[Bug c/98502] New: Optimised memcpy breaks __scalar_storage_order__

[noring at nocrew dot org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98502

            Bug ID: 98502
           Summary: Optimised memcpy breaks __scalar_storage_order__
           Product: gcc
           Version: 9.3.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: noring at nocrew dot org
  Target Milestone: ---

Compiling with -O1 or greater breaks __scalar_storage_order__ with memcpy. The
expected output of the test program below (sso.c) is "1234" but "3412" is
produced with compiler optimisations:

% gcc -Wall -o sso sso.c && ./sso
1234
% gcc -Wall -O1 -o sso sso.c && ./sso
3412

sso.c:

#include <stdint.h>
#include <stdio.h>
#include <string.h>

int main()
{
        const uint8_t b[] = { 0x12, 0x34 };
        struct __attribute__(( __scalar_storage_order__("big-endian") )) {
                uint16_t word;
        } s;

        memcpy(&s, b, sizeof(s));

        printf("%04x\n", s.word);
}