[Bug c/54907] post increasing a value pointed by p in subexpression of an expression modifying p saves the increased value in the wrong place

["joseph at codesourcery dot com" <gcc-bugzilla at gcc dot gnu dot org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54907

--- Comment #4 from joseph at codesourcery dot com <joseph at codesourcery dot com> 2012-10-12 15:40:05 UTC ---
I believe this code is well-defined.

There are two objects modified, p and *p.  p is modified by the 
assignment, and C11 6.5.16#3 says "The side effect of updating the stored 
value of the left operand is sequenced after the value computations of the 
left and right operands.".  For postfix increment, which modifies *p, "The 
value computation of the result is sequenced before the side effect of 
updating the stored value of the operand.".  Each object is modified only 
once, so there is no issue of two side effects on the same object being 
unsequenced.  And the value computation of the RHS, using p, is sequenced 
before p is updated by the assignment; furthermore, 5.1.2.3#2 says "Value 
computation for an lvalue expression includes determining the identity of 
the designated object.".  So I don't think either case of undefinedness in 
6.5#2, "If a side effect on a scalar object is unsequenced relative to 
either a different side effect on the same scalar object or a value 
computation using the value of the same scalar object, the behavior is 
undefined.", applies here.