From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752078AbaCFLe6 (ORCPT ); Thu, 6 Mar 2014 06:34:58 -0500 Received: from villa.puc.rediris.es ([130.206.18.7]:58594 "EHLO villa.puc.rediris.es" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751210AbaCFLe5 (ORCPT ); Thu, 6 Mar 2014 06:34:57 -0500 X-Envelope-From: paubert@iram.es Date: Thu, 6 Mar 2014 12:33:32 +0100 From: Gabriel Paubert To: David Laight Cc: "'Sukadev Bhattiprolu'" , Arnaldo Carvalho de Melo , Michael Ellerman , "linux-kernel@vger.kernel.org" , Stephane Eranian , "linuxppc-dev@ozlabs.org" , Paul Mackerras , Jiri Olsa Subject: Re: [PATCH 3/3] perf: Use 64-bit value when comparing sample_regs Message-ID: <20140306113332.GA7827@visitor2.iram.es> References: <1394080919-17957-1-git-send-email-sukadev@linux.vnet.ibm.com> <1394080919-17957-4-git-send-email-sukadev@linux.vnet.ibm.com> <063D6719AE5E284EB5DD2968C1650D6D0F6D3983@AcuExch.aculab.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <063D6719AE5E284EB5DD2968C1650D6D0F6D3983@AcuExch.aculab.com> User-Agent: Mutt/1.5.21 (2010-09-15) X-Spamina-Bogosity: Ham Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Mar 06, 2014 at 09:44:47AM +0000, David Laight wrote: > From: Sukadev Bhattiprolu > > When checking whether a bit representing a register is set in > > sample_regs, a 64-bit mask, use 64-bit value (1LL). > > > > Signed-off-by: Sukadev Bhattiprolu > > --- > > tools/perf/util/unwind.c | 4 ++-- > > 1 file changed, 2 insertions(+), 2 deletions(-) > > > > diff --git a/tools/perf/util/unwind.c b/tools/perf/util/unwind.c > > index 742f23b..2b888c6 100644 > > --- a/tools/perf/util/unwind.c > > +++ b/tools/perf/util/unwind.c > > @@ -396,11 +396,11 @@ static int reg_value(unw_word_t *valp, struct regs_dump *regs, int id, > > { > > int i, idx = 0; > > > > - if (!(sample_regs & (1 << id))) > > + if (!(sample_regs & (1LL << id))) > > return -EINVAL; > > > > for (i = 0; i < id; i++) { > > - if (sample_regs & (1 << i)) > > + if (sample_regs & (1LL << i)) > > idx++; > > } > > There are much faster ways to count the number of set bits, especially > if you might need to check a significant number of bits. > There might even be a function defined somewhere to do it. Indeed, look for Hamming weight (hweight family of functions) in asm/hweight.h and what is included from there. Besides that, many modern processors also have a machine instruction to perform this task. In the processor manuals the instruction is described as population count and the mnemonic starts with "popcnt" on x86 and ppc. Gabriel > Basically you just add up the bits, for 16 bit it would be: > val = (val & 0x5555) + (val >> 1) & 0x5555; > val = (val & 0x3333) + (val >> 2) & 0x3333; > val = (val & 0x0f0f) + (val >> 4) & 0x0f0f; > val = (val & 0x00ff) + (val >> 8) & 0x00ff; > As the size of the work increases the improvement is more significant. > (Some of the later masking can probably be proven unnecessary.) > > David > > _______________________________________________ > Linuxppc-dev mailing list > Linuxppc-dev@lists.ozlabs.org > https://lists.ozlabs.org/listinfo/linuxppc-dev