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COMPAILER DESIGN 10CS63 VTU NOTES UNIT-4
UNIT IV: SYNTAX ANALYSIS 3 SYLLABUS:
* Introduction to LR Parsing:
* Simple LR;
* More powerful LR parsers (excluding Efficient construction and compaction of
parsing tables) ;
* Using ambiguous grammars;
* Parser Generators.
Constructing Canonical LR(1) Parsing Tables
In SLR method, the state i makes a reduction by A???? when the current token is
a:
if the A????. in the Ii and a is FOLLOW(A)
In some situations, *A cannot be followed by the terminal a in a rightsentential
form when ???? and the state i are on the top stack. This means
that making reduction in this case is not correct.
S * AaAb S*AaAb*Aab*ab S*BbBa*Bba*ba
S * BbBa
A * * Aab * * ab Bba * * ba
B * * AaAb * Aa * b BbBa * Bb * a
LR(1) Item
To avoid some of invalid reductions, the states need to carry more information.
Extra information is put into a state by including a terminal symbol as a second
component in an item.
A LR(1) item is:
A * ??.??,a where a is the look-head of the LR(1) item
(a is a terminal or end-marker.)
When * ( in the LR(1) item A * ??.??,a ) is not empty, the look-head does not
have any affect.
When * is empty (A * ??.,a ), we do the reduction by A???? only if the next
input symbol is a (not for any terminal in FOLLOW(A)).
A state will contain A * ??.,a1 where {a1,...,an} * FOLLOW(A)
...
A * ??.,an
Canonical Collection of Sets of LR(1) Items
The construction of the canonical collection of the sets of LR(1) items are similar
to the construction of the canonical collection of the sets of LR(0) items, except
that closure and goto operations work a little bit different.
closure(I) is: ( where I is a set of LR(1) items)
every LR(1) item in I is in closure(I)
if A????.B??,a in closure(I) and B???? is a production rule of G; then
B??.??,b will be in the closure(I) for each terminal b in FIRST(??a) .
goto operation
If I is a set of LR(1) items and X is a grammar symbol (terminal or non-terminal),
then goto(I,X) is defined as follows:
If A * ??.X??,a in I then
every item in closure({A * *X.??,a}) will be in goto(I,X).
Construction of The Canonical LR(1) Collection
Algorithm:
C is { closure({S??.S,$}) }
repeat the followings until no more set of LR(1) items can be added to C.
for each I in C and each grammar symbol X
if goto(I,X) is not empty and not in C
add goto(I,X) to C
goto function is a DFA on the sets in C.
A Short Notation for The Sets of LR(1) Items
A set of LR(1) items containing the following items
A * ??.??,a1
...
A * ??.??,an
can be written as
A * ??.??,a1/a2/.../an
SLR(1) Parsing table
id * = $ S L R
0 s5 s4 1 2 3
1 acc
2 s6/r5 r5
3 r2
4 s5 s4 8 7
5 r4 r4
6 s5 s4 10 9
7 r3 r3
8 r5 r5
9 r1
Canonical LR(1) Collection Example2
S * S
1) S *L=R
2) S *R
3) L* *R
4) L * id
5) R * L
I0:S ??.S,$
S * .L=R,$
S * .R,$
L * .*R,$/=
L * .id,$/=
R ??.L,$
I1:S *S.,$
I2:S * L.=R,$
R *L.,$
I3:S * R.,$
I4:L ??*.R,$/=
R ??.L,$/=
L* .*R,$/=
L * .id,$/=
I5:L *id.,$/=
I6:S * L=.R,$
R ??.L,$
L * .*R,$
L * .id,$
I7:L ??*R.,$/=
I8: R * L.,$/=
I9:S * L=R.,$
I10:R *L.,$
I11:L ??*.R,$
R ??.L,$
L* .*R,$
L * .id,$
I12:L *id.,$
I13:L ??*R.,$
to I6
to I7
to I8
to I4
to I5
to I10
to I11
to I12
to I9
to I10
to I11
to I12
to I13
id
S
L
L
L
R
R
R
id
id
id
R
L
*
*
*
*
I4 and I11
I5 and I12
I7 and I13
I8 and I10
Construction of LR(1) Parsing Tables
1. Construct the canonical collection of sets of LR(1) items for G. C??{I0,...,In}
2. Create the parsing action table as follows
If a is a terminal, A????.a??,b in Ii and goto(Ii,a)=Ij then action[i,a] is
shift j.
