gen.c

#include "defs.h"
#include "data.h"
#include "decl.h"

// Generic code generator
// Copyright (c) 2019 Warren Toomey, GPL3

// Generate and return a new label number
int genlabel(void) {
  static int id = 1;
  return (id++);
}

// Generate the code for an IF statement
// and an optional ELSE clause
static int genIF(struct ASTnode *n, int looptoplabel, int loopendlabel) {
  int Lfalse, Lend;

  // Generate two labels: one for the
  // false compound statement, and one
  // for the end of the overall IF statement.
  // When there is no ELSE clause, Lfalse _is_
  // the ending label!
  Lfalse = genlabel();
  if (n->right)
    Lend = genlabel();

  // Generate the condition code followed
  // by a jump to the false label.
  genAST(n->left, Lfalse, NOLABEL, NOLABEL, n->op);
  genfreeregs();

  // Generate the true compound statement
  genAST(n->mid, NOLABEL, looptoplabel, loopendlabel, n->op);
  genfreeregs();

  // If there is an optional ELSE clause,
  // generate the jump to skip to the end
  if (n->right)
    cgjump(Lend);

  // Now the false label
  cglabel(Lfalse);

  // Optional ELSE clause: generate the
  // false compound statement and the
  // end label
  if (n->right) {
    genAST(n->right, NOLABEL, NOLABEL, NOLABEL, n->op);
    genfreeregs();
    cglabel(Lend);
  }

  return (NOREG);
}

// Generate the code for a WHILE statement
static int genWHILE(struct ASTnode *n) {
  int Lstart, Lend;

  // Generate the start and end labels
  // and output the start label
  Lstart = genlabel();
  Lend = genlabel();
  cglabel(Lstart);

  // Generate the condition code followed
  // by a jump to the end label.
  genAST(n->left, Lend, Lstart, Lend, n->op);
  genfreeregs();

  // Generate the compound statement for the body
  genAST(n->right, NOLABEL, Lstart, Lend, n->op);
  genfreeregs();

  // Finally output the jump back to the condition,
  // and the end label
  cgjump(Lstart);
  cglabel(Lend);
  return (NOREG);
}

// Generate the code for a SWITCH statement
static int genSWITCH(struct ASTnode *n) {
  int *caseval, *caselabel;
  int Ljumptop, Lend;
  int i, reg, defaultlabel = 0, casecount = 0;
  struct ASTnode *c;

  // Create arrays for the case values and associated labels.
  // Ensure that we have at least one position in each array.
  caseval = (int *) malloc((n->intvalue + 1) * sizeof(int));
  caselabel = (int *) malloc((n->intvalue + 1) * sizeof(int));

  // Generate labels for the top of the jump table, and the
  // end of the switch statement. Set a default label for
  // the end of the switch, in case we don't have a default.
  Ljumptop = genlabel();
  Lend = genlabel();
  defaultlabel = Lend;

  // Output the code to calculate the switch condition
  reg = genAST(n->left, NOLABEL, NOLABEL, NOLABEL, 0);
  cgjump(Ljumptop);
  genfreeregs();

  // Walk the right-child linked list to
  // generate the code for each case
  for (i = 0, c = n->right; c != NULL; i++, c = c->right) {

    // Get a label for this case. Store it
    // and the case value in the arrays.
    // Record if it is the default case.
    caselabel[i] = genlabel();
    caseval[i] = c->intvalue;
    cglabel(caselabel[i]);
    if (c->op == A_DEFAULT)
      defaultlabel = caselabel[i];
    else
      casecount++;

    // Generate the case code. Pass in the end label for the breaks
    genAST(c->left, NOLABEL, NOLABEL, Lend, 0);
    genfreeregs();
  }

  // Ensure the last case jumps past the switch table
  cgjump(Lend);

  // Now output the switch table and the end label.
  cgswitch(reg, casecount, Ljumptop, caselabel, caseval, defaultlabel);
  cglabel(Lend);
  return (NOREG);
}

