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Re: Initializing a malloc'ed struct whose fields are const with run-time values

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Ben Bacarisse wrote:

> Noob wrote:
> 
>> BartC wrote:
>>
>>> Noob wrote:
>>>
>>>> You're right, I meant:
>>>>
>>>>  struct toto { const int i; const float f; void *const p; };
>>>>
>>>> And it turns out that all this work was fruitless, as gcc does not
>>>> take advantage of the const-ness of the fields, and keeps reloading
>>>> the values from memory every time they are needed...
>>>
>>> Where should it load them from instead?
>>
>> I expected the compiler to write the value to a register before
>> the loop, and read it from there henceforth.
>>
>>> Perhaps there was no opportunity to keep the values resident in registers.
>>
>> If I convince the compiler that "i" will not change during the
>> lifetime of the function by defining a copy on the stack:
>>
>>   const int i = ctx->i;
>>   while ( 1 ) { use i, and the rest of the ctx }
>>
>> then i is indeed kept in a register (of which I have 32 on this
>> platform). I was hoping to sprinkle const-qualifiers in the struct
>> definition so that I wouldn't have to make copies of every const
>> field in the ctx struct.
> 
> You may find that using restrict (if appropriate) gives better results
> than const.  For example, given
> 
>   struct toto { int i; double d; };
>    
>   double f(struct toto *ctx, double *p)
>   {
>        double s = 0;
>        for (int i = 0; i < ctx->i; i++) {
>             s += ctx->d;
>             *p += 1;
>        }
>        return s;
>   }
> 
> gcc won't keep ctx->d in a register because *p might alias it.  The code
> loads ctx->d every time even if cts points to a const struct with const
> members.  But making both ctx and p restrict qualified does what you want.

I could be wrong, but I don't think this is an aliasing problem.

The prototype for the thread's entry point is
  void *start_routine(void *arg)
(In my case, arg is, in fact, a (struct toto *))

To make your example more like mine, your function "f" should only take
a single (struct toto *) parameter, in which case I'm not sure restrict-
qualifying the parameter brings much? (And is it compatible with the
expected prototype?)

Does it make sense to restrict-qualify struct fields?

I'm off to browse ISO/IEC 9899:TC3

Regards.

