[Bug tree-optimization/55157] Missed VRP with != 0 and multiply

["amacleod at redhat dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=55157

--- Comment #6 from Andrew Macleod <amacleod at redhat dot com> ---
(In reply to Aldy Hernandez from comment #4)

> 
> The patch below does this, but it does have a 3% penalty for VRP (though no
> penalty to overall compilation).  I'm inclined to pursue this route, since
> it makes nonzero mask optimization more pervasive across the board.
> 
> What do you think Andrew?
> 

1) Why wouldn't this be done in set_range_from_nonzero_bits()?  That call is
just above that spot in the code. Or is the name misleading and it does
something else?

2) That seems expensive.. we must be doing unnecessary work.  Maybe it would
speed up if we checked if either the ctz or clz would cause it to do anything
first.  Thus avoiding creating a couple of ranges and performing a union and
intersection in cases where neither the leading nor trailing bit is a zero?

3) It also seems to me that you then only need to add the zero/union iff the
trailing bit has zeros. ie, if the are no trailing zeros, then just set the lb
to 0, and calculate the UB based on the clz.

I should think that would speed things up a bit.