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Date: Tue, 5 Jun 2018 16:02:16 -0700
From: Gleb Smirnoff <glebius@FreeBSD.org>
To: Mateusz Guzik <mjguzik@gmail.com>
Cc: Eric van Gyzen <eric@vangyzen.net>, Mateusz Guzik <mjg@freebsd.org>,
 src-committers <src-committers@freebsd.org>, svn-src-all@freebsd.org,
 svn-src-head@freebsd.org
Subject: Re: svn commit: r334545 - in head/sys: contrib/zstd/lib/freebsd kern
 netinet/libalias sys
Message-ID: <20180605230216.GD1005@FreeBSD.org>
References: <201806022220.w52MK9TT026244@repo.freebsd.org>
 <1b77e282-e24b-d14c-9811-4cf214d58280@vangyzen.net>
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On Wed, Jun 06, 2018 at 12:56:47AM +0200, Mateusz Guzik wrote:
M> > M> Thank you for the kind words. It is positive feedback like this which
M> > M> keeps me going!
M> >
M> > Btw, what was the point of checking flags || result? Most places in kernel
M> > ignore flags and just test result regerdless of M_WAITOK/M_NOWAIT.
M> >
M> > The result is already in a register, why do you think checking for absense
M> > of M_WAITOK is faster that checking for !NULL _malloc_item?
M> >
M> 
M> This part is only reachable if flags are known at compilation time. If they
M> contain
M> M_WAITOK, the flag check will get elided along (we know for a fact it
M> passes)
M> and subsequently the NULL check will be short circuited, iow for known
M> M_WAITOK|M_ZERO flags this is:
M> 
M> _malloc_item = malloc(_size, type, flags & ~ M_ZERO);
M> bzero(_malloc_item, _size);

Thanks a lot for explanation!

-- 
Gleb Smirnoff