[Bug rtl-optimization/49847] [4.7/4.8/4.9 Regression] NULL deref in fold_rtx (prev_insn_cc0 == NULL)

[rguenther at suse dot de]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49847

--- Comment #33 from rguenther at suse dot de <rguenther at suse dot de> ---
On Sun, 16 Feb 2014, law at redhat dot com wrote:

> http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49847
> 
> --- Comment #32 from Jeffrey A. Law <law at redhat dot com> ---
> The problem we're seeing is with the cc0-setter and cc0-user in different
> blocks, they're separated by a NOTE_BASIC_BLOCK.  
> 
> That causes CSE to blow up because it expects that the cc0-setter and 
> cc0-user are always consecutive.  While we're just seeing the failure in 
> CSE right now, I'm sure there's a ton of places that assume the 
> setter/user are inseparable as that has been the documented form for ~20 
> years.
> 
> From rtl.texi:
> 
>  The instruction setting the
> condition code must be adjacent to the instruction using the condition
> code; only @code{note} insns may separate them.
> 
> 
> We either need to relax that and audit all the HAVE_cc0 code to ensure it
> doesn't make that assumption, or we need to somehow restore the property that
> the setter and user are inseparable.

I think relaxing this constraint and allowing the cc0-setter and
cc0-user be separated by a fallthru-edge should be allowed
(and make sure that bb-reorder later doesn't separate the BBs)