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Code generation and data types

Code generation and data types. Translation and Address Calculation. Outline. Intermediate codes 3-address code P-code Code generation, using attribute grammar Data types Primitive data types Arrays and Strings Records Address calculation. Intermediate Codes. 3-address code P-code.

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Code generation and data types

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  1. Code generation and data types Translation and Address Calculation

  2. Outline • Intermediate codes • 3-address code • P-code • Code generation, using attribute grammar • Data types • Primitive data types • Arrays and Strings • Records • Address calculation 2301380 Chapter 6 Code Generation and Data Types

  3. Intermediate Codes 3-address code P-code 2301380 Chapter 6 Code Generation and Data Types

  4. Intermediate Code • Intermediate representation for programs • Why intermediate code • Reduce amount of work if optimization is done for intermediate code • Easy for retargeting compilers • Forms of intermediate code • Abstract syntax tree • Linearization of abstract syntax tree 2301380 Chapter 6 Code Generation and Data Types

  5. 3-address code • x = y op z • x, y, and z are addresses of • Variables (perhaps temporaries) • Locations in programs • y and z must be differed from x. • Labels can be assigned to a location. • Operators can be: • Arithmetic operators (3-address) • Relational operators (3-address) • Conditional operators (2-address) • If_false … goto … • Jump (1-address) • Input/Output • Halt 2301380 Chapter 6 Code Generation and Data Types

  6. Example: 3-address code read (x); if x>0 then { fact:=1; repeat { fact:=fact*x; x:=x-1; } until x==0; write(fact); } read x t1=x>0 if_false t1 goto L1 fact=1 label L2 t2=fact*x fact=t2 t3=x-1 x=t3 t4=x==0 if_false t4 goto L2 write fact label L1 halt 2301380 Chapter 6 Code Generation and Data Types

  7. P-code • Code for a hypothetical stack machine • For Pascal compilers • No variable name is required • Instructions • Load stack • Arithmetic and relational operators • Jumps • Operations are performed on topmost values on stack • 0-address or 1-address instructions 2301380 Chapter 6 Code Generation and Data Types

  8. P-code Instructions • Load: push stack • Load value • Load address • Load constant • Store: save top of stack in memory • Destructive store • Nondestructive store • Arithmetic operations • Add • Subtract • Multiply • Compare • Greater • Less • equal • Label • Jump • Unconditional jump • Conditional jump • I/O • Read • Write • Stop 2301380 Chapter 6 Code Generation and Data Types

  9. Example: P-code read (x); loada x read if x>0 then loadv x loadc 0 greater jumpONfalse L1 { fact:=1; loada fact loadc 1 store repeat Label L2 { fact:=fact*x; loada fact loadv fact loadv x mult store x:=x-1; loada x loadv x loadc 1 sub store } until x==0; loadv x loadc 0 equ jumpF L2 write(fact); loadv fact write } Label L1 stop 2301380 Chapter 6 Code Generation and Data Types

  10. Code Generation Using Attribute Grammar 2301380 Chapter 6 Code Generation and Data Types

  11. Code Generation Using Synthesized Attributes • An attribute is created for the sequence of characters representing generated code. • An attribute grammar is written to generate the intermediate/target code. • The value of the attribute is passed from child nodes up to their parent node to construct a larger chunk of code. 2301380 Chapter 6 Code Generation and Data Types

  12. Attribute Grammar: P-code 2301380 Chapter 6 Code Generation and Data Types

  13. Generating P-code: Example exp loada x loadv x loadc 3 adi stn loadc 4 adi aexp loada x loadv x loadc 3 adi stn loadc 4 factor aexp + loada x loadv x loadc 3 adi stn factor num loada x loadv x loadc 3 adi stn ) exp ( loadv x loadc 3 adi exp = id loadv x loadc 3 adi aexp loadc 3 loadv x + factor aexp loadv x num factor id 2301380 Chapter 6 Code Generation and Data Types

