［转载］LCC编译器的源程序分析(46)计算需要使用栈大小

从目标代码里，可以看到下面一行：

#009 sub esp, 16

在这行里是保留栈的大小，值为16。但16个字节是怎么样计算出来的呢？下面就来分析LCC的代码，看它是怎么样计算的。它是在函数gencode里进行计算的，它的代码如下：

#001 void gencode(Symbol caller[], Symbol callee[])

#002 {

#003     Code cp;

#004     Coordinate save;

#005 

#006     if (prunetemps == -1)

#007          prunetemps = !IR->wants_dag;

#008     save = src;

#009     if (assignargs)

#010     {

#011          int i;

#012          Symbol p, q;

#013          cp = codehead.next->next;

#014          codelist = codehead.next;

#015          for (i = 0; (p = callee[i]) != NULL

#016                && (q = caller[i]) != NULL; i++)

#017                if (p->sclass != q->sclass || p->type != q->type)

#018                     walk(asgn(p, idtree(q)), 0, 0);

#019          codelist->next = cp;

#020          cp->prev = codelist;

#021     }

#022 

#023     if (glevel && IR->stabsym)

#024     {

#025          int i;

#026          Symbol p, q;

#027          for (i = 0; (p = callee[i]) != NULL

#028                && (q = caller[i]) != NULL; i++)

#029          {

#030                (*IR->stabsym)(p);

#031                if (p->sclass != q->sclass || p->type != q->type)

#032                     (*IR->stabsym)(q);

#033          }

#034          swtoseg(CODE);

#035     }

#036 

#037     cp = codehead.next;

#038     for ( ; errcnt <= 0 && cp; cp = cp->next)

#039          switch (cp->kind)

#040     {

#041          case Address: assert(IR->address);

#042                (*IR->address)(cp->u.addr.sym, cp->u.addr.base,

#043                     cp->u.addr.offset); break;

#044          case Blockbeg:

#045                {

#046                      Symbol *p = cp->u.block.locals;

#047                     (*IR->blockbeg)(&cp->u.block.x);

#048                     for ( ; *p; p++)

#049                           if ((*p)->ref != 0.0)

#050                                 (*IR->local)(*p);

#051                           else if (glevel) (*IR->local)(*p);

#052                }

#053                break;

#054          case Blockend: (*IR->blockend)(&cp->u.begin->u.block.x); break;

#055          case Defpoint: src = cp->u.point.src; break;

#056          case Gen: case Jump:

#057          case Label:   

#058                if (prunetemps)

#059                     cp->u.forest = prune(cp->u.forest);

#060                fixup(cp->u.forest);

#061                cp->u.forest = (*IR->gen)(cp->u.forest); break;

#062          case Local:   

#063                (*IR->local)(cp->u.var); break;

#064          case Switch:   break;

#065          default: assert(0);

#066     }

#067     src = save;

#068 }

#069 

计算栈大小的代码是从44行开始到第53行为止。第44行是处理块代码的开始类型。

第46行是获取当前代码块的符号表。

第47行是清空环境变量。

第48行到第51行是循环地计算所有变量的需要使用栈大小。后面再接分析怎么样计算变量占用栈的大小。