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Re: do{switch(0)default:{/*break or continue*/;}/*cleanup*/}while(0);

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Francois Grieu writes:
> On 2011/22/07 12:15, August Karlstrom wrote:
>> On 2011-07-22 08:49, Francois Grieu wrote:
>>> I just came into a case where I could use this:
>>>
>>> do
>>> {
>>> // local declarations and initializations
>>> switch(0)default:
>>> {
>>> // bunch of if/else; within that:
>>> // - "break" performs cleanup;
>>> // - "continue" skips cleanup.
>>> /*..*/;
>>> }
>>> /*..*/; // cleanup
>>> }
>>> while(0);

To my mind this is too clever and loses as much in clarity as avoiding
goto/label gains.  For a block/exit construct it overloads continue and
break arbitrarily: If you'd merely said you had a block construct where
continue and break exit with/without cleanup, I'd expect "continue" to
be the soft exit with cleanup, "break" to be the hard one without.

OTOH I've sometimes written

    switch (0) default: {
        ... if (...) break; ... 
    }

but I don't mind a few labels instead, with clear purposes like 'done:'
and 'abort:'.  With lots of jumps due to error checking, I tend to
structure C code as it would be without error checking, and then put the
error handling label(s) at the end.  Just avoid gotos when convenient
and keep the remaining ones readable.  E.g. don't goto both up and down
in the same function, which makes it hard to see if there is a hidden
loop.  Labels in the middle of a function make program flow harder to
see than those near the beginning/end.  Except an occasional end_loop:.

>> I must be missing something. Why can't you just do something like this?
>>
>> cleanup = 0;
>> if (c1) {
>> ...
>> cleanup = 1;
>> } else if (c2) {
>>
>> }...
>>
>> }
>> if (cleanup) {
>>
>> }
>
> I *CAN*. But I'd rather not. I know that most if not all the compilers I
> use will not notice that the "cleanup" variable can be suppressed,

Have you looked at the assembly output to check this?  You might be
right if you were talking about code like this:

int f(int a) {
    int c = 0;
    if (a < 0) {
	c = 1;
	a *= a;
    }
    g();
    if (c) {
	h();
    }
    return a;
}

because c might be either set or reset after executing g().  But actually
gcc -Os (optimize for space) handles even that fine, and treats it as

int f(int a) {
    if (a < 0) {
	a *= a;
	g();
	h();
    } else
	g();
    return a;
}

> and
> knowing that an extra variable exists and code is here to set and test
> it is causing me brain pain. I'd rather use two gotos (or a single goto
> and a deep nesting of ifs, which often will do the job), and get
> basically the "right" executable.

Up to here, this sounds like the itch for premature optimization.  In
those cases, just Get Over It.  Really.

> If I rationalize: on low-end platforms (PIC..) that I often use, RAM
> size is a few hundred bytes and program space a few kbytes, so every
> byte counts. Even on relatively powerful CPUs (Arm, x86) and when memory
> is not an issue, using one more register may force the shift of
> something into memory rather than in a register, or/and a little extra
> code could abruptly reduce cache efficiency, and in turn slow down
> things considerably. I bet I could construct an articial demo where the
> suggested change increases execution time/power draw quite perceptibly,
> say by 30%.

OK, that makes sense.

However, when you do decide to help the compiler, take care to check
that it actually was helpful or at least that it did no harm.  Probably
not a problem in his case, but it's not unusual for such "help" to be a
pessimization, if the code becomes harder to analyze for the compiler
than more natural code would be.  A classic example is code which walks
an array, a common operation which good compilers recognize and have
lots of tricks to deal with.  But if you instead helpfully start doing
pointer arithmetic on the array base/end, the compiler may give up
on trying those tricks and instead just compile your code step by step.

