linux-kernel - Re: [PATCH v3 0/4] exec: unify native/compat code

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Date:	Wed, 2 Mar 2011 11:48:03 -0800
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	David Miller <davem@...emloft.net>
Cc:	oleg@...hat.com, akpm@...ux-foundation.org,
	kosaki.motohiro@...fujitsu.com, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org, pageexec@...email.hu, solar@...nwall.com,
	eteo@...hat.com, spender@...ecurity.net, roland@...hat.com,
	miltonm@....com
Subject: Re: [PATCH v3 0/4] exec: unify native/compat code

On Wed, Mar 2, 2011 at 11:40 AM, David Miller <davem@...emloft.net> wrote:
>
> We purposely don't do that "page table entry typedef'd to aggregate" stuff
> on sparc32 because otherwise such values get passed on the stack.
>
> Architectures can currently avoid this bad code generation for the
> page table case, but with this new code they won't be able to avoid
> pass-by-value.

Well, the thing is, on architectures that _can_ pass by value, it
avoids one indirection.

And if you do pass it on stack, then the code generated will be the
same as if we passed a pointer. So sparc may not be able to take
advantage of the optimization, but I don't think the code generation
would be worse.

For the page table case, we don't have that kind of trade-off: the
trade-off there is literally just between "pass in registers, or pass
on stack". Here the trade-off is "pass as an aggregate value or pass
as a pointer to an aggregate value".

That said, since I suspect that the main user will always just get
inlined (ie the helper function that actually fetches the pointers), I
suspect even sparc will see the advantage of the pass-by-value model.

But you might want to actually test the difference and look at the
code generation.

                      Linus
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