[Bug middle-end/60418] [4.9 Regression] 435.gromacs in SPEC CPU 2006 is miscompiled

[rguenther at suse dot de]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=60418

--- Comment #23 from rguenther at suse dot de <rguenther at suse dot de> ---
On Mon, 10 Mar 2014, hjl.tools at gmail dot com wrote:

> http://gcc.gnu.org/bugzilla/show_bug.cgi?id=60418
> 
> --- Comment #20 from H.J. Lu <hjl.tools at gmail dot com> ---
> (In reply to Richard Biener from comment #13)
> > Huh, adding a pre-header should _never_ do sth like that.  Can you produce
> > a small testcase that exhibits these kind of changes with adding/removing
> > a preheader?
> 
> copyprop2 pass removed a preheader and cunrolli pass added it back:
> 
>   <bb 3>:
>   # n_213 = PHI <1(2)> 
> 
>   <bb 8>:
>   # n_8 = PHI <n_213(3), n_218(9)>

Not sure how it manages to add this kind of PHI node for just
adding a preheader?  That looks odd to me.

> copyprop3 pass optimized it to
> 
>   <bb 3>:
>   n_213 = 1;
> 
>   <bb 4>:
>   # n_8 = PHI <1(3), n_218(7)>
>
> Then the unused n_213 disappeared in reassoc1 pass and n_213 was
> put on FREE_SSANAMES.

By dce1 probably.

That's all expected optimizations and not really dependent on
preheader add/remove.

Richard.