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Initializing a malloc'ed struct whose fields are const with run-time values Noob <root@127.0.0.1> - 2013-01-22 16:36 +0100
  Re: Initializing a malloc'ed struct whose fields are const with run-time values Shao Miller <sha0.miller@gmail.com> - 2013-01-22 11:23 -0500
    Re: Initializing a malloc'ed struct whose fields are const with run-time values Ben Bacarisse <ben.usenet@bsb.me.uk> - 2013-01-22 21:26 +0000
      Re: Initializing a malloc'ed struct whose fields are const with run-time values James Kuyper <jameskuyper@verizon.net> - 2013-01-22 16:41 -0500
    Re: Initializing a malloc'ed struct whose fields are const with run-time values William Ahern <william@wilbur.25thandClement.com> - 2013-01-22 13:25 -0800
      Re: Initializing a malloc'ed struct whose fields are const with run-time values Shao Miller <sha0.miller@gmail.com> - 2013-01-22 23:17 -0500
    Re: Initializing a malloc'ed struct whose fields are const with run-time values Ben Bacarisse <ben.usenet@bsb.me.uk> - 2013-01-23 13:09 +0000
      Re: Initializing a malloc'ed struct whose fields are const with run-time values Noob <root@127.0.0.1> - 2013-01-23 14:16 +0100
        Re: Initializing a malloc'ed struct whose fields are const with run-time values "BartC" <bc@freeuk.com> - 2013-01-23 13:25 +0000
          Re: Initializing a malloc'ed struct whose fields are const with run-time values Noob <root@127.0.0.1> - 2013-01-23 15:21 +0100
            Re: Initializing a malloc'ed struct whose fields are const with run-time values Ben Bacarisse <ben.usenet@bsb.me.uk> - 2013-01-23 14:56 +0000
              Re: Initializing a malloc'ed struct whose fields are const with run-time values Noob <root@127.0.0.1> - 2013-01-23 16:40 +0100
                Re: Initializing a malloc'ed struct whose fields are const with run-time values Ben Bacarisse <ben.usenet@bsb.me.uk> - 2013-01-23 18:26 +0000
                Re: Initializing a malloc'ed struct whose fields are const with run-time values Tim Rentsch <txr@alumni.caltech.edu> - 2013-01-23 20:39 -0800
            Re: Initializing a malloc'ed struct whose fields are const with run-time values Shao Miller <sha0.miller@gmail.com> - 2013-01-23 17:14 -0500
  Re: Initializing a malloc'ed struct whose fields are const with run-time values Ike Naar <ike@sverige.freeshell.org> - 2013-01-22 17:59 +0000
    Re: Initializing a malloc'ed struct whose fields are const with run-time values Shao Miller <sha0.miller@gmail.com> - 2013-01-22 14:09 -0500
      Re: Initializing a malloc'ed struct whose fields are const with run-time values Noob <root@localhost> - 2013-01-22 21:59 +0100
        Re: Initializing a malloc'ed struct whose fields are const with run-time values glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2013-01-22 21:19 +0000
        Re: Initializing a malloc'ed struct whose fields are const with run-time values Shao Miller <sha0.miller@gmail.com> - 2013-01-22 23:47 -0500
      Re: Initializing a malloc'ed struct whose fields are const with run-time values Ben Bacarisse <ben.usenet@bsb.me.uk> - 2013-01-22 21:05 +0000
        Re: Initializing a malloc'ed struct whose fields are const with run-time values Shao Miller <sha0.miller@gmail.com> - 2013-01-22 16:15 -0500
    Re: Initializing a malloc'ed struct whose fields are const with   run-time values Ian Collins <ian-news@hotmail.com> - 2013-01-23 09:43 +1300
      Re: Initializing a malloc'ed struct whose fields are const with   run-time values William Ahern <william@wilbur.25thandClement.com> - 2013-01-22 13:19 -0800
        Re: Initializing a malloc'ed struct whose fields are const with   run-time values William Ahern <william@wilbur.25thandClement.com> - 2013-01-22 13:35 -0800
          Re: Initializing a malloc'ed struct whose fields are const with   run-time values Ian Collins <ian-news@hotmail.com> - 2013-01-23 10:45 +1300
            Re: Initializing a malloc'ed struct whose fields are const with   run-time values Shao Miller <sha0.miller@gmail.com> - 2013-01-22 17:07 -0500
            Re: Initializing a malloc'ed struct whose fields are const with  run-time values Tim Rentsch <txr@alumni.caltech.edu> - 2013-01-22 21:29 -0800
              Re: Initializing a malloc'ed struct whose fields are const with  run-time values Ben Bacarisse <ben.usenet@bsb.me.uk> - 2013-01-23 13:54 +0000
                Re: Initializing a malloc'ed struct whose fields are const with  run-time values Shao Miller <sha0.miller@gmail.com> - 2013-01-23 17:08 -0500
                Re: Initializing a malloc'ed struct whose fields are const with    run-time values Ian Collins <ian-news@hotmail.com> - 2013-01-24 11:29 +1300
                Re: Initializing a malloc'ed struct whose fields are const with    run-time values Shao Miller <sha0.miller@gmail.com> - 2013-01-23 18:26 -0500
