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8 Intermediate code generation. Zhang Zhizheng [email protected]. 8.0 Overview. 1.Position of Intermediate code generator. Intermediate code generator. static checker. Syntax tree. Syntax tree. Token stream. Intermediate code. parser. Code generator. - PowerPoint PPT Presentation
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Zhang Zhizheng
8.0 Overview
1.Position of Intermediate code generator
parserToken stream
static
checker
Syntax tree
Intermediate code generator
Syntax tree
Intermediate code
Code generator
Zhang Zhizheng
2.Benefits for using a machine-independent intermediate form
• Retargeting is facilitated; a compiler for a different machine can be created by attaching a back end for the new machine to an existing front end.
• A machine-independent code optimizer can be applied to the intermediate representation.
Zhang Zhizheng
3.Implementation of Intermediate code generator
• Syntax-directed translation, folded into parsing
– Top-down parsing– Bottom-up parsing
Zhang Zhizheng
8. 1 Intermediate languages
1.Intermediate representations– Syntax tree
• Syntax Tree
• Directed acyclic graph(DAG)
– Postfix notation
– Three-address code
– Quadruple
Zhang Zhizheng
A syntax tree and DAG for the assignment statement :
a:=b*-c+b*-c
assign
a +
b
* *
buminus uminus
c c
assign
a +
b
*
uminus
c
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Production of Syntax Tree (and DAG)
Production Semantic rules
S id:=E id.value=E.value
E E1 + E2 E.value=E1.value+E1.value
E E1 * E2 E.value=E1.valueE1.value
E - E1 E.value=-E1.value
E ( E1) E.value=E1.value
E id E.value=id.lexival
Semantic rules on value attribution of assignment statement
Zhang Zhizheng
Production Semantic rules
S id:=E S.nptr:=maknode(‘assign’, mkleaf(id, id.place), E. nptr)
E E1 + E2 E.nptr= maknode(‘+’, E1.nptr, E1.nptr)
E E1 * E2 E.nptr= maknode(‘*’, E1.nptr, E1.nptr)
E - E1 E.nptr= maknode(‘uminus’, E1.nptr)
E ( E1) E.nptr= E1.nptr
E id E.nptr= maknode(id, id.place)
Semantic rules on producing syntax tree of assignment statement
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Representations of the syntax tree____the data structure of graph
• See Fig. 8.4
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2.Three-address code(TAC)
A sequence of statements of the general form
x= y op zHere, x, y, z are names, constants, or compiler-
generated temporaries; op stands for any operator
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Notes: 1)There is only one operator on the right si
de of a statement2) Three address code is a linearized repres
entation of a syntax tree or a DAG in which explicit names correspond to the interior nodes of the graph
3) Each three-address code statement contains three addresses, two for the operands and one for the result
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E.g, Tree address code corresponding to the above Tree and DAG
• t1:=-c• t2:=b*t1• t3:=-c• t4:=b*t3• t5:=t2+t4• a:=t5
Code for the syntax tree
• t1:=-c• t2:=b*t1• t5:=t2+t2• a:=t5
Code for the DAG
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3. Types of TAC– x:=y op z //assignment statement, op is binary arithmetic or logical operation//
– x:=op y //assignment statement, op is unary operation as minus, logical negative, conversion operator etc.//
– x:=y //Copy assignment statement//
– goto L //Unconditional jump//
– If x relop y goto L //Conditional jump: if x stands in relation relop to y, then executes the statement with label L, else executed the following statement //
– param x1
…… param xn
call p,n return y //Call procedure P with n parameters (x1,……,xn)//
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– x=y[i]
x[i]=y
– x=&y //the value of x is the location of y//
x=*y
*x=y
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E.g, a:=b*-c+b*-c can be translated into• t1:=-c• t2:=b*t1• t3:=-c• t4:=b*t3• t5:=t2+t4• a:=t5How translate??