If A????.,a is in Ii , then action[i,a] is reduce A???? where A*S.
If S*S.,$ is in Ii , then action[i,$] is accept.
If any conflicting actions generated by these rules, the grammar is not
LR(1).
3. Create the parsing goto table
for all non-terminals A, if goto(Ii,A)=Ij then goto[i,A]=j
4. All entries not defined by (2) and (3) are errors.
Initial state of the parser contains S??.S,$
LR(1) Parsing Tables (for Example2)
id * = $ S L R
0 s5 s4 1 2 3
1 acc
2 s6 r5
3 r2
4 s5 s4 8 7
5 r4 r4
6 s12 s11 10 9
7 r3 r3
8 r5 r5
9 r1
10 r5
11 s12 s11 10 13
12 r4
13 r3
no shift/reduce or
no reduce/reduce conflict
*
so, it is a LR(1) grammar
LALR Parsing Tables
LALR stands for LookAhead LR.
LALR parsers are often used in practice because LALR parsing tables are smaller
than LR(1) parsing tables.
The number of states in SLR and LALR parsing tables for a grammar G are equal.
But LALR parsers recognize more grammars than SLR parsers.
yacc creates a LALR parser for the given grammar.
A state of LALR parser will be again a set of LR(1) items.
Creating LALR Parsing Tables
Canonical LR(1) Parser LALR Parser
shrink # of states
This shrink process may introduce a reduce/reduce conflict in the resulting
LALR parser (so the grammar is NOT LALR)
But, this shrik process does not produce a shift/reduce conflict.
The Core of A Set of LR(1) Items
The core of a set of LR(1) items is the set of its first component.
Ex: S * L.=R,$ S * L.=R Core
R * L.,$ R * L.
We will find the states (sets of LR(1) items) in a canonical LR(1) parser with
same cores. Then we will merge them as a single state.
I1:L * id.,= A new state: I12: L * id.,=
L * id.,$
I2:L * id.,$ have same core, merge them
We will do this for all states of a canonical LR(1) parser to get the states of the
LALR parser.
In fact, the number of the states of the LALR parser for a grammar will be equal
to the number of states of the SLR parser for that grammar.
Creation of LALR Parsing Tables
Create the canonical LR(1) collection of the sets of LR(1) items for the given
grammar.
Find each core; find all sets having that same core; replace those sets having same
cores with a single set which is their union.
C={I0,...,In} C={J1,...,Jm} where m * n
Create the parsing tables (action and goto tables) same as the construction of the
parsing tables of LR(1) parser.
Note that: If J=I1 * ... * Ik since I1,...,Ik have same cores
cores of goto(I1,X),...,goto(I2,X) must be same.
So, goto(J,X)=K where K is the union of all sets of items having same
cores as goto(I1,X).
If no conflict is introduced, the grammar is LALR(1) grammar. (We may
only introduce reduce/reduce conflicts; we cannot introduce a shift/reduce
conflict)
Shift/Reduce Conflict
We say that we cannot introduce a shift/reduce conflict during the shrink process
for the creation of the states of a LALR parser.
Assume that we can introduce a shift/reduce conflict. In this case, a state of LALR
parser must have:
A * ??.,a and B * ??.a??,b
This means that a state of the canonical LR(1) parser must have:
A * ??.,a and B * ??.a??,c
But, this state has also a shift/reduce conflict. i.e. The original canonical LR(1)
parser has a conflict.
(Reason for this, the shift operation does not depend on lookaheads)
Reduce/Reduce Conflict
But, we may introduce a reduce/reduce conflict during the shrink process for the
creation of the states of a LALR parser.
