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Date: Sun, 31 Jul 2016 19:35:27 +0300
From: Konstantin Belousov <kostikbel@gmail.com>
To: Bruce Evans <brde@optusnet.com.au>
Cc: Mateusz Guzik <mjg@freebsd.org>, src-committers@freebsd.org,
 svn-src-all@freebsd.org, svn-src-head@freebsd.org
Subject: Re: svn commit: r303583 - head/sys/amd64/amd64
Message-ID: <20160731163527.GZ83214@kib.kiev.ua>
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On Sun, Jul 31, 2016 at 11:11:25PM +1000, Bruce Evans wrote:
> On Haswell, "rep stos" takes about 25 cycles to start up, and the function
> call overhead is in the noise.  25 cycles is a lot.  Haswell can move
> 32 bytes/cycle from L2 to L2, so it misses moving 800 bytes or 1/5 of a
> page in its startup overhead.  Oops, that is for "rep movs".  "rep stos"
> is similar.
> 
The commit message contained a probable explanation of the reason why
the change demonstrated measurable improvement in non-microbenchmark load.

That said, the only thing I am answering and asking there is the above
claim about 25 cycles overhead of rep;stosq on hsw. I am curious how
the overhead was measured. Note: Agner Fog' tables state that fast mode
takes <2n uops and has reciprocal throughput of 0.5n worst case and do
not demostrate any setup overhead for hsw.