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Date: Fri, 17 Aug 2007 16:59:12 -0700 From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com> To: Linus Torvalds <torvalds@...ux-foundation.org> Cc: Nick Piggin <nickpiggin@...oo.com.au>, Paul Mackerras <paulus@...ba.org>, Segher Boessenkool <segher@...nel.crashing.org>, heiko.carstens@...ibm.com, horms@...ge.net.au, linux-kernel@...r.kernel.org, rpjday@...dspring.com, ak@...e.de, netdev@...r.kernel.org, cfriesen@...tel.com, akpm@...ux-foundation.org, jesper.juhl@...il.com, linux-arch@...r.kernel.org, zlynx@....org, satyam@...radead.org, clameter@....com, schwidefsky@...ibm.com, Chris Snook <csnook@...hat.com>, Herbert Xu <herbert.xu@...hat.com>, davem@...emloft.net, wensong@...ux-vs.org, wjiang@...ilience.com Subject: Re: [PATCH 0/24] make atomic_read() behave consistently across all architectures On Thu, Aug 16, 2007 at 08:50:30PM -0700, Linus Torvalds wrote: > Just try it yourself: > > volatile int i; > int j; > > int testme(void) > { > return i <= 1; > } > > int testme2(void) > { > return j <= 1; > } > > and compile with all the optimizations you can. > > I get: > > testme: > movl i(%rip), %eax > subl $1, %eax > setle %al > movzbl %al, %eax > ret > > vs > > testme2: > xorl %eax, %eax > cmpl $1, j(%rip) > setle %al > ret > > (now, whether that "xorl + setle" is better than "setle + movzbl", I don't > really know - maybe it is. But that's not thepoint. The point is the > difference between > > movl i(%rip), %eax > subl $1, %eax > > and > > cmpl $1, j(%rip) gcc bugzilla bug #33102, for whatever that ends up being worth. ;-) Thanx, Paul - To unsubscribe from this list: send the line "unsubscribe netdev" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
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