[Bug rtl-optimization/72855] Long compile time due to integrity checking during dataflow analysis per loop

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=72855

--- Comment #2 from Richard Biener <rguenth at gcc dot gnu.org> ---
If we have release checking enabled then we shuould hit

static void
df_analyze_1 (void)
{
...
#ifndef ENABLE_DF_CHECKING
  if (df->changeable_flags & DF_VERIFY_SCHEDULED)
#endif
    df_verify ();

so I wonder why df->changeable_flags & DF_VERIFY_SCHEDULED is ever true
for release checking.  Can you track that down?  I can't reproduce
df_verify being called on the 4.9 branch with release checking on x86_64
(OTOH x86_64 doesn't have a doloop pattern).  Compile-time is 186s for 4.9,
main offenders:

 df reaching defs        :  24.61 (13%) usr 
 alias stmt walking      :  13.72 ( 7%) usr  
 dominator optimization  :  33.26 (18%) usr
 expand vars             :  48.89 (26%) usr

GCC 5 seems to be quite a bit worse with 400s:

 df reaching defs        :  13.16 ( 3%) usr
 alias stmt walking      : 216.73 (54%) usr
 expand vars             :  25.86 ( 6%) usr  
 load CSE after reload   :  83.96 (21%) usr  

GCC 6 uses a _load_ more memory on this testcase (I end up swapping with 8GB
ram
and finally get killed).  So even on x86_64 the testcase looks interesting.

Note that

static bool
doloop_optimize (struct loop *loop)
{
...
  iv_analysis_loop_init (loop);

  /* Find the simple exit of a LOOP.  */
  desc = get_simple_loop_desc (loop);


performs iv_analysis_loop_init and thus df_analyze_loop twice ...


But yes, performing df_verify for each loop in a function is excessive,
we seem to lack ever clearing said flag.  Does

Index: gcc/df-core.c
===================================================================
--- gcc/df-core.c       (revision 239276)
+++ gcc/df-core.c       (working copy)
@@ -1833,6 +1833,7 @@ df_verify (void)
   if (df_live)
     df_live_verify_transfer_functions ();
 #endif
+  df->changeable_flags &= ~DF_VERIFY_SCHEDULED;
 }

 #ifdef DF_DEBUG_CFG

help in the non-DF-checking case?