save_reg

把r寄存器保存在栈上,  释放寄存器

/* save r to the memory stack, and mark it as being free */
ST_FUNC void save_reg(int r)
{
    int l, saved, size, align;
    SValue *p, sv;
    CType *type;

    /* modify all stack values */
    saved = 0;
    l = 0;
    for(p=vstack;p<=vtop;p++) {
        if ((p->r & VT_VALMASK) == r ||
            ((p->type.t & VT_BTYPE) == VT_LLONG && (p->r2 & VT_VALMASK) == r)) {
            确实有在被使用才会真正的保存，否则保存备有意义
            /* must save value on stack if not already done */
            if (!saved) {
                /* NOTE: must reload 'r' because r might be equal to r2 */
                r = p->r & VT_VALMASK;
                /* store register in the stack */
                type = &p->type;
                if ((p->r & VT_LVAL) ||
                    (!is_float(type->t) && (type->t & VT_BTYPE) != VT_LLONG))
#ifdef TCC_TARGET_X86_64
                    type = &char_pointer_type;
#else
                    type = &int_type;
#endif
                用于计算大小，从而计算放在栈上的什么位置上，loc标记栈分配的位置
                size = type_size(type, &align);
                loc = (loc - size) & -align;
                sv.type.t = type->t;
                VT_LVAL是多余的
                sv.r = VT_LOCAL | VT_LVAL;
                sv.c.ul = loc;
                store 函数生成指令保存
                store(r, &sv);
                x86 浮点寄存器形成栈，释放栈顶st0
#if defined(TCC_TARGET_I386) || defined(TCC_TARGET_X86_64)
                /* x86 specific: need to pop fp register ST0 if saved */
                if (r == TREG_ST0) {
                    o(0xd8dd); /* fstp %st(0) */
                }
#endif
#ifndef TCC_TARGET_X86_64
                /* special long long case */
                if ((type->t & VT_BTYPE) == VT_LLONG) {
                    sv.c.ul += 4;
                    store(p->r2, &sv);
                }
#endif
                l = loc;
                saved = 1;
            }
            释放寄存器, 在需要时应该重新分配寄存器并load
            /* mark that stack entry as being saved on the stack */
            if (p->r & VT_LVAL) {
                /* also clear the bounded flag because the
                   relocation address of the function was stored in
                   p->c.ul */
                p->r = (p->r & ~(VT_VALMASK | VT_BOUNDED)) | VT_LLOCAL;
            } else {
                p->r = lvalue_type(p->type.t) | VT_LOCAL;
            }

            p->r2 = VT_CONST;
            p->c.ul = l;
        }
    }
}