// Generate the code to copy the arguments of a
// function call to its parameters, then call the
// function itself. Return the register that holds 
// the function's return value.
static int gen_funccall(struct ASTnode *n) {
  struct ASTnode *gluetree = n->left;
  int reg;
  int numargs = 0;

  // If there is a list of arguments, walk this list
  // from the last argument (right-hand child) to the
  // first
  while (gluetree) {
    // Calculate the expression's value
    reg = genAST(gluetree->right, NOLABEL, NOLABEL, NOLABEL, gluetree->op);
    // Copy this into the n'th function parameter: size is 1, 2, 3, ...
    cgcopyarg(reg, gluetree->size);
    // Keep the first (highest) number of arguments
    if (numargs == 0)
      numargs = gluetree->size;
    genfreeregs();
    gluetree = gluetree->left;
  }

  // Call the function, clean up the stack (based on numargs),
  // and return its result
  return (cgcall(n->sym, numargs));
}

// Given an AST, an optional label, and the AST op
// of the parent, generate assembly code recursively.
// Return the register id with the tree's final value.
int genAST(struct ASTnode *n, int iflabel, int looptoplabel,
	   int loopendlabel, int parentASTop) {
  int leftreg, rightreg;

  // We have some specific AST node handling at the top
  // so that we don't evaluate the child sub-trees immediately
  switch (n->op) {
    case A_IF:
      return (genIF(n, looptoplabel, loopendlabel));
    case A_WHILE:
      return (genWHILE(n));
    case A_SWITCH:
      return (genSWITCH(n));
    case A_FUNCCALL:
      return (gen_funccall(n));
    case A_GLUE:
      // Do each child statement, and free the
      // registers after each child
      if (n->left != NULL) genAST(n->left, iflabel, looptoplabel, loopendlabel, n->op);
      genfreeregs();
      if (n->right != NULL) genAST(n->right, iflabel, looptoplabel, loopendlabel, n->op);
      genfreeregs();
      return (NOREG);
    case A_FUNCTION:
      // Generate the function's preamble before the code
      // in the child sub-tree
      cgfuncpreamble(n->sym);
      genAST(n->left, NOLABEL, NOLABEL, NOLABEL, n->op);
      cgfuncpostamble(n->sym);
      return (NOREG);
  }

  // General AST node handling below

  // Get the left and right sub-tree values
  if (n->left)
    leftreg = genAST(n->left, NOLABEL, NOLABEL, NOLABEL, n->op);
  if (n->right)
    rightreg = genAST(n->right, NOLABEL, NOLABEL, NOLABEL, n->op);

