netdev - Re: [PATCH] tg3 : avoid an expensive divide

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Message-ID: <p73ireemgwm.fsf@bingen.suse.de>
Date:	07 Feb 2007 10:54:49 +0100
From:	Andi Kleen <ak@...e.de>
To:	David Miller <davem@...emloft.net>
Cc:	dada1@...mosbay.com, mchan@...adcom.com, netdev@...r.kernel.org
Subject: Re: [PATCH] tg3 : avoid an expensive divide

David Miller <davem@...emloft.net> writes:
> 
> Because I've seen gcc optimize this properly before (at least on
> sparc64), it means that either:
> 
> 1) There is a GCC bug where the properties of the constants
>    do not propagate.
> 
> 2) GCC really thinks the divide is cheaper (code density vs.
>    cycle count tradeoffs etc.)

Probably Eric compiled with the now default CONFIG_CC_OPTIMIZE_FOR_SIZE/-Os.
With that gcc decides to use the shorter hardware divide instruction, even
though it is significantly slower than an expanded optimized sequence
for constant dividend.

We've seen this in a few other cases while during performance regression
testing between kernels that still used -O2 vs the newer -Os.

No good solution found unfortunately.

-Andi
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