Yinan Zhang c9d56cddf2 Optimize meld in qr module
The goal of `qr_meld()` is to change the following four fields
`(a->prev, a->prev->next, b->prev, b->prev->next)` from the values
`(a->prev, a, b->prev, b)` to `(b->prev, b, a->prev, a)`.

This commit changes

```
a->prev->next = b;
b->prev->next = a;
temp = a->prev;
a->prev = b->prev;
b->prev = temp;
```

to

```
temp = a->prev;
a->prev = b->prev;
b->prev = temp;
a->prev->next = a;
b->prev->next = b;
```

The benefit is that we can use `b->prev->next` for `temp`, and so
there's no need to pass in `a_type`.

The restriction is that `b` cannot be a `qr_next()` macro, so users
of `qr_meld()` must pay attention.  (Before this change, neither `a`
nor `b` could be a `qr_next()` macro.)
2020-04-06 09:50:27 -07:00
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