https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91672
Bug ID: 91672
Summary: wrong amount of storage allocated for initialized
structs with flexible array members
Product: gcc
Version: 9.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
Assignee: unassigned at gcc dot gnu.org
Reporter: msebor at gcc dot gnu.org
Target Milestone: ---
Even though GCC takes advantage of tail padding when laying out structures with
flexible array members, in C mode it allocates excess storage for statically
initialized objects of such structs.
In the test case below, thanks to tail padding, sizeof (struct A) is
sufficiently large to store each of a0 through a3. But as the assembly output
shows, the variables occupy more storage than necessary.
$ cat a.c && gcc -O2 -S -Wall -Wextra -o /dev/stdout -xc a.c | grep -e \.size
struct A
{
__INT64_TYPE__ i64;
__INT16_TYPE__ i16;
__INT16_TYPE__ a16[];
};
struct A a0 = { 0, 1 };
struct A a1 = { 1, 1, { 1 } };
struct A a2 = { 2, 1, { 1, 2 } };
struct A a3 = { 3, 1, { 1, 2, 3 } };
struct A a4 = { 4, 1, { 1, 2, 3, 4 } };
.size a4, 24
.size a3, 22
.size a2, 20
.size a1, 18
.size a0, 16
G++ on the other hand, allocates the same amount of space regardless of the
size of the initializer:
$ gcc -O2 -S -Wall -Wextra -o /dev/stdout -xc++ a.c | grep -e \.size
.size a4, 16
.size a3, 16
.size a2, 16
.size a1, 16
.size a0, 16
Clang allocates just the right amount, i.e., 16 bytes for a0 through a3 and 24
bytes for a4.
