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

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

[noring at nocrew dot org]

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);
}

[Bug middle-end/98502] Optimised memcpy breaks __scalar_storage_order__

[pinskia at gcc dot gnu.org]

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

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |wrong-code
          Component|c                           |middle-end
             Status|UNCONFIRMED                 |RESOLVED
   Target Milestone|---                         |11.0
         Resolution|---                         |FIXED

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
I reported this in April 2020 (but not to a bug report because I did not know
if it was a bug or not):
https://gcc.gnu.org/pipermail/gcc/2020-April/000407.html

Fixed in GCC 11 by the patch in the referenced mailing list threads.
https://gcc.gnu.org/pipermail/gcc-patches/2020-June/546934.html
which continued the discussion to:
https://gcc.gnu.org/pipermail/gcc-patches/2020-July/549596.html

[Bug middle-end/98502] Optimised memcpy breaks __scalar_storage_order__

[noring at nocrew dot org]

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

--- Comment #2 from Fredrik Noring <noring at nocrew dot org> ---
Thanks, Andrew. I felt certain it's a GCC bug. :) Will the patch be backported
to GCC 9.x and 10.x? It'll take a while before 11 is commonly available, I
think.