  14. Attribute Grammar: 3-address code 2301380 Chapter 6 Code Generation and Data Types

  15. Generating 3-address Code: Example t2 t1=x+3 x=t1 t2=t1+4 aexp t1 t1=x+3 x=t1 4 aexp factor + 4 t1 t1=x+3 x=t1 factor num t1 t1=x+3 x=t1 exp ) ( t1 t1=x+3 exp id = t1 t1=x+3 aexp x 3 + aexp factor x factor num id 2301380 Chapter 6 Code Generation and Data Types

  16. Code Generation: Tree Traversal proceduregenCode (T:node) { if (T != null) then { generate code to prepare for code of left child; genCode(T.leftChild); generate code to prepare for code of right child; genCode(T.rightChild); generate code to implement the action of T; } } 2301380 Chapter 6 Code Generation and Data Types

  17. Code Generation: P-code gencode(T:Tree) { if (T is not null) { switch (T.type) caseplusnode: { gencode(T.lchild); gencode(T.rchild); emitCode(“add”); } caseasgnnode: { emitcode(“loada”,T.strval); gencode(T.rchild); emitcode(“stn”); } caseconstnode: { emitcode(“loadc”,t.strval); } caseidnode: { emitcode(“loadv”,t.strval); } default: { emitcode(“error”); } } } 2301380 Chapter 6 Code Generation and Data Types

  18. Control Statements: code generation If statements While loops Logical expressions 2301380 Chapter 6 Code Generation and Data Types

  19. Code Generation for If Statements 3-address code P-code <code evaluating E and assigning to t1> if_false t1 goto L1 <code for S1> goto L2 label L1 <code for S2> label L2 <code evaluating E> jumpF L1 <code for S1> jump L2 label L1 <code for S2> label L2 IF ( E ) S1 ELSE S2 2301380 Chapter 6 Code Generation and Data Types

  20. Code Generation for While Loops 3-address code P-code label L1 <code evaluating E and assigning to t1> if_false t1 goto L2 <code for S> goto L1 label L2 label L1 <code evaluating E> jumpF L2 <code for S> jump L1 label L2 WHILE ( E ) S 2301380 Chapter 6 Code Generation and Data Types

  21. Generating Labels label L1 <code for E> jumpFL2 <code for S> jump L1 labelL2 • Forward jump • Label must be generated before defining the label • For intermediate code • Generate label at the jump instruction • Store the label until the actual location is found • For target code (assembly) • Leave the destination address in the jump instruction • When the destination address is found, go back and fill the address (backpatching) • Short jump or long jump? • Leave space enough for long jump • If only short jump is required, the extra space is filled with NOP. 2301380 Chapter 6 Code Generation and Data Types

  22. Code Generation for Logical Expressions • Data types and operators • Use Boolean data type and operators if included in the target/intermediate language • Use integer 0/1 and bitwise operators • Short-circuit evaluation • IF a AND b THEN S • If a is false, • the whole exp is false • there is no need to evaluate b • IF a OR b THEN S • If a is true, • the whole exp is true • there is no need to evaluate b • Intermediate code for IF a AND b THEN S <code for a> if_falseagoto L1 <code for b> if_falseb goto L1 <code for S> Label L1 • Intermediate code for IF a OR b THEN S <code for a> if_falseagoto L1 goto L2 Label L1 <code for b> if_falseb goto L3 Label L2 <code for S> Label L3 2301380 Chapter 6 Code Generation and Data Types

  23. Data Types Primitive data types Strings Arrays Records 2301380 Chapter 6 Code Generation and Data Types

  24. Primitive Data Types • Integer • Short, long, signed, unsigned • Floating Point • IEEE floating point format • Decimal • Boolean • Character • ASCII, Unicode 2301380 Chapter 6 Code Generation and Data Types