Zhang Zhizheng
Production Semantic Rules
Sid:=E S.code:=E.code||gen(id.place ‘:=’ E.place)
E E1+E2 E.place:=newtemp();
E.code:=E1.code||E2.code||
gen(E.place,’:=’,E1.place ‘+’ E2.place)E E1*E2 E.place:=newtemp(); E.code:=E1.code||E2.code|| gen(E.place,’:=’,E1.place ‘*’ E2.place)E -E1 E.place:=newtemp(); E.code:=E1.code|| gen(E.place,’:=’, ‘uminus’ E2.place)
E id E.place:=id.place
E.code:=‘’E.place,the name that will hold the value of E
E.code, the sequence of three-address statements evaluating E.
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Production Semantic Rules
Swhile E do S1 S.begin=newlabel();
S.after=newlabel();
S.code=gen(S.begin ‘:’)||E.code||
gen(‘if’ E.place ‘=‘ ‘0’ ‘goto’ S.after) ||
S1.code ||
gen(‘goto’ S.begin) ||
gen(S.after ‘:’)
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Production Semantic Rules
Sif E then S1 S.after=newlabel();
S.code=E.code||
gen(‘if’ E.place ‘=‘ ‘0’ ‘goto’ S.after) ||
S1.code ||
gen(S.after ‘:’)
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Production Semantic Rules
Sif E then S1 S.after=newlabel();
else S2 E.false=newlabel();
S.code=E.code||
gen(‘if’ E.place ‘=‘ ‘0’ ‘goto’ E.false) ||
S1.code ||
gen(‘goto’ S.after) ||
gen(E.false ‘:’) ||
S2.code ||
gen(S.after ‘:’)
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5.Addressing array elements 1)One-dimensional array
Addr(A[i])=base+(i-low)*w=i*w+(base-low*w)
Notes:1)Here, we assume the width of each array element i
s w and the start address of the array block is base.
2)The array is defined as array[low..upper] of type
3)The sub-expression c=base-low*w can be evaluated when the declaration of the array is seen and we assume that c is saved in the symbol table entry for the array.
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2)two-dimensional array
(1) row-major form
Addr(A[i1, i2])=base+((i1-low1)*n2+i2-low2)*w
=(i1*n2+i2)*w+base-(low1*n2+low2)*w
Where n2=upper2-low2+1
t1=low1*n2 t2=t1+low2 t3=t2*w t4=base-t3
t5=i1*n2 t6=t5+i2 t7=t6*w
t4[t7]=x x=t4[t7]
(2) column-major form
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3)n-dimensional array
Array[l1:u1,, l2:u2,… ln:un]
Let di=ui-li+1,i=1,2,…n, the width of each dimension is m
D=a+((i1-l1)d2d3…dn+ (i2-l2)d3d4…dn + (in-1-ln-1)dn + (i
n-ln))m
Change into D=conspart+varpart
conspart=a-C
C=((…(l1d2+l2 )d3+ l3) d3…+ ln-1) dn+ ln)m
varpart= ((…(i1d2+i2 )d3+ i3) d3…+ in-1) dn+ in)m
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6.Short-circuit code of Boolean expressions• Translate a boolean expression into intermediate c
ode without evaluating the entire expression.
