LCC编译器的源程序分析(42)赋值表达式的有向无环图

 

上一次说到赋值表达式转换为有向无环图的函数listnodes，下面继续来分析这个函数代码。

当赋值树处理时，就运行下面的分支来处理：

#256  case ASGN: 

#257         {

#258               assert(tlab == 0 && flab == 0);

#259               if (tp->kids[0]->op == FIELD)

#260               {

#261                    Tree x = tp->kids[0]->kids[0];

#262                    Field f = tp->kids[0]->u.field;

#263                    assert(generic(x->op) == INDIR);

#264                    reset();

#265                     l = listnodes(lvalue(x), 0, 0);

#266 

#267                    if (fieldsize(f) < 8*f->type->size)

#268                    {

#269                          unsigned int fmask = fieldmask(f);

#270                          unsigned int mask = fmask<<fieldright(f);

#271                          Tree q = tp->kids[1];

#272                          if (q->op == CNST+I && q->u.v.i == 0

#273                               || q->op == CNST+U && q->u.v.u == 0)

#274                               q = bittree(BAND, x, cnsttree(unsignedtype, (unsigned long)~mask));

#275                          else if (q->op == CNST+I && (q->u.v.i&fmask) == fmask

#276                               ||       q->op == CNST+U && (q->u.v.u&fmask) == fmask)

#277                               q = bittree(BOR, x, cnsttree(unsignedtype, (unsigned long)mask));

#278                          else

#279                          {

#280                               listnodes(q, 0, 0);

#281                               q = bittree(BOR,

#282                                     bittree(BAND, rvalue(lvalue(x)),

#283                                     cnsttree(unsignedtype, (unsigned long)~mask)),

#284                                     bittree(BAND, shtree(LSH, cast(q, unsignedtype),

#285                                     cnsttree(unsignedtype, (unsigned long)fieldright(f))),

#286                                     cnsttree(unsignedtype, (unsigned long)mask)));

#287                          }

#288                          r = listnodes(q, 0, 0);

#289                          op = ASGN + ttob(q->type);

#290                    }

#291                    else

#292                    {

#293                          r = listnodes(tp->kids[1], 0, 0);

#294                          op = ASGN + ttob(tp->kids[1]->type);

#295                    }

#296 

#297               }

#298               else

#299               {

#300                     l = listnodes(tp->kids[0], 0, 0);

#301                    r = listnodes(tp->kids[1], 0, 0);

#302               }

#303 

第259行是处理结构里字段操作，在这个赋值表达式里没有字段操作，先把它放下。

第300行处理非字段操作的左子树，然后生成有向无环图的左子树，这里递归调用listnodes函数来实现下一层树，其实这里是采用后序遍历二叉树的算法来构造DAG图。

第301行递归处理右子树的节点。

 

#304               list(newnode(tp->op == ASGN+B ? tp->op : op, l, r, NULL));

#305               forest->syms[0] = intconst(tp->kids[1]->type->size);

#306               forest->syms[1] = intconst(tp->kids[1]->type->align);

#307              

#308               if (isaddrop(tp->kids[0]->op)

#309                    && !tp->kids[0]->u.sym->computed)

#310                    killnodes(tp->kids[0]->u.sym);

#311               else

#312                    reset();

#313 

#314               p = listnodes(tp->kids[1], 0, 0);

#315         }

#316         break;

第304行是创建一个DAG森林的根节点，保存在全局变量forest那里。

第305行和第306行设置符号属性。

第308行和第309行判断是否需要调用函数killnodes来删除公共表达式。如果不需要就直接调用函数reset复位。

第314行调用函数listnodes递归处理兄弟树节点。

…

上面是采用后序遍历的算法把所有树节点转换到DAG节点，并构造成有向无环图。下一次分析处理赋值树的几个类型操作树节点。