[Bug tree-optimization/97623] [9/10/11 Regression] Extremely slow O2 compile (>>O(n^2))

["rguenth at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97623

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |needs-bisection

--- Comment #13 from Richard Biener <rguenth at gcc dot gnu.org> ---
OK, on that testcase the good news is that GCC 10 is much better than GCC 9
resulting in -O2 -fno-code-hoisting

 tree PRE                           :   2.25 ( 50%)   0.05 ( 20%)   2.32 ( 49%)
  31794 kB ( 20%)
 TOTAL                              :   4.47          0.25          4.72       
 161356 kB

compared to GCC 9 where I can indeed reproduce

 tree PRE                           :  37.81 ( 95%)   0.04 ( 13%)  37.96 ( 95%)
  32604 kB ( 20%)
 TOTAL                              :  39.63          0.30         39.99       
 161584 kB

suspicious are diffs in insertions and insert iterations:

Insertions: 1258
insert iterations == 3: 1
Eliminated: 1553

vs bad:

Insertions: 8616
insert iterations == 128: 1
Eliminated: 8955

I wonder what triggered this change ...

On patched trunk we're in the same ballpark as GCC 10 but without requiring
-fno-code-hoisting.  The testcase is still an interesting one for PRE
(50% of the compile spent there is a bit much, even if just 2 seconds).