Just for the record:
$ cat sizer.c
#include <stdio.h>
int main() {
printf(" sizeof(long long) = %2u
__alignof__(long long) = %2u\n", (unsigned) sizeof(long long), (unsigned)
__alignof__(long long));
printf(" sizeof(struct {long long N;}) = %2u __alignof__(struct {long
long N;}) = %2u\n", (unsigned) sizeof(struct {long long N;}), (unsigned)
__alignof__(struct {long long N;}));
return 0;
}
$ gcc -Wall -o sizer sizer.c
$ ./sizer
sizeof(long long) = 8 __alignof__(long long) = 8
sizeof(struct {long long N;}) = 8 __alignof__(struct {long long N;}) = 4
Yes, this was a surprise.
I knew that structures can cause padding to be introduced, and arrays can have
higher alignment constraints than their elements.
But lowering alignment constraints?
Is __alignof__() telling the truth here?
Or is it giving a truthful less-than-useful answer?
(Because what matters to MoarVM is the alignment of, and therefor padding
needed for, structure members)
Nicholas Clark