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7. Translation methods of Flow of control statements in Short-circuit code
1)Associate E with two labels
• E.true– The label to which control flows if E is true
• E.false– The label to which control flows if E is false
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2)Associate S with a label
• S.next– Following S.code is a jump to some label
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Production Semantic RulesSif E then S1 E.true=newlabel(); E.false=S.next; S1.next=S.next; S.code=E.code ||gen(E.true ‘:’) ||S1.codeSif E then S1 else S2 E.true=newlabel(); E.false=newlabel(); S1.next=S.next S2.next=S.next S.code=E.code ||gen(E.true ‘:’) ||S1.code||gen(‘goto’ S.next)|| gen(E.false ‘:’)||S2.code
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Production Semantic RulesSwhile E do S1 S.begin=newlabel(); E.true=newlabel(); E.false=S.next; S1.next=S.begin S.code=gen(S.begin ‘:’)||E.code ||gen(E.true ‘:’) ||S1.code||gen(‘goto’ S.begin)
Zhang Zhizheng
Production Semantic RulesEE1 or E2 E1.true=E.true; E1.false=newlabel(); E2.true=E.true; E2.false=E.false E.code=E1.code ||gen(E1.false ‘:’) ||E2.codeEE1 and E2 E1.true=newlabel(); E1.false=E.false; E2.true=E.true; E2.false=E.false E.code=E1.code ||gen(E1.true ‘:’) ||E2.codeE id1 relop id2 E.code=gen(‘if’ id1.place relop.op id2.place ‘goto’ E.true)||gen(‘goto’ E.false)
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3)Examples (1)a<b or c<d and e<f if a<b goto Ltrue goto L1 L1:if c<d goto L2 goto Lfalse L2:if e<f goto Ltrue goto Lfalse Here, we assume that the true and false exits for the enti
re expression are Ltrue and Lfalse respectively
Zhang Zhizheng
(2)while a<b do
if c<d then x=y+z
else x=y-z
L1: if a<b goto L2 goto LnextL2: if c<d goto L3 goto L4L3:t1=y+z x=t1 goto L1L4:t2=y-z x=t2 goto L1Lnext:
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8.Implementations of three-address statements• Quadruples
– (op, arg1,arg2,result)
• Triples
– (n) (op,arg1,arg2)
– (m) (op,(n),arg)
Notes: A three-address statement is an abstract form of intermediate codes
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9.Advantages of quadruples• Easy to generate target code
• Good for optimizing
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ExercisesPlease translate the following program fragment into three-address code using the form of short circuit code. i=2;loop=0;while (loop==0 && i<=10) { j=1; while (loop ==0 && j<i) if (a[i,j] == x) loop=1; else j=j+1; if (loop==0) i=i+1;}Notes: Here we assume that the declaration of array A is array [1..10,1..10], each data element of array A would use 2 storage units, and the start address of array A’s storage area is addrA.
Zhang Zhizheng
Translate the following program fragment into three-address code. i=2;m=0;loop=0;while (loop==0 && i<=10) { j=1; while (loop ==0 && j<=i) if (a[i,j] != a[j,i]) //”!=” means “not equal to” { loop=1;m=1;}else j=j+1; if (loop==0) i=i+1;}Notes: Here we assume that the declaration of array A is array [1..10,1..10], each data element of array A would only use 1 storage unit, and the start address of array A’s storage area is addrA.
Zhang Zhizheng
8. 2 Assignment statements
1 、 Assignment statements with only id
1) functions
NEWTEMP()
GEN(OP,ARG1,ARG2,RESULT)
2)Semantic rules for quadruple code generation
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(1)A i=E {GEN(=, E•PLACE ,_, i.entry}
(2)E -E (1) {T=NEWTEMP();
GEN(@, E(1)•PLACE ,_,T);
E•PLACE =T }
(3)E E (1)*E(2) {T=NEWTEMP();
GEN(*, E(1)•PLACE , E(2)•PLACE ,T);
E•PLACE =T }
(4)E E (1) + E(2) {T=NEWTEMP();
GEN(+, E(1)•PLACE , E(2)•PLACE ,T);
E•PLACE =T }
(5)E (E (1)) {E•PLACE =E(1)•PLACE }
(6)E i {E•PLACE = i.entry}
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iput SYM PLACE quadruples
A=-B*(C+D)#
=-B*(C+D)# i －-B*(C+D)# i= －－B*(C+D)# i=- －－－*(C+D)# i=-i －－－*(C+D)# i=-E －－－ B
*(C+D)# i=E －－ T1 (@,B,－ ,T1)
(C+D)# i=E* －－ T1 －C+D)# i=E*( －－ T1 －－+D)# i=E*(i －－ T1 －－
C
+D)# i=E*(E
－－ T1 －－C
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3.The translation scheme for addressing array elements
1) grammar
AV:=E
V i[Elist] | i
Elist Elist,E | E
E E op E | (E) | V
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3.The translation scheme for addressing array elements
2) Rewriting of the grammar
AV:=E
V Elist] | i
Elist Elist(1),E | i[ E
E E op E | (E) | V
Notes: This rewriting aims that the various dimensional limits nj of the array be available as we group index expressions into an Elist.
