Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752018AbYHTU5V (ORCPT ); Wed, 20 Aug 2008 16:57:21 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1750851AbYHTU5M (ORCPT ); Wed, 20 Aug 2008 16:57:12 -0400 Received: from casper.infradead.org ([85.118.1.10]:46872 "EHLO casper.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750828AbYHTU5L (ORCPT ); Wed, 20 Aug 2008 16:57:11 -0400 Subject: Re: VolanoMark regression with 2.6.27-rc1 From: Peter Zijlstra To: Ray Lee Cc: Nick Piggin , adobriyan@gmail.com, Ingo Molnar , "Zhang, Yanmin" , Dhaval Giani , LKML , Srivatsa Vaddagiri , Aneesh Kumar KV , Balbir Singh , Chris Friesen In-Reply-To: <1219264236.8651.76.camel@twins> References: <1912217169.25608.228.camel@ymzhang> <2c0942db0808200929r640b3a1cj33efc56cfd6db9b3@mail.gmail.com> <1219252901.8651.63.camel@twins> <200808210355.55597.nickpiggin@yahoo.com.au> <2c0942db0808201115v3025e5f6r6c882783fa9e29f3@mail.gmail.com> <1219264236.8651.76.camel@twins> Content-Type: text/plain Date: Wed, 20 Aug 2008 22:56:38 +0200 Message-Id: <1219265798.8651.84.camel@twins> Mime-Version: 1.0 X-Mailer: Evolution 2.22.3.1 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 3158 Lines: 82 On Wed, 2008-08-20 at 22:30 +0200, Peter Zijlstra wrote: > On Wed, 2008-08-20 at 11:15 -0700, Ray Lee wrote: > > On Wed, Aug 20, 2008 at 10:55 AM, Nick Piggin wrote: > > > On Thursday 21 August 2008 03:21, Peter Zijlstra wrote: > > >> Ok, so one last time (I hope!).. > > >> > > >> Everybody happy with this? > > > > > > > > >> Index: linux-2.6/include/linux/kernel.h > > >> =================================================================== > > >> --- linux-2.6.orig/include/linux/kernel.h > > >> +++ linux-2.6/include/linux/kernel.h > > >> @@ -367,6 +367,12 @@ static inline char *pack_hex_byte(char * > > >> (void) (&_max1 == &_max2); \ > > >> _max1 > _max2 ? _max1 : _max2; }) > > >> > > >> +#define avg(x, y) ({ \ > > >> + typeof(x) _avg1 = (x); \ > > >> + typeof(y) _avg2 = (y); \ > > >> + (void) (&_avg1 == &_avg2); \ > > >> + _avg1 + (_avg2 - _avg1)/2; }) > > > > > > That's not going to work with unsigned types. > > > > Uhm, I think it works fine, even with unsigned, even where _avg2 is > > smaller than _avg1. Underflow is a good thing here. And I mocked up a > > little test harness and it gives the correct answers for a half dozen > > sets of values I tossed at it > > > > But maybe I'm forgetting an obscure unsigned or signed int type > > widening rule, so, care to elaborate? > > Nick is right, try: > > int main(int argc, char **argv) > { > unsigned int x = 7, y = 5; > printf("%d\n", avg(x,y)); > return 0; > } > > It fails because 5-7 = -2, which needs a signed division or sign > extending right shift. > > we'd need something like: > > #define avg(x, y) ({ \ > typeof(x) _avg1 = (x); \ > typeof(y) _avg2 = (y); \ > (void) (&_avg1 == &_avg2); \ > _avg1 + (signed typeof(x))(_avg2 - _avg1)/2; }) > > except that typeof() doesn't work that way. > > #define avg(x, y) ({ \ > typeof(x) _avg1 = (x); \ > typeof(y) _avg2 = (y); \ > (void) (&_avg1 == &_avg2); \ > _avg1 + (long)(_avg2 - _avg1)/2; }) > > works for the above example, but when I make it long long, so as to > match the longest supported type, it goes boom again - for as of yet > unknown reasons. Ok, people pointed out I got my promotion rules mixed up, I casted the result of the division to signed, instead of ending up with a signed division. #define avg(x, y) ({ \ typeof(x) _avg1 = (x); \ typeof(y) _avg2 = (y); \ (void) (&_avg1 == &_avg2); \ (typeof(x))(_avg1 + ((long long)_avg2 - _avg1)/2); }) seems to work. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/