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Re: How to copy a string safely and efficiently in C?

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In article <8b37afbd-a536-4d35-ac35-2cec84d0a0f3@googlegroups.com>,
 <supercat@casperkitty.com> wrote:
>On Tuesday, September 13, 2016 at 3:03:40 PM UTC-5, Dan Cross wrote:
>> As I mentioned earlier, GCC 5.4 generates the same machine code for both
>> the strncpy() and memcpy()+compound literal variants.  Clang 3.7 calls
>> strncpy() but optimizes out the call to memcpy().  So not only are there
>> at least two platforms where the generated code is less bulky than the
>> strncpy() version, on at least one of those it is somewhat even lighter.
>
>That's curious; the code generation of 6.2 seems to be a step backward there.
>Given
>
>    #define MSG_SIZE 12
>    void test(char *dest)
>    {
>      memcpy(dest,(char[MSG_SIZE]){"Test"}, MSG_SIZE);
>    }
>
>The 86-64 GCC 6.2 on godbolt.org wrote out 12 consecutive single-byte
>move instructions; if the size were increased to the point where in-
>line expansion would become impractical (e.g. 256) it seems to create
>a temporary array on the stack though it only subtract 136 from SP, which
>I find odd.

Interesting.  I wrote it for arm64 and ran it against gcc 6.1, which
is the most recent compiler I have on my dev machine for the aarch64
port of the Harvey OS.  On that platform, the strncpy() version emits
a call to strncpy(), as on x86 with Clang, while your code optimizes
out the call to memcpy() and emits four moves and twelve STRB instructions
(this makes sense as presumably the function in question doesn't know
the alignment of `dest` at compile time).

Somewhat more surprising to me was that if I changed the code to write
into e.g. a global variable and I changed the type of that to be e.g.
an int, one still sees the same number of single-byte loads.  I would
have expected the compiler to recognize that these were aligned and
load registers with constants containing more than a single byte of
text, and then write to memory with word-storing instructions.

	- Dan C.

