From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Jan Beulich" Subject: Re: [PATCH] x86: fix variable_test_bit() asmconstraints Date: Fri, 14 Mar 2008 14:18:49 +0000 Message-ID: <47DA9759.76E4.0078.0@novell.com> References: <47DA6E53.76E4.0078.0@novell.com> <47DA90EE.76E4.0078.0@novell.com> <20080314135938.GN4358@implementation.uk.xensource.com> Mime-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: quoted-printable Return-path: In-Reply-To: <20080314135938.GN4358@implementation.uk.xensource.com> Content-Disposition: inline List-Unsubscribe: , List-Post: List-Help: List-Subscribe: , Sender: xen-devel-bounces@lists.xensource.com Errors-To: xen-devel-bounces@lists.xensource.com To: Samuel Thibault Cc: xen-devel@lists.xensource.com, Keir Fraser List-Id: xen-devel@lists.xenproject.org >>> Samuel Thibault 14.03.08 14:59 >>> >Jan Beulich, le Fri 14 Mar 2008 13:51:26 +0000, a =C3=A9crit : >> >memory-clobber would be needed for the atomic ops, but a dummy memory >> >operand would suffice for non-atomic ops. I used memory clobber = everywhere >>=20 >> Atomic ops imply a barrier (otherwise the compiler can defeat the >> purpose of the atomic operation). The non-atomic ones don't need a >> dummy operand, but one that precisely describes the place in memory >> that changes. > >Mmm, won't that unnecessarily make the compiler put code to compute that >place? Not generally - if both the 'used' and the 'fake' operands are apart just by a constant offset, the same base/index expression will be used by the compiler, just with different offsets. If the bit position is not a constant that might well be true, though (and I ought to check) - since in that case the compiler can't predict which part of the array changes*, splitting the two cases with a __builtin_constant_p() should be possible, though. The constant section then would need two operands (one of them fake), while the variable section would need just one. * In theory, the compiler could even then detect certain cases of accesses to the same field. If we're afraid of that, the we'd need to use precise operand descriptions here or, as Keir suggested, just live with using a memory barrier instead of precise operand constraints. Jan