Re: [Patch 4/4] Tell GCC 4.1 to move unlikely() code to a separate section

[Bill Davidsen <davidsen@tmr.com>]

Andi Kleen wrote:
> On Monday 27 February 2006 16:31, Arjan van de Ven wrote:
> > This patch is more controversial I assume; it offers the option 
> > to use the gcc 4.1 option to move unlikely() code to a separate section.
> > On the con side, this means that longer byte sequences are needed to jump
> > to this code, on the Pro side it means that the unlikely() code isn't sharing
> > icache cachelines and tlbs anymore.
>
> I don't think this will do anything because the default Makefile
> still has
>
> CFLAGS += -fno-reorder-blocks 
>
> That was me because it made assembly debugging much easier. I would be willing
> to reconsider this if you can give me some hard data just from this change:
> - benchmark changes
> - .text size increase
>
> Also I don't like it being an separate CONFIG options. We already have too many
> obscure ones. Either it should be on by default or not there at all.

I think you just made the case for an option, if it does produce 
substantially better code and justify being done, you still would want a 
way to kill it for debugging.

I'd like to see what it gains in general, and how much it depends on 
processor type and cache size.
--
   -bill davidsen (davidsen@tmr.com)
"The secret to procrastination is to put things off until the
 last possible moment - but no longer"  -me

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