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From: "rguenth at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug tree-optimization/104632] Missed optimization about reading
 backwards
Date: Tue, 22 Feb 2022 10:39:43 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D104632

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
     Ever confirmed|0                           |1
   Last reconfirmed|                            |2022-02-22
             Status|UNCONFIRMED                 |NEW

--- Comment #3 from Richard Biener <rguenth at gcc dot gnu.org> ---
The bswap pass is supposed to handle these but has

bool
find_bswap_or_nop_load (gimple *stmt, tree ref, struct symbolic_number *n)
{
...
      /* Avoid returning a negative bitpos as this may wreak havoc later.  =
*/
      if (maybe_lt (bit_offset, 0))
        {

commenting the code produces the desired

load_le32_backwards:
.LFB0:
        .cfi_startproc
        movl    -4(%rdi), %eax
        ret

the code is present since the introduction of memory source to the bswap pa=
ss.
It needs to be investigated what exactly the "havoc" is but clearly the "ha=
voc"
should be mitigated closer to the offenders since the above case seems to w=
ork
just fine.=