Date: Thu, 8 Jan 2009 19:46:30 -0800 (PST)
From: Linus Torvalds <torvalds@linux-foundation.org>
To: "H. Peter Anvin" <hpa@zytor.com>
cc: Ingo Molnar <mingo@elte.hu>, Chris Mason <chris.mason@oracle.com>,
       Peter Zijlstra <peterz@infradead.org>,
       Steven Rostedt <rostedt@goodmis.org>, paulmck@linux.vnet.ibm.com,
       Gregory Haskins <ghaskins@novell.com>, Matthew Wilcox <matthew@wil.cx>,
       Andi Kleen <andi@firstfloor.org>,
       Andrew Morton <akpm@linux-foundation.org>,
       Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
       linux-fsdevel <linux-fsdevel@vger.kernel.org>,
       linux-btrfs <linux-btrfs@vger.kernel.org>,
       Thomas Gleixner <tglx@linutronix.de>, Nick Piggin <npiggin@suse.de>,
       Peter Morreale <pmorreale@novell.com>,
       Sven Dietrich <SDietrich@novell.com>
Subject: Re: [PATCH -v7][RFC]: mutex: implement adaptive spinning
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On Thu, 8 Jan 2009, H. Peter Anvin wrote:
> 
> Right.   gcc simply doesn't have any way to know how heavyweight an
> asm() statement is

I don't think that's relevant.

First off, gcc _does_ have a perfectly fine notion of how heavy-weight an 
"asm" statement is: just count it as a single instruction (and count the 
argument setup cost that gcc _can_ estimate).

That would be perfectly fine. If people use inline asms, they tend to use 
it for a reason.

However, I doubt that it's the inline asm that was the biggest reason why 
gcc decided not to inline - it was probably the constant "switch()" 
statement. The inline function actually looks pretty large, if it wasn't 
for the fact that we have a constant argument, and that one makes the 
switch statement go away.

I suspect gcc has some pre-inlining heuristics that don't take constant 
folding and simplifiation into account - if you look at just the raw tree 
of the function without taking the optimization into account, it will look 
big.

			Linus
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