[Bug c++/87373] New: Packed structs are not handled properly on ARM architecture even with misaligned access is enabled

[murat.ursavas at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=87373

            Bug ID: 87373
           Summary: Packed structs are not handled properly on ARM
                    architecture even with misaligned access is enabled
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: murat.ursavas at gmail dot com
  Target Milestone: ---

Created attachment 44731
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=44731&action=edit
Minimum Test Case

Hi,

I've faced a weird behavior on my ARM MCU about a year ago and reported it on
the launchpad page. In the meantime I tried to report the issue at here but
your register process is longer than expected and here I'm reporting this issue
almost a year later.

We'd discussed the issue on this page:

https://bugs.launchpad.net/gcc-arm-embedded/+bug/1738730

I think this looks like a bug, but with my limited internal GCC knowledge, I
don't want to be rude and call it like that directly.

If I should summarize the discussion;

The GCC versions from 5.2 to 7, the compiler generates unexpected code while
handling packed structs. Normally the behavior should be like 4.9 but somehow
it got broken since then. Misaligned access should help but this does not seem
to be the case.

I've attached a minimum example which reproduces the issue.

About the seriousness; I can not update my compiler and still use 4.9 for our
products. This is a annoying drawback for me because I know GCC has received
many improvements since then. The IDE which I use is still deployed with 4.9
and this is unacceptable from my point of view.

MCU Core: ARM Cortex M3