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From: "pinskia at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug tree-optimization/98908] Failure to optimize arithmetic
 involving struct members into operating on the entire struct
Date: Fri, 03 Sep 2021 00:59:28 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D98908

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|WAITING                     |RESOLVED
   Target Milestone|---                         |9.0
         Resolution|---                         |FIXED

--- Comment #7 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
(In reply to Gabriel Ravier from comment #6)
> Also the second example wasn't misoptimized, on the contrary it was the m=
ost
> reasonable portable function I could write that would work equivalently to
> the first *and* that GCC would optimize ideally.

GCC 7.1.0 produces:
f(reg):
.LFB0:
        .cfi_startproc
        mov     edx, edi
        xor     eax, eax
        mov     ecx, edi
        and     edx, -2
        mov     al, dl
        movzx   edx, ch
        and     edx, -128
        mov     ah, dl
        ret
f1(reg):
.LFB1:
        .cfi_startproc
        and     di, -32514
        xor     eax, eax
        movzx   edx, di
        mov     al, dil
        sar     edx, 8
        mov     ah, dl
        ret

f is your first example and f1 is the second.
As you can see GCC before GCC 8 did neither.
In GCC 8, the second function produces:
  _1 =3D x.l;
  _2 =3D (signed short) _1;
  _3 =3D x.h;
  _4 =3D (int) _3;
  _5 =3D _4 << 8;
  _6 =3D (signed short) _5;
  _7 =3D _2 | _6;
  _8 =3D (short unsigned int) _7;
  tmp_14 =3D _8 & 33022;
  MEM[(unsigned char *)&D.2500] =3D tmp_14;

And is only opimitized in GCC 9 with bswap producing what I mentioned before

So fixed for GCC 9.=