[Bug c++/80297] [6/7 Regression] Compiler time crash: type mismatch in binary expression

[rguenther at suse dot de]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=80297

--- Comment #7 from rguenther at suse dot de <rguenther at suse dot de> ---
On Tue, 4 Apr 2017, jakub at gcc dot gnu.org wrote:

> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=80297
> 
> --- Comment #6 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
> (In reply to Richard Biener from comment #5)
> > We can experiment with adding unshare_exprs to genmatch and see in how many
> > patterns that triggers.  But as said, the general issue looks latent to me.
> > Folding during gimplification is dangerous.
> 
> It isn't just folding during gimplification, we have hundreds of fold_build*
> calls in tons of GIMPLE passes too, and at that point unsharing is also needed.

Well, any GENERIC created there needs to go through force_gimple_operand
again.  Hmm, but that doesn't unshare either.

> I'll try to implement the unsharing and we can see how many patterns it affects
> and based on that decide if we want some cfun flag to control the unsharing or
> not, ok?

Ok.  But given the above we have no choice..

What are the exact rules for GENERIC?  ISTR it was that 
sub-expression sharing is allowed, right?  Or is that just for
frontend GENERIC?  OTOH it would mean that fold-const.c has to cope
with tree sharing but may not generate "new" sharing?  That would be
quite a difficult thing to verify with a verifier...