CH4.1 CSE244 Intermediate Code Generation Aggelos Kiayias Computer Science & Engineering Department The University of Connecticut 371 Fairfield Road, Unit 1155 Storrs, CT

CH4.2 CSE244 Intermediate Code Generation  Translating source program into an “intermediate language.”  Simple  CPU Independent,  …yet, close in spirit to machine language.  Or, depending on the application other intermediate languages may be used, but in general, we opt for simple, well structured intermediate forms.  (and this completes the “Front-End” of Compilation).

CH4.3 CSE244 Types of Intermediate Languages  Graphical Representations.  Consider the assignment a:=b*-c+b*-c: assign a+ ** uminus b cc b assign a + * uminus b c

CH4.4 CSE244 Syntax Dir. Definition for Gr. Reps. PRODUCTIONSemantic Rule S  id := E{ S.nptr = mknode (‘assign’, mkleaf(id, id.entry), E.nptr) } E  E 1 + E 2 {E.nptr = mknode(‘+’, E 1.nptr,E 2.nptr) } E  E 1 * E 2 {E.nptr = mknode(‘*’, E 1.nptr,E 2.nptr) } E  - E 1 {E.nptr = mknode(‘uminus’, E 1.nptr) } E  ( E 1 ) {E.nptr = E 1.nptr } E  id {E.nptr = mkleaf(id, id.entry) }

CH4.5 CSE244 Three Address Code  Statements of general form x:=y op z  No built-up arithmetic expressions are allowed.  As a result, x:=y + z * w should be represented as t 1 :=z * w t 2 :=y + t 1 x:=t 2  Observe that given the syntax-tree or the dag of the graphical representation we can easily derive a three address code for assignments as above.  In fact three-address code is a linearization of the tree.  Three-address code is useful: related to machine- language/ simple/ optimizable.

CH4.6 CSE244 Example of 3-address code  t 1 :=- c t 2 :=b * t 1 t 3 :=- c t 4 :=b * t 3 t 5 :=t 2 + t 4 a:=t 5  t 1 :=- c t 2 :=b * t 1 t 5 :=t 2 + t 2 a:=t 5

CH4.7 CSE244 Types of Three-Address Statements.  x:=y op z  x:=op z  x:=z  goto L  if x relop y goto L  Push/pop (stack) more advanced:  param x 1 param x 2 … param x n call p,n  x:=y[i]x[i]:=y  x:=&yx:=*y*x:=y

CH4.8 CSE244 Syntax-Directed Translation into 3- address code.  First deal with assignments.  Use attributes  E.place to hold the name of the “place” that will hold the value of E  Identifier will be assumed to already have the place attribute defined.  For example, the place attribute will be of the form t0, t1, t2, … for identifiers and v0,v1,v2 etc. for the rest.  E.code to hold the three address code statements that evaluate E (this is the `translation’ attribute).  Use function newtemp that returns a new temporary variable that we can use.  Use function gen to generate a single three address statement given the necessary information (variable names and operations).

CH4.9 CSE244 Syntax-Dir. Definition for 3-address code PRODUCTIONSemantic Rule S  id := E{ S.code = E.code||gen(id.place ‘=’ E.place ‘;’) } E  E 1 + E 2 {E.place= newtemp ; E.code = E 1.code || E 2.code || || gen(E.place‘:=’E 1.place‘+’E 2.place) } E  E 1 * E 2 {E.place= newtemp ; E.code = E 1.code || E 2.code || || gen(E.place‘=’E 1.place‘*’E 2.place) } E  - E 1 {E.place= newtemp ; E.code = E 1.code || || gen(E.place ‘=’ ‘uminus’ E 1.place) } E  ( E 1 ) {E.place= E 1.place ; E.code = E 1.code} E  id {E.place = id.entry ; E.code = ‘’ } e.g. e.g. a := b * - (c+d)

