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From: Ranier Vilela <ranier.vf@gmail.com>
Date: Mon, 9 Mar 2026 13:25:20 -0300
Message-ID: 
 <CAEudQAptRymgvmd5hQb2mk-Ft89XcSo_xvC74kv4JBA9v=D4Sg@mail.gmail.com>
Subject: Re: Avoid multiple calls to memcpy (src/backend/access/index/genam.c)
To: Bryan Green <dbryan.green@gmail.com>
Cc: Pg Hackers <pgsql-hackers@postgresql.org>
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Em seg., 9 de mar. de 2026 =C3=A0s 11:47, Bryan Green <dbryan.green@gmail.c=
om>
escreveu:

> I created an example that is a little bit closer to the actual code and
> changed the compiler from C++ to C.
>
> It is interesting the optimization that the compiler has chosen for
> version 1 versus version 2.  One calls
> memcpy and one doesn't.  There is a good chance the inlining of memcpy as
> SSE+scalar per iteration
> will be faster for syscache scans-- which I believe are usually small (1-=
4
> keys?).
>
I doubt the inline version is better.
Clang is supported too and the code generated is much better with memcpy
one call outside of the loop.


>
> Probably the only reason to do this patch would be if N is normally large
> or if this is considered an
> improvement in code clarity without a detrimental impact on small N
> syscache scans.
> I realize you only said "possible small optimization".  It might be
> worthwhile to benchmark the code for
> different values of n to determine if there is a tipping point either way=
?
>
 In your opinion, shouldn't this be considered an optimization, even a
small one?

best regards,
Ranier Vilela

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<div dir=3D"ltr"><div dir=3D"ltr"><br></div><br><div class=3D"gmail_quote g=
mail_quote_container"><div dir=3D"ltr" class=3D"gmail_attr">Em seg., 9 de m=
ar. de 2026 =C3=A0s 11:47, Bryan Green &lt;<a href=3D"mailto:dbryan.green@g=
mail.com">dbryan.green@gmail.com</a>&gt; escreveu:<br></div><blockquote cla=
ss=3D"gmail_quote" style=3D"margin:0px 0px 0px 0.8ex;border-left:1px solid =
rgb(204,204,204);padding-left:1ex"><div dir=3D"ltr">I created an example th=
at is a little bit closer to the actual code and changed the compiler from =
C++ to C.=C2=A0<div><br></div><div>It is interesting the optimization that =
the compiler has chosen for version 1 versus version 2.=C2=A0 One calls</di=
v><div>memcpy and one doesn&#39;t.=C2=A0 There is a good chance the inlinin=
g of memcpy as SSE+scalar per iteration</div><div>will be faster for syscac=
he scans-- which I believe are usually small (1-4 keys?).=C2=A0=C2=A0</div>=
</div></blockquote><div>I doubt the inline version is better.</div><div>Cla=
ng is supported too and the code generated is much better with memcpy one c=
all outside of the loop.</div><div>=C2=A0</div><blockquote class=3D"gmail_q=
uote" style=3D"margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,2=
04);padding-left:1ex"><div dir=3D"ltr"><div><br></div><div>Probably the onl=
y reason to do this patch would be if N is normally large or if this is con=
sidered an</div><div>improvement in code clarity without a detrimental=C2=
=A0impact on small N syscache scans.=C2=A0=C2=A0</div><div>I realize you on=
ly said &quot;possible small optimization&quot;.=C2=A0 It might be worthwhi=
le to benchmark the code for=C2=A0</div><div>different values of n to deter=
mine if there is a tipping point either way?</div></div></blockquote><div>=
=C2=A0In your opinion, shouldn&#39;t this be considered an optimization, ev=
en a small one?</div><div><br></div><div>best regards,</div><div>Ranier Vil=
ela</div></div></div>

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