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From: "jakub at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug tree-optimization/98856] [11 Regression] botan AES-128/XTS is
 slower by ~17% since r11-6649-g285fa338b06b804e72997c4d876ecf08a9c083af
Date: Fri, 05 Feb 2021 16:29:54 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D98856

--- Comment #13 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
Looking at what other compilers emit for this, ICC seems to be completely
broken, it emits logical right shifts instead of arithmetic right shift, and
LLVM trunk emits for >> 63 what this patch emits, for >> 17 it emits
        vpsrad  $17, %xmm0, %xmm1
        vpsrlq  $17, %xmm0, %xmm0
        vpblendd        $10, %xmm1, %xmm0, %xmm0
instead of
        vpxor   %xmm1, %xmm1, %xmm1
        vpcmpgtq        %xmm0, %xmm1, %xmm1
        vpsrlq  $17, %xmm0, %xmm0
        vpsllq  $47, %xmm1, %xmm1
        vpor    %xmm1, %xmm0, %xmm0
the patch emits.  For >> 47 it emits:
        vpsrad  $31, %xmm0, %xmm1
        vpsrad  $15, %xmm0, %xmm0
        vpshufd $245, %xmm0, %xmm0
        vpblendd        $10, %xmm1, %xmm0, %xmm0
etc.
So, in summary, for >> 63 with SSE4.2 I think what the patch does looks bes=
t,
for >> 63 and SSE2 we can emit psrad $31 instead and permute the odd elemen=
ts
into even ones (i.e. __builtin_shuffle ((v4si) x >> 31, { 1, 1, 3, 3 })).
For >> cst where cst < 32, do a psrad and psrlq by that cst and permute such
that
we get the even SI elts from the psrlq result and odd from psrad result.
For >> 32, do a psrad $31 and permute to get the even SI elts from odd elts=
 of
the source and odd SI elts from odd results of psrad $31.
For >> cst where cst > 32, do psrad $31 and psrad $(cst-32) and permute
such that even SI elts come from odd elts of the latter and odd elts come f=
rom
odd elts of the former.=