compiler lectures

1. 1
Compiler Construction
(CS-636)
Muhammad Bilal Bashir
UIIT, Rawalpindi

2. Outline
1. Bottom-Up Parsing
2. LR(0) Items
3. Finite Automata of LR(0) Items
4. LR(0) Parsing
5. LR(0) Parsing Table
6. Summary
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3. Bottom-Up Parsing
Lecture: 15-16
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4. Bottom-Up Parsing
 A bottom-up parser uses an explicit stack to perform
a parse, similar to LL(1) parser
 The parsing stack will contain both tokens and
nonterminals and also some extra state information
 The stack is empty at the beginning of a bottom-up
parse and will contain the start symbol at the end of
a successful parse
 A bottom-up parse has two possible actions;
1. Shift a terminal from front of the input to top of the stack
2. Reduce a string α at the top of the stack to a nonterminal
A, given the BNF choice A → α
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5. Bottom-Up Parsing (Continue…)
 A bottom-up parser is sometimes called a shift-
reduce parser
 Shift actions are indicated by writing the word shift
 Reduce actions are indicated by writing the word
reduce and giving the BNF choice used in reduction
 One further feature of bottom-up parsing is that
grammars are always augmented with a new start
symbol
 Suppose S was the start symbol of the grammar;
S/
→ S
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6. Bottom-Up Parsing (Continue…)
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 Grammar: S’ → S
S → ( S ) S | ε
 Input: ()
No Parsing Stack Input Action
1 $ ( ) $ Shift
2 $ ( ) $ reduce S → ε
3 $ ( S ) $ shift
4 $ ( S ) $ reduce S → ε
5 $ ( S ) S $ reduce S → ( S ) S
6 $ S $ reduce S’ → S
7 $ S’ $ accept

7. Bottom-Up Parsing (Continue…)
 A bottom-up parser can shift input symbols onto the
stack until it determines what action to perform
 The parser may need to look deeper into the stack then just
the top in order to determine what action to perform
 Thus bottom-up parsing requires arbitrary ‘stack
lookahead’, which is not as serious as input
lookahead because parser builds the stack and can
arrange for the appropriate information to be
available
 The mechanism that will do this is a deterministic
finite automaton of “items”
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8. LR(0) Items
 The LR(0) item (or just item) of a CFG is a
production choice with a distinguished position on its
right hand side
 We use period “.” symbol to indicate distinguished position
 If A → α is a production choice, and if β and γ are
any two strings of symbols (including ε) such that βγ
= α, then A → β.γ is an LR(0) item
 Example:
 Write down LR(0) items for the following grammar:
S’ → S
S → ( S ) S | ε
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9. Finite Automata of LR(0) Items
 In first pass a NFA is generated for LR(0) items
 In second pass NFA is converted into DFA of LR(0)
items
 States in the DFA are numbered
 The transitions in symbols can be terminals,
nonterminals, and ε (only in NFA)
 There is an initial state
 There is one or more accepting states depending
upon the grammar
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10. Construction of NFA &
DFA of LR(0) Items
S’ → S
S → ( S ) S | ε
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11. LR(0) Parsing
 LR(0) is a bottom-up parsing algorithm
 The “L” indicates the input will be processed from left to
right
 The “R” indicates rightmost derivation is produced
 The “0” means no lookahead token is used
 LR(0) uses DFA of items to perform parsing
 The basic actions remain the same including shift
and reduce
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12. LR(0) Parsing (Continue…)
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 Grammar: S’ → S
S → ( S ) S | ε
 Input: ()
No Parsing Stack Input Action
1 $ 0 ( ) $ shift
2 $ 0 ( 2 ) $ reduce S → ε
3 $ 0 ( 2 S 3 ) $ shift
4 $ 0 ( 2 S 3 ) 4 $ reduce S → ε
5 $ 0 ( 2 S 3 ) 4 S 5 $ reduce S → ( S ) S
6 $ 0 S 1 $ accept

13. LR(0) Parsing Table
 A LR(0) parsing table can be constructed against
the DFA of items of a given grammar with columns
 State
 Action
 Rule
 Input
 Goto
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14. 14
Summary
Any Questions?