gcc 4.2.2: unexpected difference in behavior between -O0 and -Ox for x = 1, 2, 3

[Tim Prince]

Benj FitzPatrick wrote:
> Hello there,
> I wrote a program that numerically integrates a morse
> potential, given starting and ending points, using
> simpsons rule.  It turns out I forgot to include
>
> return(integral);
>
> at the end of the function.  What I noticed was that
> if I used gcc -O0 the program would work fine, but if
> I used any level of optimization it would give near
> zero for the integral when it should be between 1 and
> 9.  Also, if I put in a printf("%8.3e\n", integral) at
> the end the program gives the correct answers when
> optimizations are turned on.  
>
> My question is why does the buggy code give the
> correct answer with no optimizations, but an incorrect
> answer with optimizations turned on?
>   
_________________________________________________
> The un-optimized code apparently happens to calculate the result in the same register where the calling function expects it.  You certainly can't count on this.   The effect would depend on target identity as well as optimization level, as you have already been reminded.
>