https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114344
Bug ID: 114344
Summary: [arm/mips] __alignof__ report a member packed struct
as 1, while normal load/store instruction is used
Product: gcc
Version: unknown
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: target
Assignee: unassigned at gcc dot gnu.org
Reporter: syq at gcc dot gnu.org
Target Milestone: ---
#include <stdio.h>
volatile
struct s
{
char c[4];
int i;
long long l;
float f;
double d;
} __attribute__ ((packed)) s;
int main() {
s.i = 0x12345678;
printf ("%zd\n", __alignof__ (s.i));
}
For this code, `1` is printed as the align of s.i. While on MIPS and ARM,
normal instructions are emitted.
$ mipsel-linux-gnu-gcc -Wall -mabi=32 -c -O3 xx.c -mips32r2
$ objdump -d xx.o
...
34: ac620004 sw v0,4(v1)
...
If `__alignof__ (s.i)` reports correctly, `swl/swr` should be used instead of
`sw`.
And for
$ mipsel-linux-gnu-gcc -c -O3 xx.c -mips32r6 -mno-unaligned-access
$ arm-linux-gnueabihf-gcc -c -O3 xx.c -mno-unaligned-access
4 store_byte instruction should be used, while in fact, `sw/str` is used here.
