https://gcc.gnu.org/bugzilla/show_bug.cgi?id=116016
Bug ID: 116016
Summary: enhancement: add __builtin_set_counted_by(P->FAM,
COUNT) or equivalent
Product: gcc
Version: 15.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
Assignee: unassigned at gcc dot gnu.org
Reporter: kees at outflux dot net
Target Milestone: ---
With the wonderful addition of the 'counted_by' attribute and its wide roll-out
within the Linux kernel, we have found a use case that would be very nice to
have for object allocators: being able to set the counted_by counter variable
without knowing its name.
For example, given:
struct foo {
...
int counter;
...
struct bar array[] __attribute__((counted_by(counter)));
} *p;
one thought was to have __builtin_set_counted_by(P->FAM, COUNT), which would
have the behavior of:
if has_counted_by_attribute(P->FAM):
P->COUNT_MEMBER = COUNT
else
do nothing
The existing Linux object allocators are roughly:
#define alloc(P, FAM, COUNT) ({ \
size_t __size = sizeof(*P) + sizeof(*P->FAM) * COUNT; \
kmalloc(__size, GFP); \
})
Right now, any addition of a counted_by annotation must also include an
open-coded assignment of the counter variable after the allocation:
p = alloc(p, array, how_many);
p->counter = how_many;
Instead, the central allocator could be updated to:
#define alloc(P, FAM, COUNT) ({ \
typeof(P) __p; \
size_t __size = sizeof(*P) + sizeof(*P->FAM) * COUNT; \
__p = kmalloc(__size, GFP); \
__builtin_set_counted_by(__p->FAM, COUNT); \
__p; \
})
And now structs can gain the counted_by attribute without needing additional
open-coded counter assignments for each struct, and unannotated structs could
still use the same allocator. (i.e. we are able to more cleanly continue to
migrate FAM structs.)
