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7 Intermediate Representation
Between the source code and the target code. Simpler than the source code, more complex than the target code Boundary between the front-end and the back-end，convenient for porting to new platforms Possible to design optimizer for IR Parser Static checker IR generator IR Token Code generator 1/34
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Return value and parameters
Code Static Ex: compiling phases procedure sort var a: array[0..9] of integer; x:integer; Begin x:= a[3]; End; Lexical Syntactic Token stream Syntax tree Type checking IR generation Construct/ query symbol table Return value and parameters Control link Access link & status 5 a[0] a[1] a[2] … a[9] 15 x 运行栈 sort Query the symbol table, whether a variable is declared Null Head “a” Array 5 “x” Integer 15 Symbol table for sort ↑15= ↑8+3
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Ex: After compilation, before execution
。。。。 ↑15= ↑8+3 There’s only executable code, no symbol table or run-time stack 。。。。 Executable code
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Return value and parameters
Ex: Run-time 。。。。 Execution sequence ↑15= ↑8+3 load 。。。。 Code Static Executable code Return value and parameters Control link Access link and status 5 a[0] a[1] a[2] … a[9] 15 x Run-time stack
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7 Intermediate Representation
Topics Introduce several common IR design: postfix representation, graph representation, and three address code Use syntax directed definition and translation scheme to explain how the structures of programming languages are translated to IRs 5/34
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7.1 Intermediate language
Postfix Graph Three address 7.1.1 Postfix Representation Postfix representation of expression E could be defined recursively: If E is constant or variable, E's postfix representation is E If E is an expression E1 op E2, E’s postfix representation is E1 E2 op, where E1 and E2 are the postfix representations of E1 and E2, respectively If E is represented by (E1), then the postfix representation of E1 is E’s postfix representation Brackets are not necessary (8 4) + 2 => 8 4 2 +
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7.1 Intermediate language
后缀表示 图形表示 三地址代码 7.1.1 Postfix Representation Convenient for computer processing (8 4) + 2 => 2 + 返回值和参数 控制链 访问链和机器状态 局部数据临时数据 top_sp base_sp 栈 运行时！注意区分运行与编译的区别。 8
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7.1 Intermediate language
后缀表示 图形表示 三地址代码 7.1.1 Postfix Representation Convenient for computer processing (8 4) + 2 => 2 + 返回值和参数 控制链 访问链和机器状态 局部数据临时数据 top_sp base_sp 栈 8 4
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7.1 Intermediate language
后缀表示 图形表示 三地址代码 7.1.1 Postfix Representation Convenient for computer processing (8 4) + 2 => 2 + 返回值和参数 控制链 访问链和机器状态 局部数据临时数据 top_sp base_sp 栈 84- 即 8-4=4 8 4 4
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7.1 Intermediate language
后缀表示 图形表示 三地址代码 7.1.1 Postfix Representation Convenient for computer processing (8 4) + 2 => 2 + 返回值和参数 控制链 访问链和机器状态 局部数据临时数据 top_sp base_sp 栈 4 10/34 2
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7.1 Intermediate language
后缀表示 图形表示 三地址代码 7.1.1 Postfix Representation Convenient for computer processing (8 4) + 2 => 2 + 返回值和参数 控制链 访问链和机器状态 局部数据临时数据 top_sp base_sp 栈 即 8-4+2=6 4 6 2
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7.1 Intermediate language
后缀表示 图形表示 三地址代码 7.1.2 Graph Representation Syntax Tree assign a + b c d uminus (a) Syntax Tree a := (b + cd ) + cd’s graph representation
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7.1 Intermediate language
后缀表示 图形表示 三地址代码 7.1.2 Graph Representation Syntax tree is a graph representation Directed Acyclic Graph (DAG) is as well assign a + b c d uminus (a) Syntax Tree (b) dag a := (b + cd ) + cd’s graph representation
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7.1 Intermediate language
后缀表示 图形表示 三地址代码 Syntax directed definition to generate declaration statement’s syntax tree Production Semantic Rules S id :=E S.nptr := mknode(‘assign’, mkleaf (id, id.entry), E.nptr) E E1 +E2 E.nptr := mknode( ‘+’, E1.nptr, E2.nptr) E E1 E2 E.nptr := mknode( ‘’, E1.nptr, E2.nptr) E E1 E.nptr := mkunode( ‘uminus’, E1.nptr) E (E1) E.nptr := E1.nptr F id E.nptr := mkleaf (id, id.entry)
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7.1 Intermediate language
后缀表示 图形表示 三地址代码 7.1.3 Three address code General form：x := y op z Translate x + y z into three address code: t1 := y z t2 := x + t1 Ret and params Local and temp Control link Access link and status stack Temporary variable After the local variables in the activation records 15/34
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7.1 Intermediate language
后缀表示 图形表示 三地址代码 Three address Code is a linear representation of syntax tree or dag a := (b + cd ) + cd Syntax tree’s code dag’s code t1 := b t1 := b t2 := c d t2 := c d t3 := t1 + t2 t3 := t1 + t2 t4 := c d t4 := t3 + t2 t5 := t3 + t4 a := t4 a := t5 assign a + b c d uminus (a) Syntax tree assign a + b c d uminus (b) dag
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7.1 Intermediate language
Examples of three address code Declaration x := y op z, x := op y, x := y Goto goto L If if x relop y goto L Procedural call param x call p , n Return return y Access via index x := y[i] x[i] := y Assign with address x := &y，x := y x := y
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7.2 Declaration Statements
Generate symbol table entry for local names Assign storage units The symbol table contains the information of the type and the address of the storage units
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7.2 Declaration Statements
7.2.1 Declaration inside procedures Compute the type and address of the declared names P {offset := 0} D S 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 real {T.type := real; T.width := 8 } T array [ num ] of T1 {T.type := array (num.val, T1.type); T.width := num.val T1.width} T T1 {T.type := pointer (T1.type); T.width := 4 }
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7.2 Declaration Statements
7.2.2 Storage of the scope information If procedural declaration could be recursive, the processing of the declaration statements is different: P D S D D ; D | id : T | proc id ; D ; S 20/34
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7.2 Declaration Statements
7.2.2 Storage of the scope information program sort(input,output) var a:array[0..10] of integer; x:integer; prcedure readarray; var i:integer; begin …a … end{readarray}; procedure exchange(i,j:integer); begin x:=a[i]; a[i]:=a[j];a[j]:=x;end{exchange}; procedure quicksort(m,n:integer) var k,v:integer; function partition(y,z:integer):integer; begin …a… …v… …exchange(i,j)… end{partition}; begin…end{quicksort}; begin … end{sort}.
