linux-kernel - Re: [RFC] page-table walkers vs memory order

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Message-ID: <20120730192140.GU25459@jl-vm1.vm.bytemark.co.uk>
Date:	Mon, 30 Jul 2012 20:21:40 +0100
From:	Jamie Lokier <jamie@...reable.org>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	Hugh Dickins <hughd@...gle.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Rik van Riel <riel@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Nick Piggin <npiggin@...nel.dk>,
	Andrea Arcangeli <aarcange@...hat.com>,
	linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
	linux-mm@...ck.org
Subject: Re: [RFC] page-table walkers vs memory order

Paul E. McKenney wrote:
> > Does some version of gcc, under the options which we insist upon,
> > make such optimizations on any of the architectures which we support?
> 
> Pretty much any production-quality compiler will do double-fetch
> and old-value-reuse optimizations, the former especially on 32-bit
> x86.  I don't know of any production-quality compilers that do value
> speculation, which would make the compiler act like DEC Alpha hardware,
> and I would hope that if this does appear, (1) we would have warning
> and (2) it could be turned off.  But there has been a lot of work on
> this topic, so we would be foolish to rule it out.

GCC documentation for IA-64:

   -msched-ar-data-spec
   -mno-sched-ar-data-spec
     (En/Dis)able data speculative scheduling after reload. This results
     in generation of ld.a instructions and the corresponding check
     instructions (ld.c / chk.a). The default is 'enable'.

I don't know if that results in value speculation of the relevant kind.

-- Jamie
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