Prof. Roy Levow

COP 4620 / 5625Programming Language Translation /

Compiler WritingFall 2003

Lecture 5, 09/25/2003

Prof. Roy Levow

LL(1) Parser Generation

• Roadmap1. LL(1) parsing

2. LL(1) conflicts as an asset

3. LL(1) conflicts as a liability

4. The LL(1) push-down automaton

5. LL(1) error handling

6. LL gen

LL(1) Parsing

• To determine choice of production, we use FIRST and FOLLOW sets

• FIRST set– Contains tokens that can occur as the start of

a production or the tail of a production

• FOLLOW set– Contains tokens that can follow a nullable

prefix in a production

FOLLOW Set

FIRST & FOLLOW Setsfor Sample Grammar

LL(1) Conflicts

• First/First conflict– Two alternatives share some first choice

• First/Follow conflict– There is a non-null alternative that shares a

choice with the follow for a nullable alternative

• Two nullable alternatives• We do not consider Follow/Follow conflicts

since they will be seen also as one of the other conflict cases

Conditions to be LL(1)

• No First/First conflicts

• No First/Follow conflicts

• No multiple nullable alternatives

• If we can remove all conflicts, we may be able to produce a LL(1) grammar from one that is not

Making a Grammar LL(1)

• Left-factoring– Factor out the common prefix into a separate

production– Example:

term -> IDENT | IDENT ‘[‘ expr ‘]’ | …

Becomes

term -> IDENT (‘[‘ expr ‘]’ )? | …

Making a Grammar LL(1).2

• Substitution– Replace non-terminal on RHS with its – Example (indirect First/First conflict)

term -> IDENT | indexed | …

indexed -> IDENT ‘[‘ expr ‘]’

Substitute and then left factor

Making a Grammar LL(1).3

• Left Recursion Removal– Direct: a -> a b– Indirect: a -> b c; b -> a d– Hidden: a -> c a; c -> ε

• In many cases, left recursion can be replaced by right recursion either by using a standard transformation or by studying the construct and developing an alternative

Correcting Semantic Effects

• Restructuring a grammar may change the implied semantics, interfering with translation

• Can sometimes resolve the problem by adding “marker rules” that are transformed along with the original grammar– See p.131

Automatic Conflict Resolution

• Always choose first alternative

• Use LL(2), look ahead an extra token

• Dynamic resolution– Add decision rule to first alternative– Example

else-tail-option -> %if (1) ‘else’ stmt | ε

Table Driven LL(1) Parsing

• Uses a push-down automaton– Finite State Automaton with a stack– Start with start symbol on stack– When processing a non-terminal

• Pop non-terminal• Push selected RHS in reverse order

– When processing a terminal• Match with input

Table Driven LL(1) Parsing.2

LL(1) PDA Operation

LL(1) Error Handling

• Concerns– Avoid corrupt syntax tree– Avoid infinite loops

• Possible approaches– Insert needed token

• May loop

– Skip input until match– Pop items from stack until match

• May produce corrupt syntax tree

LL(1) Error Handling.2

• Acceptable set method is a combination of skipping input and popping items from stack– Discard input until acceptable continuation

token is found– Resynchronize parser based on token found– Acceptable sets are restart points for each

production• Set used is union for pending productions

LLgen

• Parser generator from– Amsterdam Compiler Kit

• Tanenbaum et al., 1983