netdev - Re: [PATCH net-next] fast_hash: clobber registers correctly for inline function use

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Date:	Fri, 14 Nov 2014 07:33:01 -0800
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Hannes Frederic Sowa <hannes@...essinduktion.org>
Cc:	netdev@...r.kernel.org, ogerlitz@...lanox.com, pshelar@...ira.com,
	jesse@...ira.com, jay.vosburgh@...onical.com,
	discuss@...nvswitch.org
Subject: Re: [PATCH net-next] fast_hash: clobber registers correctly for
 inline function use

On Fri, 2014-11-14 at 16:13 +0100, Hannes Frederic Sowa wrote:
> > 
> > 
> > Thats a lot of clobbers.
> 
> Yes, those are basically all callee-clobbered registers for the
> particular architecture. I didn't look at the generated code for jhash
> and crc_hash because I want this code to always be safe, independent of
> the version and optimization levels of gcc.
> 
> > Alternative would be to use an assembly trampoline to save/restore them
> > before calling __jhash2
> 
> This version provides the best hints on how to allocate registers to the
> optimizers. E.g. it could avoid using callee-clobbered registers but use
> callee-saved ones. If we build a trampoline, we need to save and reload
> all registers all the time. This version just lets gcc decide how to do
> that.
> 
> > __intel_crc4_2_hash2 can probably be written in assembly, it is quite
> > simple.
> 
> Sure, but all the pre and postconditions must hold for both, jhash and
> intel_crc4_2_hash and I don't want to rewrite jhash in assembler.

We write optimized code for current cpus.

With current generation of cpus, we have crc32 support.

The fallback having to save/restore few registers, we don't care, as the
fallback has huge cost anyway.

You don't have to write jhash() in assembler, you misunderstood me.

We only have to provide a trampoline in assembler, with maybe 10
instructions.

Then gcc will know that we do not clobber registers for the optimized
case.



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