[Bug rtl-optimization/99847] New: Optimization breaks alignment on CPU32

["m.frohiky at gmail dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99847

            Bug ID: 99847
           Summary: Optimization breaks alignment on CPU32
           Product: gcc
           Version: 10.2.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: rtl-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: m.frohiky at gmail dot com
  Target Milestone: ---

For CPU32 architecture optimizes two byte accesses into a single word access,
without any regard that the original array may be unaligned.

So this code:

void ntoh(const uint8_t *idata, uint16_t *odata) {
    *odata = ((uint16_t)idata[0] << 8) | idata[1];
}

Compiled with -Os or -O2 produces:

        move.l 4(%sp),%a1
        move.l 8(%sp),%a0
        move.w (%a1),(%a0)
        rts

And if idata (address in register a1) is not aligned, the CPU will crash.

The compiler I'm using is m68k-linux-gnu-gcc (g++ has the same problem) which
comes from Debian repository. The exact version is "(Debian 10.2.1-6) 10.2.1
20210110"

Even if it's relying on exception handler to handle the unaligned data it makes
no sense because it's sooo much slower.