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7.2 Declaration Statements
program sort(input,output) var a:array[0..10] of integer; x:integer; “a”, type info，address in the frame sort Null Head a x readarray exchange quicksort Symbol table of sort
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7.2 Declaration Statements
program sort(input,output) var a:array[0..10] of integer; x:integer; prcedure readarray; var i:integer; begin …a … end{readarray}; exchange readarray x a Head Null sort quicksort readarrary i
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7.2 Declaration Statements
program sort(input,output) …… prcedure readarray;…. procedure exchange(i,j:integer); begin x:=a[i]; a[i]:=a[j];a[j]:=x; end{exchange}; sort Null Head a x readarray Point to readarray exchange Point to exchange quicksort readarrary exchange Head Head i
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7.2 Declaration Statements
program sort(input,output) var a:array[0..10] of integer; x:integer; prcedure readarray; procedure exchange(i,j:integer); procedure quicksort(m,n:integer) var k,v:integer; sort Null Head a x readarray To readarray exchange To exchange quicksort readarrary quicksort exchange Head Head Head i k v partition 25/34 partition
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7.2 Declaration Statements
exchange readarray x a Head Null sort quicksort To readarray partition v k readarrary i To exchange program sort(input,output) prcedure readarray; procedure exchange(i,j:integer); procedure quicksort(m,n:integer) function partition(…. var k,v:integer; begin …a… …v… … end{partition};
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7.2 Declaration Statements
7.2.2 Storage of the scope information The grammar P D S D D ; D | id : T | proc id ; D ; S Functions in the semantic rules mktable(previous) enter(table, name, type, offset) addwidth(table, width) enterproc(table, name, newtable) exchange readarray x a Head Null sort quicksort readarrary i enterproc(↑sort,readarray,↑readarray) mktable(↑sort) enter(table, name, type, offset)
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7.2 Declaration Statements
P M D S {addwidth (top (tblptr), top (offset) ); pop(tblptr); pop (offset) } M {t := mktable (nil); push(t, tblprt); push (0, offset) } D D1 ; D2 D proc id ; N D1; S {t := top(tblptr); addwidth(t, top(offset) ); pop(tblptr); pop(offset); enterproc(top(tblptr), id.name, t) } Did : 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) }
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7.3 Assignment Statements
7.3.1 Names in the symbol table S id := E {p := lookup(id.name); if p nil then emit ( p, ‘:=’, E.place) else error } E E1 + E2 {E.place := newtemp; emit (E.place, ‘:=’, E1.place, ‘+’, E2.place) } Generate a new temporary name, put the name into the symbol table, and return the address of the entry Output the three address code, similar to printf
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7.3 Assignment Statements
7.3.1 Names in the symbol table (cont) E E1 {E.place := newtemp; emit (E.place, ‘:=’, ‘uminus’, E1.place) } E (E1) {E.place := E1.place } E id {p := lookup(id.name); if p nil then E.place := p else error } 30/34
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7.3 Assignment Statements
7.3.2 Reuse of temporary names Suitable for optimization Expansion of the symbol table Larger activation record
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7.3 Assignment Statements
7.3.2 Reuse of temporary names Suitable for optimization Expansion of the symbol table Larger activation record E E1 + E2 generate code like Calculate E1 as t1 Calculate E2 as t2 t3 := t1 + t2 ( ( ) ) ( ( ( ) ( ) ) ( ) ) The lifespan of temporary variables is similar as matched brackets
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7.3 Assignment Statements
Number of temp names x := a b + c d e f Statement C value $0 := a b 1 $1 := c d 2 $0 := $0 + $1 $1 := e f $0 := $0 $1 x := $0
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Exercise 7.1 34/34
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