CH4.10 CSE244 What about things that are not assignments?  E.g. while statements of the form “while E do S” (intepreted as while the value of E is not 0 do S) Extension to the previous syntax-dir. Def. PRODUCTION S  while E do S 1 Semantic Rule begin = newlabel; after = newlabel ; S.code = gen(begin ‘:’) || E.code || gen(‘if’ E.place ‘=’ ‘0’ ‘goto’ after) || || S 1.code || gen(‘goto’ begin) || gen(after ‘:’)

CH4.11 CSE244 Dealing with Procedures P  procedure ident ‘;’ block ‘;’ Semantic Rule begin = newlabel; Enter into symbol-table in the entry of the procedure name the begin label. P.code = gen(begin ‘:’) || block.code || gen(‘pop’ return_address) || gen(“goto return_address”) S  call ident Semantic Rule Look up symbol table to find procedure name. Find its begin label called proc_begin return = newlabel; S.code = gen(‘push’return); gen(goto proc_begin) || gen(return “:”)

CH4.12 CSE244 Implementations of 3-address statements  Quadruples t 1 :=- c t 2 :=b * t 1 t 3 :=- c t 4 :=b * t 3 t 5 :=t 2 + t 4 a:=t 5 oparg1arg2result (0)uminusc t1t1t1t1 (1)*b t1t1t1t1 t2t2t2t2 (2)uminusc (3)*b t3t3t3t3 t4t4t4t4 (4)+ t2t2t2t2 t4t4t4t4 t5t5t5t5 (5):= t5t5t5t5a

CH4.13 CSE244 Implementations of 3-address statements, II  Triples t 1 :=- c t 2 :=b * t 1 t 3 :=- c t 4 :=b * t 3 t 5 :=t 2 + t 4 a:=t 5 oparg1arg2 (0)uminusc (1)*b(0) (2)uminusc (3)*b(2) (4)+(1)(3) (5)assigna(4)

CH4.14 CSE244 Other types of 3-address statements  e.g. ternary operations like x[i]:=yx:=y[i]  require two or more entries. e.g. oparg1arg2 (0) [ ] = xi (1)assign(0)y oparg1arg2(0) yi (1)assignx(0)

CH4.15 CSE244 Implementations of 3-address statements, III  Indirect Triples oparg1arg2 (14)uminusc (15)*b(14) (16)uminusc (17)*b(16) (18)+(15)(17) (19)assigna(18)op(0)(14) (1)(15) (2)(16) (3)(17) (4)(18) (5)(19)

CH4.16 CSE244Declarations Using a global variable offset PRODUCTIONSemantic Rule P  M D { } M   {offset:=0 } D  id : T{ addtype(id.entry, T.type, offset) offset:=offset + T.width } T  char{T.type = char; T.width = 4; } T  integer {T.type = integer ; T.width = 4; } T  array [ num ] of T 1 {T.type=array(1..num.val,T 1.type) T.width = num.val * T 1.width} T  ^T 1 {T.type = pointer(T 1.type); T 1.width = 4}

CH4.17 CSE244 Keeping Track of Scope Information Consider the grammar fraction: P  D D  D ; D | id : T | proc id ; D ; S Each procedure should be allowed to use independent names. Nested procedures are allowed.

CH4.18 CSE244 Keeping Track of Scope Information (a translation scheme) P  M D{ addwidth(top(tblptr), top(offset)); pop(tblptr); pop(offset) } M   { t:=mktable(null); push(t, tblptr); push(0, offset)} D  D 1 ; D 2... D  proc id ; N D ; S{ t:=top(tblpr); addwidth(t,top(offset)); pop(tblptr); pop(offset); enterproc(top(tblptr), id.name, t)} N   {t:=mktable(top(tblptr)); push(t,tblptr); push(0,offset);} D  id : T {enter(top(tblptr), id.name, T.type, top(offset); top(offset):=top(offset) + T.width Example: proc func1; D; proc func2 D; S; S