[Bug c/46619] gcc thinks line of code can be removed.

[eskil at obsession dot se]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=46619

--- Comment #16 from Eskil Steenberg <eskil at obsession dot se> 2010-11-23 22:31:14 UTC ---
Hi again!

> http://gcc.gnu.org/bugzilla/show_bug.cgi?id=46619
>
> --- Comment #15 from Andrew Pinski <pinskia at gcc dot gnu.org> 2010-11-23
> 22:09:44 UTC ---
>>Not true. it is not undefined, it is implementation specific.
>
> Huh?  Why do you think that is true?  The C standard is explicit when it
> comes
> to signed integer overflow is undefined behavior.  In fact when talking
> about
> an undefined behavior at the very beginning of the standard, it lists it.

I see this now, i was looking at this:
--------------------
3.2.1.2 Signed and unsigned integers

   When an unsigned integer is converted to another integral type, if
the value can be represented by the new type, its value is unchanged.

   When a signed integer is converted to an unsigned integer with
equal or greater size, if the value of the signed integer is
nonnegative, its value is unchanged.  Otherwise: if the unsigned
integer has greater size, the signed integer is first promoted to the
signed integer corresponding to the unsigned integer; the value is
converted to unsigned by adding to it one greater than the largest
number that can be represented in the unsigned integer type. /22/

   When an integer is demoted to an unsigned integer with smaller
size, the result is the nonnegative remainder on division by the
number one greater than the largest unsigned number that can be
represented in the type with smaller size.  When an integer is demoted
to a signed integer with smaller size, or an unsigned integer is
converted to its corresponding signed integer, if the value cannot be
represented the result is implementation-defined.

-----------------------
Still one could argue that this text:

Undefined behavior --- behavior, upon use of a nonportable or
   erroneous program construct, of erroneous data, or of
   indeterminately-valued objects, for which the Standard imposes no
   requirements.  Permissible undefined behavior ranges from ignoring the
   situation completely with unpredictable results, to behaving during
   translation or program execution in a documented manner characteristic
   of the environment (with or without the issuance of a diagnostic
   message), to terminating a translation or execution (with the issuance
   of a diagnostic message).

...Allows the undefined behavior, to either doing something (witch it
does), doing nothing, or terminating (the compilation or execution). It
does not allow the implementation to change the behavior of things defined
in the spec.

Cheers

E