http://gcc.gnu.org/bugzilla/show_bug.cgi?id=47557
Summary: Effect of aligned attribute on arrays
Product: gcc
Version: unknown
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
AssignedTo: [email protected]
ReportedBy: [email protected]
The following test case demonstrates what the effect aligned on the size of
arrays:
typedef __attribute__((aligned(2))) struct {
char a[3];
} T;
unsigned x1 = sizeof(T); // sizeof is 3
unsigned x2 = sizeof(T[1]); // sizeof is 4
unsigned x3 = sizeof(T[2]); // sizeof is 6
unsigned x4 = sizeof(T[2][1]); // sizeof is 8
unsigned x5 = sizeof(T[1][2]); // sizeof is 6
The result is that type 'T' is not aligned by the array-of-T is.
Padding arrays seems like a mistake, I couldn't find any mention in the
standards about padding in arrays. C++ 5.3.3p2 (expr.sizeof) says:
"When applied to an array, the result is the total number of bytes in the
array. This implies that the size of an array of n elements is n times the size
of an element."
C99 6.5.3.4p6 (sizeof) implies the same with "Another use of the sizeof
operator is to compute the number of elements in an array: sizeof array /
sizeof array[0]"
Note that in both standards, in those sections they specifically mention
padding for classes/structures.
IMO the reasonable thing is to fully respect the alignment of T and pad it to 4
bytes so that the elements in array-of-T are aligned, e.g. second element of
array-of-T is not aligned as it is now.
So, to sum up, I think the test case should be like this:
typedef __attribute__((aligned(2))) struct {
char a[3];
} T;
unsigned x1 = sizeof(T); // sizeof is 4
unsigned x2 = sizeof(T[1]); // sizeof is 4
unsigned x3 = sizeof(T[2]); // sizeof is 8
unsigned x4 = sizeof(T[2][1]); // sizeof is 8
unsigned x5 = sizeof(T[1][2]); // sizeof is 8
