[Bug tree-optimization/77498] [7 regression] Performance drop after r239414 on spec2000/172mgrid

[rguenther at suse dot de]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=77498

--- Comment #10 from rguenther at suse dot de <rguenther at suse dot de> ---
On Mon, 20 Mar 2017, thopre01 at gcc dot gnu.org wrote:

> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=77498
> 
> --- Comment #9 from Thomas Preud'homme <thopre01 at gcc dot gnu.org> ---
> Sadly I could not come up with a minimal testcase so far. What I can see from
> the code is that tree code hoisting increases the live range of some values
> which then translates into more spilling in reload.
> 
> As an approximation I'm wondering if the maximum distance (computer in number
> of blocks traversed) from the definition to the use could be used to limit when
> the optimization is applied when optimizing for speed.

Sadly the data-flow used to compute the opportunities is not suitable
for determining this.  It would probaly require "aging" of exprs in
the hoistable sets when propagating the dataflow for ANTIC_IN (in
principle PRE would have a similar issue).  We already restrict
"distance" by requiring at least one successor of the hoisting point
to provide the value directly but we do not limit proximity further.

See do_hoist_insertion.