SyntaxDirected Translation

1
Syntax-Directed Translation
Section 2.3 Chapter 5

• Attributes
• Semantic Rules
• Evaluation Order

Token Stream
Parser
Intermediate Code
Well associate code with productions to create
an intermediate code.
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Syntax-directed Translation
Syntax-directed Translation is done by attaching
rules or program fragments to productions in a
grammar.
Well use subscripts to disambiguate.
E ? E1 T compute E1 compute T Es value is
E1s value Ts value
Note that the parse tree would have E1 and T
under this production
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Attributes
Attributes are values associated with terminals
or non-terminals. The most basic attribute The
value of a terminal.
e ? e t t t ? t f f f ? ( e ) id num
f.val is the value of the symbol or value of the
number for f ? id num f.val is the value of the
computed expression for f ? ( e )
We can name attributes whatever we want. Some
names are common .lexval for the string from
lexical analysis.
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Syntax-directed definition (SDD)
A style for specifying the computation of an
attributed grammar.
The subscripts allow us to differentiate
occurrences of the same symbol in a production.
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Visualizing an SDDs translation
3 5
e
e1
t
t
f
f
num
num
A parse tree - No big surprise here.
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Annotated parse tree
3 5
e.val 8
e1.val 3
t.val 5
t.val 3
f.val 5
f.val 3
num.lexval 5
num.lexval 3
We added the annotations to the parse tree.
TT
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Question
3 5
e.val 8
e1.val 3
t.val 5
t.val 3
f.val 5
f.val 3
num.lexval 5
num.lexval 3
In what order should this parse tree be evaluated?
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One possible order (are there others?)
3 5
e.val 8
e1.val 3
t.val 5
t.val 3
f.val 5
f.val 3
num.lexval 5
num.lexval 3
Order is pretty easy for this particular parse
tree.
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How about this grammar?
D ? T L T ? int T ? float L ? L1 , id L ? id
What does this grammar do?
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How about this grammar?
D ? T L T ? int T ? float L ? L1 , id L ? id
What will the semantic rules be for each
production?
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How about this grammar?
D ? T L T ? int T ? float L ? L1 , id L ? id
Where do we get the type from?
What will the semantic rules be for each
production?
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Types of attributes
Synthesized attributes Computed based only on
the node and its children. Inherited attributes
Computed by the parent of a node.
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Inherited attribute example
D ? T L T ? int T ? float L ? L1 , id L ? id
Inherited attribute
Note that the rule L ? id is dependent on its
parent.
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What about order of evaluation now?
D ? T L T ? int T ? float L ? L1 , id L ? id
float x
D
D.inh float
T
L
L.inh float
T.type float
float
id
float
createVariable(id.lexval, float)
Parse tree
Annotated parse tree
What is a valid evaluation order for this tree?
TT
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Order of evaluation
D ? T L T ? int T ? float L ? L1 , id L ? id
float x
D.inh float
L.inh float
T.type float
float
createVariable(id.lexval, float)
How can we determine this?
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Inherited attribute example
D ? T L T ? int T ? float L ? L1 , id L ? id
Inherited attribute
TT
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Dependency Graphs
float x
D.inh float
L.inh float
T.type float
float
createVariable(id.lexval, float)
What is dependent on what here?
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Dependency Graphs
float x
D.inh float
L.inh float
T.type float
float
createVariable(id.lexval, float)
How do I determine the dependencies?
The arrow is the order the nodes must be
evaluated.
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Dependency Graphs
float x
D.inh float
L.inh float
T.type float
float
createVariable(id.lexval, float)
Heres how it works.
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Dependencies
If we use a childs synthesized attribute, were
dependent on the child. If we use a parents
inherited attribute, were dependent on the
parent.
These should be pretty obvious
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What about this case?
A
B
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What about this case?
A
B
There are some syntax directed definitions for
which no evaluation order exists.
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Classes of SDDs

• An SDD is S-attributed if every attribute is
  synthesized.
• An SDD is L-attributed if each attribute is
  either
• 1. Synthesized
• Inherited but with this limiting rule Suppose
  there is a production A ? X1X2...Xn and there is
  an inherited attribute Xi.a computed by a rule
  associated with the production. Then the rule
  may only use
• a) Inherited attributes associated with the head
  A.
• b) Inherited or synthesized attributes to the
  left of Xi
• c) Inherited or synthesized attributes
  associated with Xi itself, but only if no cycles
  in the dependency graph are formed.

TT
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Building Syntax Trees
Suppose I want to build a syntax tree? How would
I go about doing it?
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Building Syntax Trees
TT
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Moving these ideas to Lex/Yacc
/ Lex file for expression.y / include
ltstdio.hgt include "y.tab.h" 0-9
yylval atoi(yytext) return NUM \t
\n return '\n' . return
yytext0 int yywrap() return 1
This should look relatively familiar
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Yacc
/ Expression evaluator / include
"stdio.h" define YYSTYPE int token
NUM S E printf("answer d\n", 1) S
S '\n' S S '\n' E printf("answer2
d\n", 3) E E '' T 1 3 E T
1 T T '' NUM 1 3 T
NUM 1
Production
Semantic Rule
What does this grammar do (why the S rules?)