  25. Ordinal Types • Integers • User-defined ordinal types: range of possible values can be associated with I+ • Enumeration types • user-defined set of possible values. • Subrange types • Ordered contiguous subsequence of an ordinal type. 2301380 Chapter 6 Code Generation and Data Types

  26. Strings • Can be either • Primitive type: Ada, FORTRAN, Basic • Array: Pascal, C, C++ • Class: Java (String) • Operations • Assignment • Comparison • Catenation • Substring reference • Pattern Matching 2301380 Chapter 6 Code Generation and Data Types

  27. Descriptor for Strings • Descriptor is the information stored in a compiler. • Static string • Type: static string • Length • Address • Limited dynamic string • Type: dynamic string • Length: maximum and current length • Address 2301380 Chapter 6 Code Generation and Data Types

  28. Arrays • Types of subscripts • Implementation 2301380 Chapter 6 Code Generation and Data Types

  29. Subscripts Types of subscripts Range of subscripts • Integers • FORTRAN, C, Java • Ordinal types • Enumeration types • Pascal, Ada • Static • C, FORTRAN • Java Arrays • Dynamic • Ada • FORTRAN ALLOCATBLE • Perl, JavaScript • Java ArrayList 2301380 Chapter 6 Code Generation and Data Types

  30. Implementation • Number of subscripts • Most languages have no limit, but some versions of FORTRAN have. • Descriptor • Type: Array • Element type • Index type • Number of dimensions • Index ranges, for each dimension • Address 2301380 Chapter 6 Code Generation and Data Types

  31. Array: Address calculation s Address of A[i, j] = s + ((i-1)n + j)*size(elementType) 2301380 Chapter 6 Code Generation and Data Types

  32. Records • heterogeneous aggregate of data elements in which the individual elements are identified by names. • Example: record student { int id; char firstname[40]; char lastname[40]; float grade; } 2301380 Chapter 6 Code Generation and Data Types

  33. Implementation • Descriptor • Type: record • For each field: • Name, type, offset • Address • Address Calculation • S + field offset Field 1 Field n 2301380 Chapter 6 Code Generation and Data Types

  34. Address Calculation Addressing operations Array reference Record reference 2301380 Chapter 6 Code Generation and Data Types

  35. Addressing Operations 3-address code P-code • Address of x • &x • Indirect address • *x • Address of x • loada x • Indirect load • ind x • (load *(top+x)) • Indexed address • ixa x • (load top*x+(top-1)) 2301380 Chapter 6 Code Generation and Data Types

  36. Array References 3-address code P-code • x=a[i] t1=i*elesize(a) t2=&a+t1 x=*t2 • a[i]=x t1=i*elesize(a) t2=&a+t1 *t2=x • x=a[i] loada x loada a loadvi ixaelesize(a) ind 0 sto • a[i]=x loada a loadvi ixaelesize(a) loadv x sto Find offset Find address Find address Find content Find offset Find address Find address 2301380 Chapter 6 Code Generation and Data Types

  37. More Complex Array References 3-address code P- code • a[i+1]=a[j+2]+3 t1=j+2 t2=t1*elesize(a) t3=&a+t2 t4=*t3 t5=t4+3 t6=i+1 t7=t6*elesize(a) t8=&a+t7 *t8=t5 • a[i+1]=a[j+2]+3 loada a loadvi loadc 1 adi ixaelesize(a) loada a loadv j loadc 2 adi ixaelesize(a) ind 0 loadc 3 adi sto push &a[i+1] t4=a[t1] push a[j+2] a[t6]=t5 2301380 Chapter 6 Code Generation and Data Types

  38. Record References 3-address code P-code • Address of a field t1=&x+offset(x,j) • Content of a field t1=&x+offset(x,j) t2=*t1 • Address of a field loada x loadc offset(x.j) ixa 1 • Content of a field loada x ind offset(x.j) x.j x.c Offset of x.j x.i base address of x Offset of x.c 2301380 Chapter 6 Code Generation and Data Types

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