Semantic Analysis

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Semantic Analysis

• Mooly Sagiv
• html//www.cs.tau.ac.il/msagiv/courses/wcc04.html
  

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Outline

• What is Semantic Analysis
• Why is it needed?
• Scopes and type checking for imperative languages
  (Chapter 6)
• Attribute grammars (Chapter 3)

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Semantic Analysis

• The meaning of the program
• Requirements related to the context in which a
  construct occurs
• Context sensitive requirements - cannot be
  specified using a context free grammar(Context
  handling)
• Requires complicated and unnatural context free
  grammars
• Guides subsequent phases

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Basic Compiler Phases
Source program (string)
Front-End
lexical analysis
Tokens
syntax analysis
Abstract syntax tree
semantic analysis
Back-End
Fin. Assembly
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Example Semantic Condition

• In C
• break statements can only occur inside switch or
  loop statements

6
Partial Grammar for C
Stm ? Exp
Stm ? if (Exp) Stm
StList ? StList Stm
Stm ? if (Exp) Stm else Stm
StList ? ?
Stm ? while (Exp) do Stm
Stm ? break
Stm? StList
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Refined Grammar for C
Stm?Exp
Stm ? if (Exp) Stm
StList ? StList Stm
Stm ? if (Exp) Stm else Stm
StList ? ?
Stm? while (Exp) do LStm
Stm? StList
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A Possible Abstract Syntax for C
typedef struct A_St_ A_St struct A_St enum
A_if, A_while, A_break, A_block, ... kind
A_pos pos union struct
A_Exp e A_St st1 A_St st2
if_st struct A_Exp e
A_St st while_st struct
A_St st1 A_St st2 block_st
... u
A_St A_IfStm(A_Exp, A_St, A_St) A_St
A_WhileStm(A_Exp A_St) A_St A_BreakStm(void) A_S
t A_BlockStm(A_St, A_St)
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Partial Bison Specification
stm IF ( exp ) stm A_IfStm(3, 5,
NULL) IF ( exp ) stm ELSE stm
A_IfStm(3, 5, 7) WHILE ( exp
) stm A_WhileStm(3, 5)
stmList 2 BREAK '
A_BreakStm() stmList
stmList st A_BlockStm(1, 2)
/ empty / NULL
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A Semantic Check(on the abstract syntax tree)
void check_break(A_St st) switch (st-gtkind)
case A_if check_break(st-gt u.if_st.st1)
check_break(st-gtu.if_st.st2)
break case A_while break case
A_break error(Break must be enclosed within a
loop,
st-gtpos) break case A_block
check_break(st-gtu.block_st.st1)
check_break(st-gtu.block_st.st2)
break
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Syntax Directed Solution
static int loop_count 0 stm exp
IF ( exp ) stm IF ( exp
) stm ELSE stm WHILE ( exp ) m stm
loop_count-- stmList
BREAK if (!loop_count)
error(Break must be enclosed within a loop,
line_count) stmList
stmList st / empty / m / empty /
loop_count
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Problems with Syntax Directed Translations

• Grammar specification may be tedious (e.g., to
  achieve LALR(1))
• May need to rewrite the grammar to incorporate
  different semantics
• Modularity is impossible to achieve
• Some programming languages allow
  forwarddeclarations (Algol, ML and Java)

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Example Semantic Condition Scope Rules

• Variables must be defined within scope
• Dynamic vs. Static Scope rules
• Cannot be coded using a context free grammar

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Dynamic vs. Static Scope Rules
procedure p var x integer procedure q
begin q x
end q procedure r var x
integer begin r q end r
begin p q r end p
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Example Semantic Condition

• In Pascal Types in assignment must be
  compatible'

