[Bug tree-optimization/83991] [8 regression] gcc.dg/vect/pr79347.c fail

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=83991

Martin Liška <marxin at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|ASSIGNED                    |NEW
                 CC|                            |amker at gcc dot gnu.org
           Assignee|marxin at gcc dot gnu.org          |unassigned at gcc dot gnu.org

--- Comment #5 from Martin Liška <marxin at gcc dot gnu.org> ---
There's diff of the problematic revision one the one before:

$ diff -u pr79347.c.161t.vect ~/Programming/gcc2/objdir/gcc/pr79347.c.161t.vect
--- pr79347.c.161t.vect 2018-01-25 13:57:56.262964977 +0100
+++ /home/marxin/Programming/gcc2/objdir/gcc/pr79347.c.161t.vect       
2018-01-25 12:47:23.368570520 +0100
@@ -350,8 +350,8 @@
 Applying pattern match.pd:90, generic-match.c:50
 /home/marxin/Programming/gcc/gcc/testsuite/gcc.dg/vect/pr79347.c:9:3: note:
niters for prolog loop: (unsigned int) -((unsigned long) a.0_1 >> 1) & 7
 Applying pattern match.pd:90, gimple-match.c:65
-;; Scaling loop 1 with scale 90.0% (guessed) bounding iterations to 0 from
guessed 9
-;; guessed iterations are now 9
+;; Scaling loop 1 with scale 90.0% (guessed) bounding iterations to 0 from
guessed 7
+;; guessed iterations are now 6
 Removing basic block 11
 ;; basic block 11, loop depth 0
 ;;  pred:       9
@@ -367,8 +367,8 @@
 Applying pattern match.pd:1476, generic-match.c:571
 Applying pattern match.pd:1479, generic-match.c:622
 Applying pattern match.pd:1486, generic-match.c:642
-;; Scaling loop 2 with scale 87.5% (guessed) bounding iterations to 7 from
guessed 10
-;; guessed iterations are now 1
+;; Scaling loop 2 with scale 87.5% (guessed) bounding iterations to 7 from
guessed 7
+;; guessed iterations are now 6
 /home/marxin/Programming/gcc/gcc/testsuite/gcc.dg/vect/pr79347.c:9:3: note:
=== vect_update_inits_of_dr ===
 Applying pattern match.pd:99, generic-match.c:13248
 Applying pattern match.pd:99, generic-match.c:13248

Note that the second loop is not changed from guessed '10' to '7' and I think
it exposes a latent bug in scale_loop_profile
function.

Amker may I please ask you for help?
The loop looks simple, it would be easier for you to find the problem.

And one another note/observation:
The first loop is loop prologue where we align the pointer to 8.
Isn't this situation where expected # of iterations is expected to be let's say
8/2 ?
I would skip guessed loop iteration predictor for that?
Thanks