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Computer Science 332

Compiler Construction

Chapter 8: Intermediate CodeGeneration

Section 8.2: Procedure Declarations

8.2 Declarations : Single Procedure

Each local name has a symbol-table entry

Entry contains

Type : .type attribute

Relative address in procedure's activation record (e.g, Java

inttakes four bytes, doubletakes eight) : .widthattribute

Code-generating part of compiler uses a global variable offset

that is reset to zero as new procedures are declared

Code-generator proc. enter (name, type, width) creates symbol-

table entry with this info

Consider just declarations part of program....

P D { offset := 0}// initializaion

D D ;D

D id: T { enter(id.name, T.type, offset);

offset := offset + T.width) }

T integer { T.type := integer; T.width := 4 }

T double { T.type := double; T.width := 8 }

Problem : (look at first production)

offset := 0 code will be performed as last action, instead offirst

Solution: "dummy" non-terminal:

P M DM { offset := 0 }

8.2 Declarations : Nested Procedures

ML Example:

let fun f(x) =

let fun g(x, y) = x + y

in f(g(3, 4)) end end

Abstractly:

P DD D, D | id : T | fun id (D ) =S

Each procedure gets its own symbol table

Table for "inner" declaration points to table for "outer" linked list, searched from end to beginning (recalenvironmentsfrom CS312)

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8.2 Declarations : Nested Procedures

nil

f:x

g

g:x

y

8.2 Declarations : Nested Procedures

Semantic rules are a little more complicated:

mktable (previous) creates and returns a new s.t., pointing to

previously created one

enter (table, name, type, offset) acts like enter, but using a

specified table

addwidth (table, width) records cumulative width of all

entries in table

enterproc (table, name, newtable) creates new entry for

procedure namein existing table, and assigns newtable to be

the s.t. for this new proc.

Can do it all in one pass, if we use stacks tblptr, offset to keep

track of symbol tables and offsets:

8.2 Declarations : Nested ProceduresP M D { addwidth(top(tblptr), top(offset));

pop(tblptr);pop(offset); } // done afterM's

// actionsM { t := mktable(nil);

push(t, tblptr);push(0, offset) }D D,D

D fun idN (D1) =S { t := top (tblptr);

addwidth (t, top ( offset)); pop(tblptr);pop(offset);

enterproc( top( tblptr), id.name, t ) }

continues ...

8.2 Declarations : Nested Procedures

D id: T { enter (top (tblptr), id.name, T.type, top ( offset));

top(offset) := top( offset) + T.width) }

N { t := mktable(top( tblptr) );

push(t, tblptr);push(0, offset) }

Let's try it with

let fun f(x:int) =

let fun g(x:int, y:int) = x + y

in f(g(3, 4)) end end