Issues in Code Generation CPSC 388 Ellen Walker Hiram College

Address Calculation Variables –Look up the “address” in the symbol table Arrays & Structures –Look up base address, compute and add offset Pointers –Generate code for indirect reference

Address Calculation Tools in 3- Address Code Operators from C –&xaddress of x –*xvalue contained in location x Addressing Modes –None data as before (x) –Address address of data (&x) –Indirect pointer to data (*x)

3-Address Example: x=A[i] Assume: int A[i], sizeof(int) = 2 Allow the operator t = a[x] (array ref op) t1 = i * 2compute index t2 = &A + t1compute array addr t3 = *t2get the value x = t3assign to x

Address Calculation Tools in PCode IND x –Add x to top of stack, use result as an address of item to push onto the stack IXA x –Calculate address as (top)*x+(top-1), push address onto stack

Pcode Example: x=A[i] Assume: int A[i], sizeof(int) = 2 LDA xload address of x (for later LDA Aload address of A LOD iload value of i IXA 2calculate (&A+2*i) IND 0Load item at that address STOstore it in x

Pcode Generation for Arrays Add subscript ([]) operation to syntax tree (OpKind = subs) Add a parameter (isAddr) to the code generation function: –falsereturn the value of the expression –true return the address of the expression You get C-style multi-D arrays for free (arrays of arrays…)

New Case in GenCode case ‘[]’: //Array index cout first->name; Gen_code(t->first->next, false); cout first->name << “)”; if (!isAddr) cout << “IND 0” break;

Revised to allow Multi- dimensional Arrays case ‘[]’: //Array index //push address gen_code(t->first, true); //push index gen_code(t->first->next, false); cout first->name << “)”; if (!isAddr) cout << “IND 0” break;

Write Pcode for... A[i+1] = 2*A[i] x = A[y][z] Assume: –double A –sizeof(double) =4 –sizeof (double *) = 1

Class/Struct References Address of element is base + offset Base is address of class Offset is sum of sizes of all elements previous (offset of 1st item is 0) Pointer can be treated as struct with base 0, offset = pointer value!

Address computation Assume sizeof int=2, sizeof double=4 –class st{ int A1, double A2, double A3}; –St x; Addresses: –&(x.A1) = &x –&(x.A2) = &x + 2 –&(x.A3) = &x + 6

Offset computation Add function: field_offset(st,field) returns the integer offset of field from the beginning of st Computing the address of x.j –t1 = &x + field_offset (x,j)(3 addr) –LDA x; LOD field_offset(x,j); IXA 1 (Pcode)

Example (pointer & class) class tN{ int val; class tN *left; class tN *right; }; tN *p; p = p->right;

Pcode (p = p->right) LDA p//for storing result LOD p//push p (an address) IND fieldoffset(tN,right); //*(p + offset) STN// store in p

Generating Control Code All control statements can be constructed from IF and WHILE Two additional instructions are needed –FJPjump if falseconditional jump –UJPjump unconditional jump Need to be able to generate and assign labels to statements

Code for IF if else In pcode: –Code to compute and push E –FJP Label1 –Code to execute S1 –UJP Label 2 –Label1: code to execute S2 –Label2:

Code for While while In pcode: –Label1: Code to compute and push E –FJP Label2 –Code to execute S1 –UJP Label 1 –Label2: