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.