netdev - RE: [PATCH] of_mdio: merge branch tails in of_phy_register_fixed

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Date:   Tue, 15 Aug 2017 11:18:50 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'David Miller' <davem@...emloft.net>,
        "sergei.shtylyov@...entembedded.com" 
        <sergei.shtylyov@...entembedded.com>
CC:     "andrew@...n.ch" <andrew@...n.ch>,
        "f.fainelli@...il.com" <f.fainelli@...il.com>,
        "robh+dt@...nel.org" <robh+dt@...nel.org>,
        "frowand.list@...il.com" <frowand.list@...il.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>
Subject: RE: [PATCH] of_mdio: merge branch tails in
 of_phy_register_fixed_link()

From: David Miller
> Sent: 14 August 2017 04:09
> From: Sergei Shtylyov <sergei.shtylyov@...entembedded.com>
> Date: Sun, 13 Aug 2017 00:03:06 +0300
> 
> > Looks  like gcc isn't always able to figure  out that 3 *if* branches in
> > of_phy_register_fixed_link() calling fixed_phy_register() at their ends
> > are similar enough and thus can be merged. The "manual" merge saves 40
> > bytes of the object code (AArch64 gcc 4.8.5), and still saves 12 bytes
> > even  if gcc was able to merge the branch tails (ARM gcc 4.8.5)...
> >
> > Signed-off-by: Sergei Shtylyov <sergei.shtylyov@...entembedded.com>
> 
> Applied, but if two instances of the "same" compiler just with
> different targets changes the optimization, it could be because of a
> tradeoff which is specific to parameters expressed in that target's
> backend.
> 
> So in the future we should probably back away from trying to "help"
> the compiler in this way.

Probably a trade off between code size and execution speed.
I've had 'fun' trying to stop gcc merging tail code paths
in order to avoid the cost of the branch instruction.

	David