  switch (n->op) {
    case A_ADD:
      return (cgadd(leftreg, rightreg));
    case A_SUBTRACT:
      return (cgsub(leftreg, rightreg));
    case A_MULTIPLY:
      return (cgmul(leftreg, rightreg));
    case A_DIVIDE:
      return (cgdiv(leftreg, rightreg));
    case A_AND:
      return (cgand(leftreg, rightreg));
    case A_OR:
      return (cgor(leftreg, rightreg));
    case A_XOR:
      return (cgxor(leftreg, rightreg));
    case A_LSHIFT:
      return (cgshl(leftreg, rightreg));
    case A_RSHIFT:
      return (cgshr(leftreg, rightreg));
    case A_EQ:
    case A_NE:
    case A_LT:
    case A_GT:
    case A_LE:
    case A_GE:
      // If the parent AST node is an A_IF or A_WHILE, generate
      // a compare followed by a jump. Otherwise, compare registers
      // and set one to 1 or 0 based on the comparison.
      if (parentASTop == A_IF || parentASTop == A_WHILE)
	return (cgcompare_and_jump(n->op, leftreg, rightreg, iflabel));
      else
	return (cgcompare_and_set(n->op, leftreg, rightreg));
    case A_INTLIT:
      return (cgloadint(n->intvalue, n->type));
    case A_STRLIT:
      return (cgloadglobstr(n->intvalue));
    case A_IDENT:
      // Load our value if we are an rvalue
      // or we are being dereferenced
      if (n->rvalue || parentASTop == A_DEREF) {
	if (n->sym->class == C_GLOBAL) {
	  return (cgloadglob(n->sym, n->op));
	} else {
	  return (cgloadlocal(n->sym, n->op));
	}
      } else
	return (NOREG);
    case A_ASSIGN:
      // Are we assigning to an identifier or through a pointer?
      switch (n->right->op) {
	case A_IDENT:
	  if (n->right->sym->class == C_GLOBAL)
	    return (cgstorglob(leftreg, n->right->sym));
	  else
	    return (cgstorlocal(leftreg, n->right->sym));
	case A_DEREF:
	  return (cgstorderef(leftreg, rightreg, n->right->type));
	default:
	  fatald("Can't A_ASSIGN in genAST(), op", n->op);
      }
    case A_WIDEN:
      // Widen the child's type to the parent's type
      return (cgwiden(leftreg, n->left->type, n->type));
    case A_RETURN:
      cgreturn(leftreg, Functionid);
      return (NOREG);
    case A_ADDR:
      return (cgaddress(n->sym));
    case A_DEREF:
      // If we are an rvalue, dereference to get the value we point at,
      // otherwise leave it for A_ASSIGN to store through the pointer
      if (n->rvalue)
	return (cgderef(leftreg, n->left->type));
      else
	return (leftreg);
    case A_SCALE:
      // Small optimisation: use shift if the
      // scale value is a known power of two
      switch (n->size) {
	case 2:
	  return (cgshlconst(leftreg, 1));
	case 4:
	  return (cgshlconst(leftreg, 2));
	case 8:
	  return (cgshlconst(leftreg, 3));
	default:
	  // Load a register with the size and
	  // multiply the leftreg by this size
	  rightreg = cgloadint(n->size, P_INT);
	  return (cgmul(leftreg, rightreg));
      }
    case A_POSTINC:
    case A_POSTDEC:
      // Load and decrement the variable's value into a register
      // and post increment/decrement it
      if (n->sym->class == C_GLOBAL)
	return (cgloadglob(n->sym, n->op));
      else
	return (cgloadlocal(n->sym, n->op));
    case A_PREINC:
    case A_PREDEC:
      // Load and decrement the variable's value into a register
      // and pre increment/decrement it
      if (n->left->sym->class == C_GLOBAL)
	return (cgloadglob(n->left->sym, n->op));
      else
	return (cgloadlocal(n->left->sym, n->op));
    case A_NEGATE:
      return (cgnegate(leftreg));
    case A_INVERT:
      return (cginvert(leftreg));
    case A_LOGNOT:
      return (cglognot(leftreg));
    case A_TOBOOL:
      // If the parent AST node is an A_IF or A_WHILE, generate
      // a compare followed by a jump. Otherwise, set the register
      // to 0 or 1 based on it's zeroeness or non-zeroeness
      return (cgboolean(leftreg, parentASTop, iflabel));
    case A_BREAK:
      cgjump(loopendlabel);
      return (NOREG);
    case A_CONTINUE:
      cgjump(looptoplabel);
      return (NOREG);
    default:
      fatald("Unknown AST operator", n->op);
  }
  return (NOREG);		// Keep -Wall happy
}

void genpreamble() {
  cgpreamble();
}
void genpostamble() {
  cgpostamble();
}
void genfreeregs() {
  freeall_registers();
}
void genglobsym(struct symtable *node) {
  cgglobsym(node);
}
int genglobstr(char *strvalue) {
  int l = genlabel();
  cgglobstr(l, strvalue);
  return (l);
}
int genprimsize(int type) {
  return (cgprimsize(type));
}
int genalign(int type, int offset, int direction) {
  return (cgalign(type, offset, direction));
}