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How to copy a string safely and efficiently in C? will hunt <glovepm@gmail.com> - 2016-09-01 18:50 -0700
  Re: How to copy a string safely and efficiently in C? John Gordon <gordon@panix.com> - 2016-09-02 01:58 +0000
    Re: How to copy a string safely and efficiently in C? will hunt <glovepm@gmail.com> - 2016-09-01 19:40 -0700
      Re: How to copy a string safely and efficiently in C? Geoff <geoff@invalid.invalid> - 2016-09-01 20:29 -0700
        Re: How to copy a string safely and efficiently in C? James Kuyper <jameskuyper@verizon.net> - 2016-09-02 12:29 -0400
      Re: How to copy a string safely and efficiently in C? asetofsymbols@gmail.com - 2016-09-12 13:09 -0700
        Re: How to copy a string safely and efficiently in C? Keith Thompson <kst-u@mib.org> - 2016-09-12 15:30 -0700
    Re: How to copy a string safely and efficiently in C? Keith Thompson <kst-u@mib.org> - 2016-09-01 23:08 -0700
    Re: How to copy a string safely and efficiently in C? Malcolm McLean <malcolm.mclean5@btinternet.com> - 2016-09-04 23:59 -0700
  Re: How to copy a string safely and efficiently in C? Joe Pfeiffer <pfeiffer@cs.nmsu.edu> - 2016-09-01 21:12 -0600
    Re: How to copy a string safely and efficiently in C? <william@wilbur.25thandClement.com> - 2016-09-01 22:12 -0700
  Re: How to copy a string safely and efficiently in C? Malcolm McLean <malcolm.mclean5@btinternet.com> - 2016-09-01 23:58 -0700
  Re: How to copy a string safely and efficiently in C? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2016-09-02 10:42 +0100
    Re: How to copy a string safely and efficiently in C? Tim Rentsch <txr@alumni.caltech.edu> - 2016-09-04 22:23 -0700
      Re: How to copy a string safely and efficiently in C? Keith Thompson <kst-u@mib.org> - 2016-09-04 23:20 -0700
        Re: How to copy a string safely and efficiently in C? mark.bluemel@gmail.com - 2016-09-05 01:03 -0700
          Re: How to copy a string safely and efficiently in C? Keith Thompson <kst-u@mib.org> - 2016-09-05 01:46 -0700
        Re: How to copy a string safely and efficiently in C? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2016-09-05 10:30 +0100
          Re: How to copy a string safely and efficiently in C? Tim Rentsch <txr@alumni.caltech.edu> - 2016-09-10 11:25 -0700
        Re: How to copy a string safely and efficiently in C? Tim Rentsch <txr@alumni.caltech.edu> - 2016-09-11 06:23 -0700
          Re: How to copy a string safely and efficiently in C? supercat@casperkitty.com - 2016-09-11 15:41 -0700
            Re: How to copy a string safely and efficiently in C? Tim Rentsch <txr@alumni.caltech.edu> - 2016-09-11 23:40 -0700
              Re: How to copy a string safely and efficiently in C? Ian Collins <ian-news@hotmail.com> - 2016-09-12 21:04 +1200
                Re: How to copy a string safely and efficiently in C? Melzzzzz <Melzzzzz@zzzzz.com> - 2016-09-12 09:21 +0000
                Re: How to copy a string safely and efficiently in C? Ian Collins <ian-news@hotmail.com> - 2016-09-12 21:33 +1200
                Re: How to copy a string safely and efficiently in C? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2016-09-12 10:41 +0100
                Re: How to copy a string safely and efficiently in C? Melzzzzz <Melzzzzz@zzzzz.com> - 2016-09-12 10:48 +0000
                Re: How to copy a string safely and efficiently in C? Tim Rentsch <txr@alumni.caltech.edu> - 2016-09-12 07:04 -0700
  Re: How to copy a string safely and efficiently in C? Richard Heathfield <rjh@cpax.org.uk> - 2016-09-02 12:36 +0100
    Re: How to copy a string safely and efficiently in C? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2016-09-02 14:24 +0100
      Re: How to copy a string safely and efficiently in C? Richard Heathfield <rjh@cpax.org.uk> - 2016-09-02 14:51 +0100
      Re: How to copy a string safely and efficiently in C? Tim Rentsch <txr@alumni.caltech.edu> - 2016-09-04 23:42 -0700
  Re: How to copy a string safely and efficiently in C? Richard Heathfield <rjh@cpax.org.uk> - 2016-09-02 18:05 +0100
    Re: How to copy a string safely and efficiently in C? Ike Naar <ike@iceland.freeshell.org> - 2016-09-02 19:48 +0000
  Re: How to copy a string safely and efficiently in C? cross@spitfire.i.gajendra.net (Dan Cross) - 2016-09-13 01:45 +0000
    Re: How to copy a string safely and efficiently in C? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2016-09-13 11:06 +0100
      Re: How to copy a string safely and efficiently in C? cross@spitfire.i.gajendra.net (Dan Cross) - 2016-09-13 14:09 +0000
        Re: How to copy a string safely and efficiently in C? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2016-09-13 15:30 +0100
          Re: How to copy a string safely and efficiently in C? cross@spitfire.i.gajendra.net (Dan Cross) - 2016-09-13 14:52 +0000