I1 : A * ??.,a I2: A * ??.,b
B * ??.,b B * ??.,c
*
I12: A * ??.,a/b reduce/reduce conflict
B * ??.,b/c
Canonical LALR(1) Collection Example2
S * S
1) S *L=R
2) S *R
3) L* *R
4) L * id
5) R * L
I0:S ??.S,$
S * .L=R,$
S * .R,$
L * .*R,$/=
L * .id,$/=
R ??.L,$
I1:S *S.,$
I2:S * L.=R,$
R *L.,$
I3:S * R.,$
I411:L ??*.R,$/=
R * .L,$/=
L* .*R,$/=
L * .id,$/=
I512:L *id.,$/=
I6:S * L=.R,$
R ??.L,$
L * .*R,$
L * .id,$
I713:L ??*R.,$/=
I810: R * L.,$/=
I9:S * L=R.,$
to I6
to I713
to I810
to I411
to I512
to I810
to I411
to I512
to I9
S
L
L
L
R
R
id
id
id
R
*
*
*
Same Cores
I4 and I11
I5 and I12
I7 and I13
I8 and I10
Using Ambiguous Grammars
All grammars used in the construction of LR-parsing tables must be un-
ambiguous.
Can we create LR-parsing tables for ambiguous grammars
Yes, but they will have conflicts.
We can resolve these conflicts in favor of one of them to disambiguate the
grammar.
At the end, we will have again an unambiguous grammar.
Why we want to use an ambiguous grammar
Some of the ambiguous grammars are much natural, and a corresponding
unambiguous grammar can be very complex.
Usage of an ambiguous grammar may eliminate unnecessary reductions.
Ex.
E * E+T | T
E * E+E | E*E | (E) | id T * T*F | F
F * (E) | id
Sets of LR(0) Items for Ambiguous Grammar
I0: E ??.E
E * .E+E
E * .E*E
E * .(E)
E * .id
I1: E *E.
E * E .+E
E * E .*E
I2: E * (.E)
E * .E+E
E * .E*E
E * .(E)
E * .id
I3: E * id.
I4: E * E +.E
E * .E+E
E * .E*E
E * .(E)
E * .id
I5: E * E *.E
E * .E+E
E * .E*E
E * .(E)
E * .id
I6: E * (E.)
E * E.+E
E * E.*E
I7: E * E+E.
E * E.+E
E * E.*E
I8: E * E*E.
E * E.+E
E * E.*E
I9: E * (E).
I5
)
E
E
E
E
*
+
+
+
+
*
*
*
(
(
(
(
id
id
id
id
I4
I2
I2
I3
I3
I4
I4
I5
I5
SLR-Parsing Tables for Ambiguous Grammar
FOLLOW(E) = { $,+,*,) }
State I8 has shift/reduce conflicts for symbols + and *.
I0 I1 I5 I7
E * E
when current token is *
shift ?* is right-associative
reduce ?* is left-associative
when current token is +
shift ?+ has higher precedence than *
reduce ?* has higher precedence than +
SLR-Parsing Tables for Ambiguous Grammar
id + * ( ) $ E
0 s3 s2 1
1 s4 s5 acc
2 s3 s2 6
3 r4 r4 r4 r4
4 s3 s2 7
5 s3 s2 8
6 s4 s5 s9
7 r1 s5 r1 r1
8 r2 r2 r2 r2
9 r3 r3 r3 r3
Action Goto
Error Recovery in LR Parsing
An LR parser will detect an error when it consults the parsing action
table and finds an error entry. All empty entries in the action table are
error entries.
Errors are never detected by consulting the goto table.
An LR parser will announce error as soon as there is no valid
continuation for the scanned portion of the input.
A canonical LR parser (LR(1) parser) will never make even a single
reduction before announcing an error.
The SLR and LALR parsers may make several reductions before
announcing an error.
But, all LR parsers (LR(1), LALR and SLR parsers) will never shift an
erroneous input symbol onto the stack.
Panic Mode Error Recovery in LR Parsing
Scan down the stack until a state s with a goto on a particular
nonterminal A is found. (Get rid of everything from the stack before this
state s).
Discard zero or more input symbols until a symbol a is found that can
legitimately follow A.
The symbol a is simply in FOLLOW(A), but this may not work for all situations.
The parser stacks the nonterminal A and the state goto[s,A], and it
resumes the normal parsing.
This nonterminal A is normally is a basic programming block (there can
be more than one choice for A).
stmt, expr, block, ...
Phrase-Level Error Recovery in LR Parsing
Each empty entry in the action table is marked with a specific error routine.
An error routine reflects the error that the user most likely will make in that case.
An error routine inserts the symbols into the stack or the input (or it deletes the
symbols from the stack and the input, or it can do both insertion and deletion).
missing operand
unbalanced right parenthesis
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