.val + T.val
E → E1 - T	E.val = E1.val - T.val
E → T	E.val = T.val
T → T1 * F	T.val = T1.val * F.val
T → T1 / F	T.val = T1.val / F.val
T → F	T.val = F.val
F → ( E )	F.val = E.val
F → num	F.val = num.lexval

Example: The SDD at the right gives a left-recursive grammar for expressions with an extra nonterminal L added as the start symbol. The terminal num is given a value by the lexer, which corresponds to the value stored in the numbers table for lab 2.

Draw the parse tree for 7+6/3 on the board and verify that L.val is 9, the value of the expression.

This example uses only synthesized attributes.

Definition: An SDD with only synthesized attributes is called S-attributed.

For these SDDs all attributes depend only on attribute values at the children.
Why?
Answer: The other possibility (depending on values at the node itself) involves inherited attributes, which are absent in an S-attributed SDD.

Thus, for an S-attributed SDD, all the rules can be evaluated by a single bottom-up (i.e., postorder) traversal of the annotated parse tree.

Inherited attributes are more complicated since the node N of the parse tree with which the attribute is associated (and which is also the natural node to store the value) does not contain the production with the corresponding semantic rule.

Definition: An inherited attribute of a nonterminal B at node N, where B is the LHS of the production, is defined by a semantic rule of the production at the parent of N (where B occurs in the RHS of the production). The value depends only on attributes at N, N's siblings, and N's parent.

Note that when viewed from the parent node P (the site of the semantic rule), the inherited attribute depends on values at P and at P's children (the same as for synthesized attributes). However, and this is crucial, the nonterminal B is the LHS of a child of P and hence the attribute is naturally associated with that child. It is normally stored there and is shown there in the diagrams below.

We will soon see examples with inherited attributes.

Definition: Often the attributes are just evaluations without side effects. In such cases we call the SDD an attribute grammar.