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Date: Thu, 8 Feb 2001 16:24:23 +0000 (GMT)
From: Hugh Dickins <hugh@veritas.com>
To: Linus Torvalds <torvalds@transmeta.com>
cc: Rik van Riel <riel@conectiva.com.br>,
        Mark Hahn <hahn@coffee.psychology.mcmaster.ca>,
        David Howells <dhowells@cambridge.redhat.com>,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH] micro-opt DEBUG_ADD_PAGE
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On Wed, 7 Feb 2001, Linus Torvalds wrote:
> On Wed, 7 Feb 2001, Hugh Dickins wrote:
> > 
> > None of those optimizes this: I believe the semantics of "||" (don't
> > try next test if first succeeds) forbid the optimization "|" gives?
> 
> No. The optimization is entirely legal - but the fact that
> "constant_test_bit()" uses a "volatile unsigned int *" is the reason why
> gcc thinks it can't optimize it.

Ah, yes, I hadn't noticed that, the "volatile" is indeed why it ends up
with three "mov"s.  But take the "volatile"s out of constant_test_bit(),
and DEBUG_ADD_PAGE still expands to three tests and three (four if 2.97)
jumps - which is what originally offended me.

But Mark (in test program in private mail) shows gcc combining bits
into one test and one jump, just as we'd hope (and I wrongly thought
forbidden).  Perhaps the inline function nature of constant_test_bit()
(which Mark didn't use) gets in the way of combining those tests.

> You could try to remove the volatile from test_bit, and see if that fixes
> it - but then we'd have to find and add the proper "rmb()" calls to people
> who do the endless loop kind of thing like above.

That is not an inviting path to me, at least not any time soon!

I think this all argues for the little patch I suggested - just avoid
test_bit() here.  But it was only intended as a quick little suggestion:
looks like our tastes differ, and you prefer taking the _tiny_ hit of
using the regular macros, to seeing "1<<PG_bitshift"s in DEBUG_ADD_PAGE.

Hugh

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