linux-kernel - Re: [RFC] gcc feature request: Moving blocks into sections

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Message-ID: <52000C76.3060601@twiddle.net>
Date:	Mon, 05 Aug 2013 10:35:02 -1000
From:	Richard Henderson <rth@...ddle.net>
To:	Jason Baron <jbaron@...mai.com>
CC:	Marek Polacek <polacek@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	LKML <linux-kernel@...r.kernel.org>, gcc <gcc@....gnu.org>,
	Ingo Molnar <mingo@...nel.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	"H. Peter Anvin" <hpa@...ux.intel.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	David Daney <ddaney.cavm@...il.com>,
	Behan Webster <behanw@...verseincode.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Herbert Xu <herbert@...dor.hengli.com.au>
Subject: Re: [RFC] gcc feature request: Moving blocks into sections

On 08/05/2013 09:57 AM, Jason Baron wrote:
> On 08/05/2013 03:40 PM, Marek Polacek wrote:
>> On Mon, Aug 05, 2013 at 11:34:55AM -0700, Linus Torvalds wrote:
>>> On Mon, Aug 5, 2013 at 11:24 AM, Linus Torvalds
>>> <torvalds@...ux-foundation.org> wrote:
>>>> Ugh. I can see the attraction of your section thing for that case, I
>>>> just get the feeling that we should be able to do better somehow.
>>> Hmm.. Quite frankly, Steven, for your use case I think you actually
>>> want the C goto *labels* associated with a section. Which sounds like
>>> it might be a cleaner syntax than making it about the basic block
>>> anyway.
>> FWIW, we also support hot/cold attributes for labels, thus e.g.
>>
>>    if (bar ())
>>      goto A;
>>    /* ... */
>> A: __attribute__((cold))
>>    /* ... */
>>
>> I don't know whether that might be useful for what you want or not though...
>>
>>     Marek
>>
> 
> It certainly would be.
> 
> That was how I wanted to the 'static_key' stuff to work, but unfortunately the
> last time I tried it, it didn't move the text out-of-line any further than it
> was already doing. Would that be expected? The change for us, if it worked
> would be quite simple. Something like:

It is expected.  One must use -freorder-blocks-and-partition, and use real
profile feedback to get blocks moved completely out-of-line.

Whether that's a sensible default or not is debatable.


r~
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