Re: "Experimental" features in releases

[Mark Mitchell <mark at codesourcery dot com>]

David Edelsohn wrote:
> 	-ftree-loop-linear enables a number of features and
> transformations.  Which part, exactly, is experimental?  You are quoting
> from the documentation for the option, but Dan may be referring to a
> particular transformation.  I thought the failure and algorithm
> correctness was related to creating perfect loop nests used by other
> transformations, not all of the tree loop linear.
> 
> 	I think this discussion is losing a lot in the summary.

I did provide nearly all of the information that I had from Dan, but
it's entirely possible that the original conversation failed to convey
the full complexity of the situation to me. :-)

Dan wrote (and gave me permission to repost):

> It only happens with -ftree-loop-linear, a flag we have marked as experimental.
> IE it is not something that users will hit without adding extra flags to their compiler.

I don't see that -ftree-loop-linear is marked as experimental, so
something seems to be confused about this situation.  That quote
suggested to me that the entire option was experimental.

However, Dan also wrote:

> I can fix almost all of these bugs by simply removing the perfect nest conversion code, 

which confirms your suggestion that it's this one transformation that's
experimental.

However, we still have the same problems as from my original email:

* We have an option which is (at least partially) experimental and
known-dangerous, but we're not conveying this to users, and:

* We need a general strategy for dealing with this situation in future,
independent of -ftree-loop-linear.

Possibly, we could solve this specific issue by creating a separate
option to enable the problematic transformations -- but I'd still like
to have a general strategy for dealing with such options, including the
one that would be created to enable the perfect nest conversion code.

-- 
Mark Mitchell
CodeSourcery
mark@codesourcery.com
(650) 331-3385 x713