.net core 的夸代扫描标记card_table的细节分析

夸代扫描，比如说回收短暂代，但是在短暂代当中有一个被一代引用的变量。

这个时候可能会造成了这个变量被回收，程序发生不可与之错误。card table 就是为了预防这种情况的一个数组。

int p = 0;
int* card_table= &p;
card_table[0] = 1;
card_table[1] = 2;
card_table[2] = 3;

card_table表在的索引位 card/32,而card为 2的8次方。而值只有两个为零或者为1，4个字节的0xFFFF的宽度

BOOL gc_heap::find_card(uint32_t* card_table,
size_t& card,
size_t card_word_end,
size_t& end_card)
{
uint32_t* last_card_word;
uint32_t card_word_value;
uint32_t bit_position;

// Find the first card which is set
last_card_word = &card_table [card_word (card)]; // 找到card_table当前索引的地址值，实质上也就是实际地址除以2的13次方之后的值
bit_position = card_bit (card); // 求余数获取到偏移的bit位
card_word_value = (*last_card_word) >> bit_position;// 这个实质上是在set_card的时候复原原值的一个过程card_table[word] = (card_table[word] | (1 << card_bit(card)));
if (!card_word_value)// 如果当前card_table为零，意思是这个宽度为FFFF的位并没有被跨代记录
{
bit_position = 0;
#ifdef CARD_BUNDLE // card 束
size_t lcw = card_word(card) + 1;// 把实际地址值除以2的13次方之后，加1，挪向下一个地址值
if (gc_heap::find_card_dword (lcw, card_word_end) == FALSE)// 查找lcw和card_word_end之间是否有被跨代表及
{
return FALSE;//没有返回
}
else //有的话
{
last_card_word = &card_table [lcw];// 设置
card_word_value = *last_card_word;// 设置
}

#else //CARD_BUNDLE.//没有定义card bundle
do // 循环实际地址值
{
++last_card_word;
}
while ((last_card_word < &card_table [card_word_end]) && !(*last_card_word)); // 当找到一个被跨代标记的

if (last_card_word < &card_table [card_word_end])
{
card_word_value = *last_card_word;
}
else
{
// We failed to find any non-zero card words before we got to card_word_end
return FALSE;
}
#endif //CARD_BUNDLE
}

// Look for the lowest bit set
if (card_word_value)
{
while (!(card_word_value & 1))
{
bit_position++;
card_word_value = card_word_value / 2;
}
}

// card is the card word index * card size + the bit index within the card
card = (last_card_word - &card_table[0]) * card_word_width + bit_position;

do
{
// Keep going until we get to an un-set card.
bit_position++;
card_word_value = card_word_value / 2;

// If we reach the end of the card word and haven't hit a 0 yet, start going
// card word by card word until we get to one that's not fully set (0xFFFF...)
// or we reach card_word_end.
if ((bit_position == card_word_width) && (last_card_word < &card_table [card_word_end]))
{
do
{
card_word_value = *(++last_card_word);
} while ((last_card_word < &card_table [card_word_end]) &&

#ifdef _MSC_VER
(card_word_value == (1 << card_word_width)-1)
#else
// if left shift count >= width of type,
// gcc reports error.
(card_word_value == ~0u)
#endif // _MSC_VER
);
bit_position = 0;
}
} while (card_word_value & 1);

end_card = (last_card_word - &card_table [0])* card_word_width + bit_position;

//dprintf (3, ("find_card: [%Ix, %Ix[ set", card, end_card));
dprintf (3, ("fc: [%Ix, %Ix[", card, end_card));
return TRUE;
}