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lib: use assume() for SA fixing, add explainer

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Literally 4 minutes after hitting merge on Mark's previous fix for this
I remembered we have an `assume()` macro in compiler.h which seems like
the right tool for this.

(... and if I'm touching it, I might as well add a little text
explaining what's going on here.)

Signed-off-by: David Lamparter <equinox@opensourcerouting.org>
This commit is contained in:
David Lamparter 2022-07-06 10:48:56 +02:00
parent 5adddad38f
commit 255a76a56a
1 changed files with 20 additions and 2 deletions
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22
lib/typesafe.h
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@ -308,9 +308,27 @@ struct dlist_head {
static inline void typesafe_dlist_add(struct dlist_head *head,
struct dlist_item *prev, struct dlist_item *item)
{
/* SA on clang-11 thinks this can happen, but in reality -assuming no
* memory corruption- it can't. DLIST uses a "closed" ring, i.e. the
* termination at the end of the list is not NULL but rather a pointer
* back to the head. (This eliminates special-casing the first or last
* item.)
*
* Sadly, can't use assert() here since the libfrr assert / xref code
* uses typesafe lists itself... that said, if an assert tripped here
* we'd already be way past some memory corruption, so we might as
* well just take the SEGV. (In the presence of corruption, we'd see
* random SEGVs from places that make no sense at all anyway, an
* assert might actually be a red herring.)
*
* ("assume()" tells the compiler to produce code as if the condition
* will always hold; it doesn't have any actual effect here, it'll
* just SEGV out on "item->next->prev = item".)
*/
assume(prev->next != NULL);
item->next = prev->next;
if (item->next)
item->next->prev = item;
item->next->prev = item;
item->prev = prev;
prev->next = item;
head->count++;