Wrong RTL instruction deleted

[Robert Dewar]

Richard Kenner wrote:
>     The lack of data is always frustrating. So far I am comparing real
>     data from a real large scale user vs Richard's intuitive guesses.
>     I prefer the former to base my judgmnents on.
> 
> We have no data on the value of strict aliasing with tree-ssa.

Yes, so there is no basis for claiming
that it is critical. If indeed -O2 -fno-strict-aliasing has
got a lot worse with tree-ssa, that is another reason to
delay using this version.

If -O2 -fstrict-aliasing has got a lot better, then that's
good news, but it does not necessarily say what the default
should be.

We are seeing an increasing number of users who with pre-
tree-ssa GNAT Pro 5 versions are seeing regressions in
large programs.

It is not at all clear that a performance increase is
worth these regressions. What we are seeing is that more
and more of our users fall into using -fno-strict-aliasing
by default, and indeed we are getting used to recommending
this as the first thing to try if people are seeing mysteries
in -O2. Other users just fall back to -O0, discouraged to
find out that gcc optimization is "dangerous". Yes, yes
I know all the language lawyer responses here :-)