linux-kernel - Re: [PATCH] vsprintf.c: Reduce sizeof struct printf

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Message-ID: <alpine.LFD.2.00.1003061818530.4033@localhost.localdomain>
Date:	Sat, 6 Mar 2010 18:24:55 -0800 (PST)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Joe Perches <joe@...ches.com>
cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Nick Andrew <nick@...k-andrew.net>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Greg Kroah-Hartman <gregkh@...e.de>, netdev@...r.kernel.org,
	Bjorn Helgaas <bjorn.helgaas@...com>
Subject: Re: [PATCH] vsprintf.c: Reduce sizeof struct printf_spec from 24 to
 8 bytes



On Sat, 6 Mar 2010, Linus Torvalds wrote:

> the 'pointer()' function has a 200+ byte stack footprint on x86-64. And 
> vsnprintf itself is about 100+ bytes. So that stack depth is way bigger 
> than I would have expected.
> 
> I'm not sure _why_ that stack footprint for 'pointer()' is so big, but I 
> bet it's due to some simple inlining, and gcc (once more) sucking at it 
> and not being able to combine stack frames. It's a damn shame.

Yeah, a few noinline's gets 'pointer()' to just save registers on the 
stack, no need for any extra buffers (which then is ok for your recursion 
case - the other subfunctions it can call have their own buffers, of 
course, but they won't be in the recursive call-path except at the leaf.

vsnprintf() itself seems less obviously fixable. I'm not sure wht gcc 
decides it needs 88 bytes of temp-space there.

		Linus
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