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

["eskil at obsession dot se" <gcc-bugzilla at gcc dot gnu dot org>]

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

--- Comment #11 from Eskil Steenberg <eskil at obsession dot se> 2010-11-23 21:29:12 UTC ---
Hi Again!

> http://gcc.gnu.org/bugzilla/show_bug.cgi?id=46619
>
> --- Comment #10 from Andrew Pinski <pinskia at gcc dot gnu.org> 2010-11-23
> 20:24:00 UTC ---
>    uv = (unsigned int)((int)x[1 + j] * (int)x[1 + i]);
>
> Is what the C standard says should be done as unsigned short is smaller
> than int so it is promoted to int.
       ^^^

Yes, but it ISNT an int, its an unsigned int. double is also bigger then
unsigned short, why not promote it to double if we are bringing in random
types?

>  So we have a 16x16 unsigned to a 31+1
> multiple which can overflow.

Assuming the silliness above is right, it still shouldn't matter.

If it can overflow it means that the product value of the two positive x
values can be negative, and then in turn would become bigger then max
signed int when it gets converted to a unsigned int (uv) from the type int
where you claim the multiplication should be made, making it illegal to
optimize away the later line.

If you read out the data after the line doing the addition it outputs a
unsinged int value larger then max signed int. So the compiler thinks
something could never happen that clearly happens when you run the code.

Cheers

E