-- 
Hallvard

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do{switch(0)default:{/*break or continue*/;}/*cleanup*/}while(0); Francois Grieu <fgrieu@gmail.com> - 2011-07-22 08:49 +0200
  Re: do{switch(0)default:{/*break or continue*/;}/*cleanup*/}while(0); "Kleuskes & Moos" <kleuske@xs4all.nl> - 2011-07-22 00:58 -0700
  Re: do{switch(0)default:{/*break or continue*/;}/*cleanup*/}while(0); gwowen <gwowen@gmail.com> - 2011-07-22 01:46 -0700
    Re: do{switch(0)default:{/*break or continue*/;}/*cleanup*/}while(0); gazelle@shell.xmission.com (Kenny McCormack) - 2011-07-22 13:16 +0000
      Re: do{switch(0)default:{/*break or continue*/;}/*cleanup*/}while(0); "Kleuskes & Moos" <kleuske@xs4all.nl> - 2011-07-22 06:35 -0700
      Re: do{switch(0)default:{/*break or continue*/;}/*cleanup*/}while(0); tom st denis <tom@iahu.ca> - 2011-07-22 08:37 -0700
      Re: do{switch(0)default:{/*break or continue*/;}/*cleanup*/}while(0); Kenneth Brody <kenbrody@spamcop.net> - 2011-07-22 12:20 -0400
    Re: do{switch(0)default:{/*break or continue*/;}/*cleanup*/}while(0); Francois Grieu <fgrieu@gmail.com> - 2011-07-22 16:41 +0200
    Re: do{switch(0)default:{/*break or continue*/;}/*cleanup*/}while(0); Kenneth Brody <kenbrody@spamcop.net> - 2011-07-22 12:10 -0400
      Re: do{switch(0)default:{/*break or continue*/;}/*cleanup*/}while(0); Ben Bacarisse <ben.usenet@bsb.me.uk> - 2011-07-22 19:39 +0100
      Re: do{switch(0)default:{/*break or continue*/;}/*cleanup*/}while(0); luser- -droog <mijoryx@yahoo.com> - 2011-07-22 21:05 -0700
        Re: do{switch(0)default:{/*break or continue*/;}/*cleanup*/}while(0); Ben Bacarisse <ben.usenet@bsb.me.uk> - 2011-07-23 13:24 +0100
          Re: do{switch(0)default:{/*break or continue*/;}/*cleanup*/}while(0); gazelle@shell.xmission.com (Kenny McCormack) - 2011-07-23 15:11 +0000
          Re: do{switch(0)default:{/*break or continue*/;}/*cleanup*/}while(0); "io_x" <a@b.c.invalid> - 2011-07-23 22:16 +0200
    Re: do{switch(0)default:{/*break or continue*/;}/*cleanup*/}while(0); Keith Thompson <kst-u@mib.org> - 2011-07-22 09:39 -0700
      Re: do{switch(0)default:{/*break or continue*/;}/*cleanup*/}while(0); Phil Carmody <thefatphil_demunged@yahoo.co.uk> - 2011-07-22 22:30 +0300
        Re: do{switch(0)default:{/*break or continue*/;}/*cleanup*/}while(0); Keith Thompson <kst-u@mib.org> - 2011-07-22 13:15 -0700
          Re: do{switch(0)default:{/*break or continue*/;}/*cleanup*/}while(0); Phil Carmody <thefatphil_demunged@yahoo.co.uk> - 2011-07-23 01:02 +0300
            Re: do{switch(0)default:{/*break or continue*/;}/*cleanup*/}while(0); Jens Schweikhardt <usenet@schweikhardt.net> - 2011-07-23 13:06 +0000
              Re: do{switch(0)default:{/*break or continue*/;}/*cleanup*/}while(0); Phil Carmody <thefatphil_demunged@yahoo.co.uk> - 2011-07-23 16:51 +0300
              Re: do{switch(0)default:{/*break or continue*/;}/*cleanup*/}while(0); Kenneth Brody <kenbrody@spamcop.net> - 2011-07-25 12:06 -0400
                Re: do{switch(0)default:{/*break or continue*/;}/*cleanup*/}while(0); Jens Schweikhardt <usenet@schweikhardt.net> - 2011-07-25 20:39 +0000
