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Re: the language (and the role of Undefined Behavior therein)

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supercat@casperkitty.com writes:
[...]
> I do dislike the prohibition on memcpy being passed the same argument for
> src and dest along with a non-zero size, however, since a lot of compilers
> do a better job of optimizing memcpy than memmove, and given code like:
>
>     void copyLong2(long *dest, long *src)
>     {
>       if (dest != src)
>         memcpy(dest, src, sizeof *dest);
>     }
>
> a compiler could only optimize away the comparison if it could prove that
> it was impossible for dest and src to both be null (since calling copyLong2
> with two null pointers would be a defined safe no-op, but reading and
> writing a null pointer would not be.)
[...]

A compiler could optimize away the comparison if it knows that
    memcpy(foo, foo, non_zero)
is safe *for that implementation*.  Since memcpy is part of the
standard library and therefore part of the implementation, a compiler
may take advantage of any knowledge it has about its behavior.

A compiler designed to work with different runtime libraries could
not in general assume anything beyond what the standard guarantees
-- but a particular implementation of <string.h> might include
gcc-specific code that could provide extra information to gcc and
be ignored by other compilers.

-- 
Keith Thompson (The_Other_Keith) kst-u@mib.org  <http://www.ghoti.net/~kst>
Working, but not speaking, for JetHead Development, Inc.
"We must do something.  This is something.  Therefore, we must do this."
    -- Antony Jay and Jonathan Lynn, "Yes Minister"

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Re: the language (and the role of Undefined Behavior therein) supercat@casperkitty.com - 2015-09-29 15:48 -0700
  Re: the language (and the role of Undefined Behavior therein) Tim Rentsch <txr@alumni.caltech.edu> - 2015-10-04 16:57 -0700
    Re: the language (and the role of Undefined Behavior therein) supercat@casperkitty.com - 2015-10-04 19:48 -0700
      Re: the language (and the role of Undefined Behavior therein) glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2015-10-05 04:50 +0000
        Re: the language (and the role of Undefined Behavior therein) Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-10-05 13:43 +0100
          Re: the language (and the role of Undefined Behavior therein) glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2015-10-05 15:26 +0000
            Re: the language (and the role of Undefined Behavior therein) Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-10-05 19:39 +0100
              Re: the language (and the role of Undefined Behavior therein) supercat@casperkitty.com - 2015-10-05 13:32 -0700
              Re: the language (and the role of Undefined Behavior therein) glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2015-10-05 22:06 +0000
                Re: the language (and the role of Undefined Behavior therein) supercat@casperkitty.com - 2015-10-05 16:08 -0700
                Re: the language (and the role of Undefined Behavior therein) Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-10-06 11:04 +0100
                Re: the language (and the role of Undefined Behavior therein) supercat@casperkitty.com - 2015-10-06 08:06 -0700
                Re: the language (and the role of Undefined Behavior therein) Rosario19 <Ros@invalid.invalid> - 2015-10-06 18:14 +0200
                Re: the language (and the role of Undefined Behavior therein) Rosario19 <Ros@invalid.invalid> - 2015-10-06 18:54 +0200
                Re: the language (and the role of Undefined Behavior therein) supercat@casperkitty.com - 2015-10-06 11:20 -0700
                Re: the language (and the role of Undefined Behavior therein) supercat@casperkitty.com - 2015-10-06 10:00 -0700
                Re: the language (and the role of Undefined Behavior therein) David Thompson <dave.thompson2@verizon.net> - 2015-10-31 06:44 -0400
                Re: the language (and the role of Undefined Behavior therein) supercat@casperkitty.com - 2015-10-31 11:08 -0700
                Re: the language (and the role of Undefined Behavior therein) Keith Thompson <kst-u@mib.org> - 2015-10-31 13:53 -0700
                Re: the language (and the role of Undefined Behavior therein) supercat@casperkitty.com - 2015-10-31 14:06 -0700
          Re: the language (and the role of Undefined Behavior therein) supercat@casperkitty.com - 2015-10-05 09:28 -0700
          Re: the language (and the role of Undefined Behavior therein) Keith Thompson <kst-u@mib.org> - 2015-10-05 11:20 -0700
            Re: the language (and the role of Undefined Behavior therein) Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-10-05 20:13 +0100
              Re: the language (and the role of Undefined Behavior therein) Tim Rentsch <txr@alumni.caltech.edu> - 2015-10-12 09:29 -0700
      Re: the language (and the role of Undefined Behavior therein) Tim Rentsch <txr@alumni.caltech.edu> - 2015-10-12 10:32 -0700
        Re: the language (and the role of Undefined Behavior therein) supercat@casperkitty.com - 2015-10-12 12:23 -0700
        Re: the language (and the role of Undefined Behavior therein) supercat@casperkitty.com - 2015-10-12 14:42 -0700
        Re: the language (and the role of Undefined Behavior therein) supercat@casperkitty.com - 2015-10-14 09:10 -0700
          Re: the language (and the role of Undefined Behavior therein) Tim Rentsch <txr@alumni.caltech.edu> - 2015-10-19 16:11 -0700

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