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Subject: [Bug middle-end/101197] __builtin_memmove does not perform constant
 optimizations
Date: Mon, 16 Aug 2021 10:54:10 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D101197

--- Comment #11 from cqwrteur <unlvsur at live dot com> ---
(In reply to Tamar Christina from comment #10)
> (In reply to cqwrteur from comment #9)
> > (In reply to Tamar Christina from comment #8)
> > > (In reply to Jakub Jelinek from comment #6)
> > > > Shouldn't that be a different PR with details?  I mean, this PR is =
that we
> > > > should expand shorter memmove inline even if the regions do overlap.
> > >=20
> > > Sure, I'm still trying to create a minimal representative example (it=
's C++
> > > and templated) unless just pointing at the github is enough.=20
> > >=20
> > > To be clear though, just inlining memmove at all will cover most of t=
he
> > > distance, it's just that you require less registers.
> >=20
> > inline things like memcpy and memmove will lead to serious binary bloat=
. The
> > compiler usually picks to emit call to libc's memcpy and memmove that is
> > usually highly optimized with assembly code.
>=20
> Yes your binary will grow, but on small memcopy and memmove. the calling
> overhead, not to mention the register allocation overhead you might get f=
rom
> having to spill your caller saves more than makes up for it.
>=20
> We already inline memcpy and memset. there's no reason not to do memmove,
> especially at -O3.

That is false. inline memcpy and memset only works when the size is constan=
t.=