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3.The translation scheme for addressing array elements
3) semantic variables
ARRAY
DIM
PLACE
OFFSET
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3.The translation scheme for addressing array elements
4) Translation code
(1)AV=E
{if (V•OFFSET=null)
GEN(=,E • PLACE,_,V•PLACE);
else GEN([ ]=,E•PLACE,_,V•PLACE[V•OFFSET])}
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(2)E E(1) op E (2) {T=NEWTEMP();
GEN(op, E(1) •PLACE, E(2) •PLACE,T);
E • PLACE =T}
(3)E (E (1)) {E • PLACE = E(1) •PLACE}
(4)E V {if (V•OFFSET=null)
E • PLACE = V •PLACE;
else
{T=NEWTEMP();
GEN(=[ ], E • PLACE[V •OFFSET],_,T);
E • PLACE =T;}}
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(5)V Elist] {if (TYPE[ARRAY]<>1)
{T=NEWTEMP();
GEN(*,Elist•PLACE,TYPE[ARRAY],T);
Elist •PLACE=T;}
V •OFFSET=Elist •PLACE;
T=NEWTEMP();
GEN(-,HEAD[ARRAY],CONS[ARRAY],T);
V •PLACE=T}
(6)V i {V •PLACE=ENTRY[i];
V •OFFSET=null}
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(7)Elist Elist(1),E {T=NEWTEMP();
k= Elist(1) •DIM+1;
dk=LIMIT(Elist(1)•ARRAY,k);
GEN(*,Elist (1) •PLACE, dk,T);
T1=NEWTEMP();
GEN(+,T,E •PLACE, T1);
Elist•ARRAY= Elist(1)•ARRAY;
Elist•PLACE= T1;
Elist•DIM=k;
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(8)Elist i[ E
{Elist•PLACE=E•PLACE;
Elist•DIM=1;
Elist•ARRAY=ENTRY(i)}
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E.g. Let A be an array:ARRAY[1:10,1:20]; the address of the beginning of the array is a, m=1.
We can get C by the computing: (low1*n2+low2)*m=(1*20+1)*1=21
The quadruples for X=A[I,J] are:
(1) (*,I,20,T1)
(2) (+, T1,J, T2)
(3) (-,a,21, T3)
(4) (=[ ], T3[T2],_, T4)
(5) (=, T4,_,X)
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8. 3 Boolean expressions
1.Primary purposes of boolean expressions
– Compute logical values
– Used as conditional expressions in statements that alter the flow of control,such as if or while statements.
2.Grammar
– E E and E | E or E | not E | (E) | i | Ea rop Ea
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3.Numerical representation
(1)EEa(1) rop Ea
(2) {T=NEWTEMP();
GEN(rop, Ea(1)•PLACE , Ea
(2)•PLACE ,T);
E•PLACE =T }
(2)E E (1) bop E(2) {T=NEWTEMP();
GEN(bop, E(1)•PLACE , E(2)•PLACE ,T);
E•PLACE =T }
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3.Numerical representation
(3)E not E (1) {T=NEWTEMP; GEN(not, E(1)•PLACE , _ ,T); E•PLACE =T }
(4)E (E (1)) {E•PLACE =E(1)•PLACE }
(5)E i {E•PLACE = ENTRY(i)}
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3.Numerical representationE.g. X+Y>Z or A and (not B or C)
(+,X,Y,T1) ;E+E
(>, T1,Z, T2) ; E >E
(not,B,_, T3) ; not E
(or, T3,C, T4) ; E or E
(and ,A, T4,T5) ; E and E
(or, T2, T5, T6) ; E or E
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4.Short-circuit code
• Translate a boolean expression into intermediate code without evaluating the entire expression.
