[Bug c/125571] compiling with gcc 380 times slower than clang (19 minutes vs 3 seconds)

[rguenth at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=125571

--- Comment #15 from Richard Biener <rguenth at gcc dot gnu.org> ---
(In reply to Drea Pinski from comment #13)
> (In reply to Drea Pinski from comment #12)
> > (In reply to Drea Pinski from comment #11)
> > > Created attachment 64612 [details]
> > > hack to gimplifier to gimplify COND_EXPR as COND_EXPR for -O0/-O1
> > > 
> > > This hacks up the gimplifier and improves comment #9 compile time a lot.
> > > Both at -O0 and -O1.
> > 
> > Seems to helps the original testcase too.
> > 60s at -O1
> > 12s at -O0
> > (this is with -fno-checking but still with checking).
> 
>  tree PTA                           :  18.63 ( 33%)  6847k (  1%)
>  tree SSA rewrite                   :   0.22 (  0%)    18M (  3%)

But it's a hack.  The following hack also helps exactly this testcase:

diff --git a/gcc/bitmap.cc b/gcc/bitmap.cc
index 91c1019cb34..90c64fbe4bb 100644
--- a/gcc/bitmap.cc
+++ b/gcc/bitmap.cc
@@ -404,8 +404,11 @@ bitmap_list_find_element (bitmap head, unsigned int indx)
   /* `element' is the nearest to the one we want.  If it's not the one we
      want, the one we want doesn't exist.  */
   gcc_checking_assert (element != NULL);
-  head->current = element;
-  head->indx = element->indx;
+  if (element != head->first)
+    {
+      head->current = element;
+      head->indx = element->indx;
+    }
   if (element->indx != indx)
     element = 0;
   return element;