            Re: How to copy a string safely and efficiently in C? cross@spitfire.i.gajendra.net (Dan Cross) - 2016-09-13 14:54 +0000
              Re: How to copy a string safely and efficiently in C? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2016-09-13 17:18 +0100
                Re: How to copy a string safely and efficiently in C? cross@spitfire.i.gajendra.net (Dan Cross) - 2016-09-13 18:15 +0000
              Re: How to copy a string safely and efficiently in C? Tim Rentsch <txr@alumni.caltech.edu> - 2016-09-13 20:30 -0700
                Re: How to copy a string safely and efficiently in C? cross@spitfire.i.gajendra.net (Dan Cross) - 2016-09-15 14:03 +0000
                Re: How to copy a string safely and efficiently in C? Tim Rentsch <txr@alumni.caltech.edu> - 2016-09-15 12:32 -0700
                Re: How to copy a string safely and efficiently in C? luser droog <luser.droog@gmail.com> - 2016-09-15 12:45 -0700
                Re: How to copy a string safely and efficiently in C? supercat@casperkitty.com - 2016-09-15 13:00 -0700
                Re: How to copy a string safely and efficiently in C? Joe Pfeiffer <pfeiffer@cs.nmsu.edu> - 2016-09-15 14:52 -0600
                Re: How to copy a string safely and efficiently in C? Keith Thompson <kst-u@mib.org> - 2016-09-15 16:04 -0700
                Re: How to copy a string safely and efficiently in C? Joe Pfeiffer <pfeiffer@cs.nmsu.edu> - 2016-09-15 15:52 -0600
                Re: How to copy a string safely and efficiently in C? Keith Thompson <kst-u@mib.org> - 2016-09-15 17:07 -0700
                Re: How to copy a string safely and efficiently in C? Keith Thompson <kst-u@mib.org> - 2016-09-15 15:53 -0700
                Re: How to copy a string safely and efficiently in C? Richard Heathfield <rjh@cpax.org.uk> - 2016-09-15 22:17 +0100
                Re: How to copy a string safely and efficiently in C? Keith Thompson <kst-u@mib.org> - 2016-09-15 17:01 -0700
                Re: How to copy a string safely and efficiently in C? supercat@casperkitty.com - 2016-09-15 16:36 -0700
                Re: How to copy a string safely and efficiently in C? Keith Thompson <kst-u@mib.org> - 2016-09-15 19:19 -0700
                Re: How to copy a string safely and efficiently in C? supercat@casperkitty.com - 2016-09-15 21:39 -0700
                Re: How to copy a string safely and efficiently in C? Tim Rentsch <txr@alumni.caltech.edu> - 2016-09-16 06:17 -0700
                Re: How to copy a string safely and efficiently in C? Malcolm McLean <malcolm.mclean5@btinternet.com> - 2016-09-16 07:04 -0700
                Re: How to copy a string safely and efficiently in C? supercat@casperkitty.com - 2016-09-16 09:11 -0700
                Re: How to copy a string safely and efficiently in C? Tim Rentsch <txr@alumni.caltech.edu> - 2016-09-22 22:24 -0700
                Re: How to copy a string safely and efficiently in C? cross@spitfire.i.gajendra.net (Dan Cross) - 2016-09-19 15:15 +0000
    Re: How to copy a string safely and efficiently in C? Richard Damon <Richard@Damon-Family.org> - 2016-09-13 08:19 -0400
      Re: How to copy a string safely and efficiently in C? cross@spitfire.i.gajendra.net (Dan Cross) - 2016-09-13 14:07 +0000
        Re: How to copy a string safely and efficiently in C? supercat@casperkitty.com - 2016-09-13 07:57 -0700
          Re: How to copy a string safely and efficiently in C? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2016-09-13 17:33 +0100
            Re: How to copy a string safely and efficiently in C? cross@spitfire.i.gajendra.net (Dan Cross) - 2016-09-13 18:13 +0000
            Re: How to copy a string safely and efficiently in C? supercat@casperkitty.com - 2016-09-13 11:30 -0700
              Re: How to copy a string safely and efficiently in C? cross@spitfire.i.gajendra.net (Dan Cross) - 2016-09-13 20:03 +0000
                Re: How to copy a string safely and efficiently in C? supercat@casperkitty.com - 2016-09-13 13:38 -0700
                Re: How to copy a string safely and efficiently in C? cross@spitfire.i.gajendra.net (Dan Cross) - 2016-09-13 21:39 +0000
              Re: How to copy a string safely and efficiently in C? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2016-09-13 23:26 +0100
            Re: How to copy a string safely and efficiently in C? Tim Rentsch <txr@alumni.caltech.edu> - 2016-09-13 16:14 -0700
    Re: How to copy a string safely and efficiently in C? BartC <bc@freeuk.com> - 2016-09-13 15:59 +0100
    Re: How to copy a string safely and efficiently in C? Tim Rentsch <txr@alumni.caltech.edu> - 2016-09-13 16:07 -0700
      Re: How to copy a string safely and efficiently in C? cross@spitfire.i.gajendra.net (Dan Cross) - 2016-09-14 00:24 +0000

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