                Re: Initializing a malloc'ed struct whose fields are const with      run-time values Ian Collins <ian-news@hotmail.com> - 2013-01-24 14:03 +1300
                Re: Initializing a malloc'ed struct whose fields are const with      run-time values Shao Miller <sha0.miller@gmail.com> - 2013-01-23 23:58 -0500
                Re: Initializing a malloc'ed struct whose fields are const with  run-time values Ben Bacarisse <ben.usenet@bsb.me.uk> - 2013-01-23 23:36 +0000
                Re: Initializing a malloc'ed struct whose fields are const with  run-time values Ian Collins <ian-news@hotmail.com> - 2013-01-24 13:55 +1300
                Re: Initializing a malloc'ed struct whose fields are const with  run-time values Ben Bacarisse <ben.usenet@bsb.me.uk> - 2013-01-24 02:44 +0000
                Re: Initializing a malloc'ed struct whose fields are const with  run-time values Tim Rentsch <txr@alumni.caltech.edu> - 2013-01-23 21:39 -0800
                Re: Initializing a malloc'ed struct whose fields are const with  run-time values Shao Miller <sha0.miller@gmail.com> - 2013-01-24 00:54 -0500
                Re: Initializing a malloc'ed struct whose fields are const with  run-time values Ben Bacarisse <ben.usenet@bsb.me.uk> - 2013-01-24 17:56 +0000
                Re: Initializing a malloc'ed struct whose fields are const with  run-time values Tim Rentsch <txr@alumni.caltech.edu> - 2013-01-24 13:58 -0800
        Re: Initializing a malloc'ed struct whose fields are const with   run-time values James Kuyper <jameskuyper@verizon.net> - 2013-01-22 16:37 -0500
          Re: Initializing a malloc'ed struct whose fields are const with  run-time values Tim Rentsch <txr@alumni.caltech.edu> - 2013-01-22 21:22 -0800
      Re: Initializing a malloc'ed struct whose fields are const with  run-time values Tim Rentsch <txr@alumni.caltech.edu> - 2013-01-22 21:06 -0800
        Re: Initializing a malloc'ed struct whose fields are const with  run-time values Ian Collins <ian-news@hotmail.com> - 2013-01-23 19:55 +1300
          Re: Initializing a malloc'ed struct whose fields are const with  run-time values Tim Rentsch <txr@alumni.caltech.edu> - 2013-01-23 00:48 -0800
            Re: Initializing a malloc'ed struct whose fields are const with  run-time values Ian Collins <ian-news@hotmail.com> - 2013-01-23 23:06 +1300
    Re: Initializing a malloc'ed struct whose fields are const with run-time values Ben Bacarisse <ben.usenet@bsb.me.uk> - 2013-01-22 21:02 +0000
      Re: Initializing a malloc'ed struct whose fields are const with run-time values Shao Miller <sha0.miller@gmail.com> - 2013-01-22 16:12 -0500
        Re: Initializing a malloc'ed struct whose fields are const with run-time values Ben Bacarisse <ben.usenet@bsb.me.uk> - 2013-01-22 22:37 +0000
          Re: Initializing a malloc'ed struct whose fields are const with run-time values Shao Miller <sha0.miller@gmail.com> - 2013-01-22 22:29 -0500
            Re: Initializing a malloc'ed struct whose fields are const with run-time values Ben Bacarisse <ben.usenet@bsb.me.uk> - 2013-01-23 04:28 +0000
              Re: Initializing a malloc'ed struct whose fields are const with run-time values Shao Miller <sha0.miller@gmail.com> - 2013-01-23 00:07 -0500
                Re: Initializing a malloc'ed struct whose fields are const with run-time values Ben Bacarisse <ben.usenet@bsb.me.uk> - 2013-01-23 14:04 +0000
    Re: Initializing a malloc'ed struct whose fields are const with run-time values Noob <root@localhost> - 2013-01-22 22:10 +0100
  Re: Initializing a malloc'ed struct whose fields are const with run-time values Ben Bacarisse <ben.usenet@bsb.me.uk> - 2013-01-22 21:13 +0000
    Re: Initializing a malloc'ed struct whose fields are const with run-time values Shao Miller <sha0.miller@gmail.com> - 2013-01-22 16:25 -0500
      Re: Initializing a malloc'ed struct whose fields are const with run-time values Ben Bacarisse <ben.usenet@bsb.me.uk> - 2013-01-22 22:21 +0000
    Re: Initializing a malloc'ed struct whose fields are const with run-time values William Ahern <william@wilbur.25thandClement.com> - 2013-01-22 13:32 -0800
      Re: Initializing a malloc'ed struct whose fields are const with run-time values Ben Bacarisse <ben.usenet@bsb.me.uk> - 2013-01-23 03:50 +0000
    Re: Initializing a malloc'ed struct whose fields are const with run-time values Nick Bowler <nbowler@draconx.ca> - 2013-01-22 21:43 +0000
      Re: Initializing a malloc'ed struct whose fields are const with run-time values Ben Bacarisse <ben.usenet@bsb.me.uk> - 2013-01-23 03:45 +0000
        Re: Initializing a malloc'ed struct whose fields are const with run-time values Keith Thompson <kst-u@mib.org> - 2013-01-24 08:23 -0800
  Re: Initializing a malloc'ed struct whose fields are const with run-time values Tim Rentsch <txr@alumni.caltech.edu> - 2013-01-22 20:48 -0800

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