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Re: speed of unaligned accesses that cross page-boundaries

From Tim Rentsch <tr.17687@z991.linuxsc.com>
Newsgroups comp.lang.c++
Subject Re: speed of unaligned accesses that cross page-boundaries
Date 2022-04-18 20:59 -0700
Organization A noiseless patient Spider
Message-ID <865yn5afkq.fsf@linuxsc.com> (permalink)
References <t17mqk$cj4$1@dont-email.me> <t1947l$1jv3$2@gioia.aioe.org>

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Juha Nieminen <nospam@thanks.invalid> writes:

> Bonita Montero <Bonita.Montero@gmail.com> wrote:
>
>> I just wanted to measure the performance of unaligned accesses.
>> For accesses within a page the difference was below the measurement
>> error.  Then I read that some processors put a penalty on unaligned
>> accesses that cross page-boundaries.  So I wrote a little benchmark
>> that tries to measure this.  But for my PCs, a Ryzen Threadripper
>> 3990X, a Ryzen 7 1800X and a Phenom X4 945 all measurements were
>> also within the measurement error.
>
> IIRC both Intel and AMD got rid of the unaligned access penalty
> at some point (relatively recently).  You would need an older CPU
> in order to get the penalty.
>
> (Btw, I assume you know that unaligned access is standardized as UB.)

I assume BM is talking about unaligned access in the actual
machine.  An access that is unaligned in the actual machine may
be an aligned access as far as the abstract machine is concerned.

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Thread

speed of unaligned accesses that cross page-boundaries Bonita Montero <Bonita.Montero@gmail.com> - 2022-03-20 18:04 +0100
  Re: speed of unaligned accesses that cross page-boundaries Bonita Montero <Bonita.Montero@gmail.com> - 2022-03-20 18:08 +0100
  Re: speed of unaligned accesses that cross page-boundaries scott@slp53.sl.home (Scott Lurndal) - 2022-03-20 18:12 +0000
    Re: speed of unaligned accesses that cross page-boundaries Bonita Montero <Bonita.Montero@gmail.com> - 2022-03-20 19:36 +0100
  Re: speed of unaligned accesses that cross page-boundaries "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2022-03-20 16:13 -0700
    Re: speed of unaligned accesses that cross page-boundaries Bonita Montero <Bonita.Montero@gmail.com> - 2022-03-21 06:58 +0100
    Re: speed of unaligned accesses that cross page-boundaries scott@slp53.sl.home (Scott Lurndal) - 2022-03-21 14:49 +0000
  Re: speed of unaligned accesses that cross page-boundaries Juha Nieminen <nospam@thanks.invalid> - 2022-03-21 05:59 +0000
    Re: speed of unaligned accesses that cross page-boundaries Bonita Montero <Bonita.Montero@gmail.com> - 2022-03-21 08:56 +0100
    Re: speed of unaligned accesses that cross page-boundaries scott@slp53.sl.home (Scott Lurndal) - 2022-03-21 14:53 +0000
      Re: speed of unaligned accesses that cross page-boundaries scott@slp53.sl.home (Scott Lurndal) - 2022-03-21 16:59 +0000
    Re: speed of unaligned accesses that cross page-boundaries Tim Rentsch <tr.17687@z991.linuxsc.com> - 2022-04-18 20:59 -0700
    Re: speed of unaligned accesses that cross page-boundaries Bonita Montero <Bonita.Montero@gmail.com> - 2022-04-19 06:45 +0200
      Re: speed of unaligned accesses that cross page-boundaries Juha Nieminen <nospam@thanks.invalid> - 2022-04-19 12:51 +0000
        Re: speed of unaligned accesses that cross page-boundaries Bonita Montero <Bonita.Montero@gmail.com> - 2022-04-19 15:17 +0200
      Re: speed of unaligned accesses that cross page-boundaries scott@slp53.sl.home (Scott Lurndal) - 2022-04-19 14:11 +0000
        Re: speed of unaligned accesses that cross page-boundaries Bonita Montero <Bonita.Montero@gmail.com> - 2022-04-19 16:15 +0200
          Re: speed of unaligned accesses that cross page-boundaries "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2022-04-27 01:52 -0700
            Re: speed of unaligned accesses that cross page-boundaries Bonita Montero <Bonita.Montero@gmail.com> - 2022-04-27 20:02 +0200
            Re: speed of unaligned accesses that cross page-boundaries Bonita Montero <Bonita.Montero@gmail.com> - 2022-04-28 08:19 +0200
              Re: speed of unaligned accesses that cross page-boundaries Bonita Montero <Bonita.Montero@gmail.com> - 2022-04-28 12:19 +0200
                Re: speed of unaligned accesses that cross page-boundaries "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2022-04-28 16:00 -0700
                Re: speed of unaligned accesses that cross page-boundaries Bonita Montero <Bonita.Montero@gmail.com> - 2022-04-29 05:52 +0200
                Re: speed of unaligned accesses that cross page-boundaries "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2022-05-04 18:06 -0700
                Re: speed of unaligned accesses that cross page-boundaries Bonita Montero <Bonita.Montero@gmail.com> - 2022-05-05 06:13 +0200
                Re: speed of unaligned accesses that cross page-boundaries "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2022-05-04 21:27 -0700
                Re: speed of unaligned accesses that cross page-boundaries Bonita Montero <Bonita.Montero@gmail.com> - 2022-05-05 07:37 +0200
                Re: speed of unaligned accesses that cross page-boundaries Öö Tiib <ootiib@hot.ee> - 2022-05-05 04:34 -0700
                Re: speed of unaligned accesses that cross page-boundaries "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2022-05-05 12:16 -0700

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