https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98908
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 most
> 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 = x.l;
_2 = (signed short) _1;
_3 = x.h;
_4 = (int) _3;
_5 = _4 << 8;
_6 = (signed short) _5;
_7 = _2 | _6;
_8 = (short unsigned int) _7;
tmp_14 = _8 & 33022;
MEM[(unsigned char *)&D.2500] = tmp_14;
And is only opimitized in GCC 9 with bswap producing what I mentioned before
So fixed for GCC 9.
