linux-kernel - Re: [3/10 PATCH] inline wake_up

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Message-Id: <20080625.172838.242554644.davem@davemloft.net>
Date:	Wed, 25 Jun 2008 17:28:38 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	vda.linux@...glemail.com
Cc:	mpatocka@...hat.com, linux-kernel@...r.kernel.org,
	sparclinux@...r.kernel.org
Subject: Re: [3/10 PATCH] inline wake_up_bit

From: Denys Vlasenko <vda.linux@...glemail.com>
Date: Wed, 25 Jun 2008 22:37:58 +0200

> On Wednesday 25 June 2008 18:01, Mikulas Patocka wrote:
> > > Ingo's suggestion to talk to gcc people to remedy
> > > insane call convention sounds as a more workable solution.
> > >
> > > BTW, i386 uses regparm call convention, is similar trick
> > > possible for sparc64?
> > 
> > Sparc64 has register windows: it passes arguments in registers, but it 
> > must allocate space for that registers. If the call stack is too deep (8 
> > levels), the CPU runs out of registers and starts spilling the registers 
> > of the function 8-levels-deep to the stack.
> > 
> > The stack usage could be reduced to 176 bytes with little work from gcc 
> > developers and to 128 bytes with more work (ABI change). If you wanted to 
> 
> Wow, it's nearly x2 reduction.
> 
> ABI change in not a problem for kernel, since it is a "freestanding
> application". Exactly like i386 switched to regparm, which is a different ABI.

Except that nobody has written this code and therefore being about to
use this unimplemented compiler facility to get correctness is not
tenable.
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