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

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Message-Id: <200806252237.58046.vda.linux@googlemail.com>
Date:	Wed, 25 Jun 2008 22:37:58 +0200
From:	Denys Vlasenko <vda.linux@...glemail.com>
To:	Mikulas Patocka <mpatocka@...hat.com>
Cc:	linux-kernel@...r.kernel.org, sparclinux@...r.kernel.org,
	davem@...emloft.net
Subject: Re: [3/10 PATCH] inline wake_up_bit

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.

> go below 128 bytes, you could use one register to indicate number of used 
> registers and modify the spill/fill handlers to load only that number of 
> registers and reduce the stack usage even more --- that would be a big 
> code change in both gcc and linux.
--
vda
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