                Re: do{switch(0)default:{/*break or continue*/;}/*cleanup*/}while(0); Kenneth Brody <kenbrody@spamcop.net> - 2011-07-26 10:43 -0400
                Re: do{switch(0)default:{/*break or continue*/;}/*cleanup*/}while(0); Keith Thompson <kst-u@mib.org> - 2011-07-26 08:58 -0700
            Re: do{switch(0)default:{/*break or continue*/;}/*cleanup*/}while(0); Kenneth Brody <kenbrody@spamcop.net> - 2011-07-25 12:03 -0400
              Re: do{switch(0)default:{/*break or continue*/;}/*cleanup*/}while(0); Phil Carmody <thefatphil_demunged@yahoo.co.uk> - 2011-07-29 01:51 +0300
  Re: do{switch(0)default:{/*break or continue*/;}/*cleanup*/}while(0); August Karlstrom <fusionfile@gmail.com> - 2011-07-22 12:15 +0200
    Re: do{switch(0)default:{/*break or continue*/;}/*cleanup*/}while(0); Francois Grieu <fgrieu@gmail.com> - 2011-07-22 16:11 +0200
      Re: do{switch(0)default:{/*break or continue*/;}/*cleanup*/}while(0); "Kleuskes & Moos" <kleuske@xs4all.nl> - 2011-07-22 08:14 -0700
        Re: do{switch(0)default:{/*break or continue*/;}/*cleanup*/}while(0); Thomas Boell <tboell@domain.invalid> - 2011-07-29 14:38 +0200
          Re: do{switch(0)default:{/*break or continue*/;}/*cleanup*/}while(0); James Kuyper <jameskuyper@verizon.net> - 2011-07-29 09:23 -0400
            Re: do{switch(0)default:{/*break or continue*/;}/*cleanup*/}while(0); Thomas Boell <tboell@domain.invalid> - 2011-07-29 15:58 +0200
              Re: do{switch(0)default:{/*break or continue*/;}/*cleanup*/}while(0); James Kuyper <jameskuyper@verizon.net> - 2011-07-29 10:42 -0400
                Re: do{switch(0)default:{/*break or continue*/;}/*cleanup*/}while(0); Willem <willem@toad.stack.nl> - 2011-07-29 14:56 +0000
                Re: do{switch(0)default:{/*break or continue*/;}/*cleanup*/}while(0); James Kuyper <jameskuyper@verizon.net> - 2011-07-29 20:47 -0400
                Re: do{switch(0)default:{/*break or continue*/;}/*cleanup*/}while(0); Willem <willem@toad.stack.nl> - 2011-07-30 09:12 +0000
                Re: do{switch(0)default:{/*break or continue*/;}/*cleanup*/}while(0); James Kuyper <jameskuyper@verizon.net> - 2011-07-30 07:30 -0400
                Re: do{switch(0)default:{/*break or continue*/;}/*cleanup*/}while(0); Thomas Boell <tboell@domain.invalid> - 2011-07-29 17:08 +0200
            Re: do{switch(0)default:{/*break or continue*/;}/*cleanup*/}while(0); Ali <abdulrazaq@gmail.com> - 2011-07-29 08:21 -0700
      Re: do{switch(0)default:{/*break or continue*/;}/*cleanup*/}while(0); Kenneth Brody <kenbrody@spamcop.net> - 2011-07-22 12:24 -0400
        Re: do{switch(0)default:{/*break or continue*/;}/*cleanup*/}while(0); Francois Grieu <fgrieu@gmail.com> - 2011-07-25 10:34 +0200
          Re: do{switch(0)default:{/*break or continue*/;}/*cleanup*/}while(0); Willem <willem@toad.stack.nl> - 2011-07-25 08:54 +0000
            Re: do{switch(0)default:{/*break or continue*/;}/*cleanup*/}while(0); Keith Thompson <kst-u@mib.org> - 2011-07-25 09:08 -0700