• Represent the value of an expression by a position in the code sequence.
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• E.g. if A or B<D then S1 else S2
(1)(jnz,A,_,(5)) ;E.true, to S1
(2)(j,__,(3)) ;E.false, look at the right of or(3)(j<,B,D,(5)) ;Ea.true, to S1 (4)(j,_,_,(P+1)) ; Ea.false, to S2 (5) S1
……(P)(j,_,_,(q)) ;jump over S2
(p+1) S2
……(q)the code after S2
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8.4 Backpatching
1.Why and what is backpatching?• When generating code for boolean expressions and flow-o
f-control statements , we may not know the labels that control must go to.
• We can get around this problem by generating a series of branching statement with the targets of the jumps temporarily left unspecified.
• Each such statement will be put on a list of goto statements whose labels will be filled in when the proper label can be determined.
• This subsequent filling in of labels is called backpatching
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2.Functions to manipulate lists of labels related to backpatching
• Makelist(i)– Creates a new list containing only i, an index into the arr
ay of quadruples; makelist returns a pointer to the list it has made.
• Merge(p1,p2)– Concatenates the lists pointed to by p1 and p2, and return
s a pointer to the concatenated list.• Backpatch(p,i)
– Inserts i as the target label for each of the statements on the list pointed to by p.
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3.Boolean expression1)Modify the grammar
E EAE | E0E | not E | (E) | i | Ea rop Ea
EA E andE0 E or
2)Semantic Rules(1) E i {E•TC=NXQ; E•FC=NXQ+1; GEN(jnz,ENTRY(i),_,0); GEN(j,_,_,0)}
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3.Boolean expression 2)Semantic Rules (2) E Ea rop Ea
{E•TC=NXQ; E•FC=NXQ+1;
GEN(jrop, Ea(1)•PLACE, Ea
(2)•PLACE,0); GEN(j,_,_,0)}(3) E (E(1)) {E•TC= E(1)•TC; E•FC= E(1)•FC}(4) E not E(1) {E•TC= E(1)•FC; E•FC= E(1)•TC}
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3.Boolean expression 2)Semantic Rules
(5)EA E(1) and {BACKPATCH(E(1)•TC,NXQ);
EA•FC= E(1)•FC;}
(6) EEAE(2)
{E•TC= E(2)•TC;
E•FC=MERG(EA•FC,E(2)•FC}
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3.Boolean expression 2)Semantic Rules
(7)E0 E(1) or
{BACKPATCH(E(1)•FC,NXQ);
E0•TC= E(1)•TC;}
(8) EE0E(2)
{E•FC= E(2)•FC;
E•TC=MERG(E0•TC,E(2)•TC}
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Translate A and B or not C
-2----# EAi or not C#
-2--# EA B or not C#
2.(j,-,-(5))-2--1-#E and B or not C#
1.(jnz,a,-,(3))-2-1#E and B or not C#
----#i and B or not C#
--#A and B or not C#
quadrupleFCTCSYMINPUT
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quadrupleFCTCSYMINPUT
3.(jnz,B, － ,0)－ 2 4－－ 3# EAE or not C ＃
success
－ 5－ 6#E #
6.(j, － , － ,3)－－ 5－ 3 6# E0 E #
5.(jnz,C, － ,0)－－－ 6－ 3 － 5# E0 notE #
－－－－－ 3 －－# E0 not i ＃－－－－ 3 －# E0 not C ＃－－－ 3# E0 not C ＃－ 4 －－ 3 －#E or or not C ＃
4.(j, － , － (5))－ 4－ 3#E or not C ＃
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4.Flow of control statements1) modify the grammarS if E then S(1) else S(2)
C if E thenT C S(1) elseS T S(2)
S if E then S(1)
C if E thenS C S(1)
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4.Flow of control statements2) Semantic Rules
C if E then {BACKPATCH(E•TC,NXQ); C•CHAIN=E•FC;}T C S(1) else {q=NXQ; GEN(j, － , － 0); BACKPATCH(C•CHAIN,NXQ); T •CHAIN=MERG(S(1)•CHAIN,q)}S T S(2)
{S•CHAIN=MERG(T•CHAIN,S(2)•CHAIN)}S C S(1) {S•CHAIN=MERG(C•CHAIN,S(1)•CHAIN)}
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e.g.
