[Fwd: Re: compiler interpretation of *mptr++ = mptr]

[Ajoy K Thamattoor <athamatt at cisco dot com>]

  A colleague of mine pointed out gcc gave warnings on the following 
constructs.
I understand a strictly conforming implementation is allowed to warn on 
anything,
but some of these are actually valid constructs. Wanted clarification on 
why gcc
wants to provide sequence-point warnings on these. Please cc me on the 
response,
since I am not subscribed to the list...

Thanks in advance,
Ajoy.

--
http://cs.stanford.edu/~ajoyk





>  a = a; /* { dg-bogus "undefined" "bogus sequence point warning" } */
>  
  There is no multiple modification here. Think of how similar this is to:
a = a + 1;

>  a = fn (a++); /* { dg-bogus "undefined" "bogus sequence point 
> warning" } */
>  
  A function call introduces a sequence point. Hence there is a 
sequence point between
the two modifications of 'a' here.

>  b++, (b + b); /* { dg-bogus "undefined" "bogus sequence point 
> warning" } */
>  
  The comma operator introduces a sequence point. This is fully 
equivalent to
b++; (b + b);

>  (a = b++), (a = b++); /* { dg-bogus "undefined" "bogus sequence point 
> warning" } */
>  
  Same thing. This is equivalent to (a = b++); (a = b++);

>  a = (b++, b++); /* { dg-bogus "undefined" "bogus sequence point 
> warning" } */
>  
   Same thing.

>  a = b++ && b++; /* { dg-bogus "undefined" "bogus sequence point 
> warning" } */
>  a = b++ || b++; /* { dg-bogus "undefined" "bogus sequence point 
> warning" } */
>  a = (b++ ? b++ : a); /* { dg-bogus "undefined" "bogus sequence point 
> warning" } */
>  
  The &&, || and ? operators introduce sequence points. The multiple 
modifications
to 'b' here are separated by a sequence point, and these are valid.

>  a = (b++ ? a : b++); /* { dg-bogus "undefined" "bogus sequence point 
> warning" } */
>  
  Same here.