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Partial Grammar for Pascal
Stm? id Assign Exp
Exp ? IntConst
Exp ? RealConst
Exp? Exp Exp
Exp? Exp -Exp
Exp? ( Exp )
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Refined Grammar for Pascal
Stm? RealId Assign RealExp
Stm?IntExpAssign IntExp
Stm?RealId Assign IntExp
RealExp ? RealConst
IntExp ? IntConst
RealIntExp ? RealId
IntExp ? IntId
RealExp? RealExp RealExp
RealExp? RealExp IntExp
IntExp? IntExp IntExp
RealExp? IntExp RealExp
IntExp? IntExp -IntExp
RealExp? RealExp -RealExp
RealExp? RealExp -RealExp
IntExp? ( IntExp )
RealExp? RealExp -IntExp
RealExp? IntExp -RealExp
RealExp? ( RealExp )
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Syntax Directed Solution
... stm id Assign exp compat_ass(lookup(1
), 3) exp exp PLUS exp
compat_op(PLUS, 1, 3)
op_type(PLUS, 1, 3) exp
MINUS exp compat_op(MINUS, 1, 3)
op_type(MINUS, 1,
3) ID lookup(1) INCONST
ty_int REALCONST ty_real
( exp ) 2
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Type Checking (Imperative languages)

• Identify the type of every expression
• Usually one or two passes over the syntax tree
• Handle scope rules

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Attribute Grammars Knuth 68

• Generalize syntax directed translations
• Every grammar symbol can have several attributes
• Every production is associated with evaluation
  rules
• Context rules
• The order of evaluation is automatically
  determined
• declarative
• Multiple visits of the abstract syntax tree

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Attribute Grammar for Types
stm? id Assign exp
compat_ass(id.type, exp.type) exp? exp
PLUS exp compat_op(PLUS,
exp1.type,exp2.type)
exp0.type op_type(PLUS, exp1.type,
exp2.type) exp? exp MINUS exp
compat_op(MINUS, exp1.type, exp2.type)
exp0.type op_type(MINUS, exp1.type,
exp2.type) exp? ID exp.type
lookup(id.repr) exp? INCONST exp.type ty_int
exp? REALCONST exp.type ty_real exp?
( exp ) exp0.type exp1.type

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Example Binary Numbers
Z ?L Z ?L.L L ?L B L ?B B ?0 B ?1
Compute the numeric value of Z
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Z ?L Z.v L.v Z ?L.L Z.v
L1.v L2.v
L ?L B L0.v L1.v B.v L ? B
L.v B.v
B ? 0 B.v 0 B ? 1 B.v ?
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Z ?L Z.v L.v Z ?L.L Z.v
L1.v L2.v
L ?L B L0.v L1.v B.v L ? B
L.v B.v
B ? 0 B.v 0 B ? 1 B.v 2B.s
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Z ?L Z.v L.v Z ?L.L Z.v
L1.v L2.v
L ?L B L0.v L1.v B.v B.s
L0.s L1.s L0.s 1 L ?
B L.v B.v B.s L.s
B ? 0 B.v 0 B ? 1 B.v 2B.s
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Z ?L Z.v L.v L.s 0 Z ?L.L
Z.v L1.v L2.v L1.s 0
L2.s?
L ?L B L0.v L1.v B.v B.s
L0.s L1.s L0.s 1 L ?
B L.v B.v B.s L.s
B ? 0 B.v 0 B ? 1 B.v 2B.s
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Z ?L Z.v L.v L.s 0 Z ?L.L
Z.v L1.v L2.v L1.s 0
L2.s-L2.l
L ?L B L0.v L1.v B.v B.s
L0.s L1.s L0.s 1 L0.l
L1.l 1 L ? B L.v B.v
B.s L.s L.l 1
B ? 0 B.v 0 B ? 1 B.v 2B.s
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Z.v1.625
Z
L.v0.625
L.v1
L.l3
L.s-3
L
.
L
L.s0
L.l1
L.v0.5
B.s-3
L.l2
B.s0
B
L
L.s-2
B
B.v1
B.v0.125
B.s-2
1
L.s-1
B
L
L.l1
B.v0
L.v0.5
1
0
B
B.v0.5
B.s-1
1
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Summary

• Several ways to enforce semantic correctness
  conditions
• syntax
• Regular expressions
• Context free grammars
• syntax directed
• traversals on the abstract syntax tree
• later compiler phases?
• Runtime?
• There are tools that automatically generate
  semantic analyzer from specification(Based on
  attribute grammars)