              Re: do{switch(0)default:{/*break or continue*/;}/*cleanup*/}while(0); Willem <willem@toad.stack.nl> - 2011-07-25 16:52 +0000
                Re: do{switch(0)default:{/*break or continue*/;}/*cleanup*/}while(0); Keith Thompson <kst-u@mib.org> - 2011-07-25 11:37 -0700
                Re: do{switch(0)default:{/*break or continue*/;}/*cleanup*/}while(0); cri@tiac.net (Richard Harter) - 2011-07-25 20:16 +0000
                Re: do{switch(0)default:{/*break or continue*/;}/*cleanup*/}while(0); Francois Grieu <fgrieu@gmail.com> - 2011-07-26 10:23 +0200
              Re: do{switch(0)default:{/*break or continue*/;}/*cleanup*/}while(0); Nick Keighley <nick_keighley_nospam@hotmail.com> - 2011-07-26 00:49 -0700
                Re: do{switch(0)default:{/*break or continue*/;}/*cleanup*/}while(0); "io_x" <a@b.c.invalid> - 2011-07-26 11:16 +0200
                Re: do{switch(0)default:{/*break or continue*/;}/*cleanup*/}while(0); Nick Keighley <nick_keighley_nospam@hotmail.com> - 2011-07-26 03:09 -0700
                Re: do{switch(0)default:{/*break or continue*/;}/*cleanup*/}while(0); "io_x" <a@b.c.invalid> - 2011-07-26 19:21 +0200
                Re: do{switch(0)default:{/*break or continue*/;}/*cleanup*/}while(0); Nick Keighley <nick_keighley_nospam@hotmail.com> - 2011-07-27 00:43 -0700
                Re: do{switch(0)default:{/*break or continue*/;}/*cleanup*/}while(0); "io_x" <a@b.c.invalid> - 2011-07-27 11:29 +0200
                Re: do{switch(0)default:{/*break or continue*/;}/*cleanup*/}while(0); Keith Thompson <kst-u@mib.org> - 2011-07-27 08:30 -0700
              Re: do{switch(0)default:{/*break or continue*/;}/*cleanup*/}while(0); Kenneth Brody <kenbrody@spamcop.net> - 2011-07-26 10:39 -0400
      Re: do{switch(0)default:{/*break or continue*/;}/*cleanup*/}while(0); Hallvard B Furuseth <h.b.furuseth@usit.uio.no> - 2011-07-26 12:37 +0200
        Re: do{switch(0)default:{/*break or continue*/;}/*cleanup*/}while(0); Francois Grieu <fgrieu@gmail.com> - 2011-07-26 12:52 +0200
          Re: do{switch(0)default:{/*break or continue*/;}/*cleanup*/}while(0); Hallvard B Furuseth <h.b.furuseth@usit.uio.no> - 2011-07-26 13:18 +0200
            Re: do{switch(0)default:{/*break or continue*/;}/*cleanup*/}while(0); Keith Thompson <kst-u@mib.org> - 2011-07-26 08:57 -0700
              Re: do{switch(0)default:{/*break or continue*/;}/*cleanup*/}while(0); Hallvard B Furuseth <h.b.furuseth@usit.uio.no> - 2011-07-27 10:18 +0200
  Re: do{switch(0)default:{/*break or continue*/;}/*cleanup*/}while(0); Ali <abdulrazaq@gmail.com> - 2011-07-23 15:38 -0700
    Re: do{switch(0)default:{/*break or continue*/;}/*cleanup*/}while(0); Ali <abdulrazaq@gmail.com> - 2011-07-24 09:25 -0700
      Re: do{switch(0)default:{/*break or continue*/;}/*cleanup*/}while(0); Nick Keighley <nick_keighley_nospam@hotmail.com> - 2011-07-26 01:01 -0700
    Re: do{switch(0)default:{/*break or continue*/;}/*cleanup*/}while(0); gwowen <gwowen@gmail.com> - 2011-07-25 03:08 -0700

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