If a then
if b then
A:=2
else A:=3
Else if c then
A=4
Else a=5
(1) (jnz,a,_,0)(2) (j,_,_,0)
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If a then
if b then
A:=2
else A:=3
Else if c then
A=4
Else a=5
(1)(jnz,a,_,(3))(2)(j,_,_,0)(3)(jnz,b,_,0)(4)(j,_,_,0)
Ca•CHAIN->2
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If a then
if b then
A:=2
else A:=3
Else if c then
A=4
Else a=5
(1)(jnz,a,_,(3))(2)(j,_,_,0)(3)(jnz,b,_,(5))(4)(j,_,_,0)
Ca•CHAIN->2
Cb•CHAIN->4
(5)(:=,2,_,A)
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If a then
if b then
A:=2
else A:=3
Else if c then
A=4
Else a=5
(1)(jnz,a,_,(3))(2)(j,_,_,0)(3)(jnz,b,_,(5))(4)(j,_,_,(7))(5)(:=,2,_,A)
Ca•CHAIN->2
Cb•CHAIN->6
(6)(j,_,_,0)
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Answer
(1)(jnz,a,_,(3)) (8)(j,_,_,6)
(2)(j,_,_,(9)) (9)(jnz,c,_,(11))
(3)(jnz,b,_,(5)) (10)(j,_,_,(13))
(4)(j,_,_,(7)) (11)(:=,4,_,A)
(5)(:=,2,_,A) (12)(j,_,_,8)
(6)(j,_,_,0) (13)(:=,5,_,A)
(7)(:=,3,_,A)S•CHAIN->6->8->12
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a
b
S1(A:=2)
S2(A:=3)
c
S3(A:=4)
S4(A:=5)
TRUEFALSE
1,2
3,4
5
7
9,10
11
13
6
8
12
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4.Flow of control statements
3) While statement
S while E do S(1)
W while
Wd W E do
S Wd S(1)
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4.flow of control statements
3) While statement
W while {W•QUAD=NXQ}
Wd W E do {BACKPATCH(E•TC,NXQ);
Wd•CHAIN=E•FC;
Wd•QUAD=W•QUAD;}
S Wd S(1){BACKPATCH(S(1)•CHAIN, Wd•QUAD);
GEN(j,_,_, Wd •QUAD);
S • CHAIN= Wd•CHAIN}
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4.flow of control statements
3) While statement
Code of E
Code of S(1)
S.CHAIN
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e.g.
While (A<B) do
if (C<D) then
X:=Y+Z;
(100) (j<,A,B,0)
(101)(j,_,_,0)
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e.g.
While (A<B) do
if (C<D) then
X:=Y+Z;
： (100) (j<,A,B,(102))
(101)(j,_,_,0)
(102)(j<,C,D,0)
(103)(j,_,_,(100))
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e.g.
While (A<B) do
if (C<D) then
X:=Y+Z; ： (100) (j<,A,B,(102))
(101)(j,_,_,0)
(102)(j<,C,D,(104))
(103)(j,_,_,(100))
(104)(+,Y,Z,T1)
(105)(:=, T1,_,X)
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e.g.
While (A<B) do
if (C<D) then
X:=Y+Z;
： (100) (j<,A,B,(102)) (106)(j,_,_,(100))
(101)(j,_,_,(107))
(102)(j<,C,D,(104))
(103)(j,_,_,(100))
(104)(+,Y,Z,T1)
(105)(:=, T1,_,X)
