[Bug tree-optimization/61034] Optimizing takes too many passes

["rguenther at suse dot de" <gcc-bugzilla@gcc.gnu.org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=61034

--- Comment #12 from rguenther at suse dot de <rguenther at suse dot de> ---
On Wed, 7 May 2014, glisse at gcc dot gnu.org wrote:

> http://gcc.gnu.org/bugzilla/show_bug.cgi?id=61034
> 
> --- Comment #11 from Marc Glisse <glisse at gcc dot gnu.org> ---
> The committed patch doesn't seem to optimize as well as the hack. fre2 (- is
> the hack and + is the new trunk, unless I confused my directories):
> 
> -  _8 = _98;
> +  _8 = MEM[(const struct I &)b_9(D)].o;
> 
> -  _224 = _201;
> +  _224 = _18->count;
> 
> etc
> 
> I assume that's because call_may_clobber_ref_p_1 sometimes says that free
> clobbers its argument while the hack was assuming it never does. "free" is
> strange, somehow here we would want call_may_clobber_ref_p_1 to return false
> (we can't be reading after a free, so we must have taken another path, this
> free didn't run and didn't clobber anything) even when stmt_kills_ref_p_1 would
> return true, but that would confuse other parts of gcc.

Yeah, I know.  call_may_clobber_ref_p_1 has to guard against sinking
a load across it as well.

I'm working on improving value-numbering in other ways at the moment,
we can come back and special-case free and va_end builtins later
in SCCVN.