[Bug tree-optimization/84111] [8 Regression] Compile time hog w/ -O2

[jakub at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=84111

--- Comment #4 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
Seems this is during
#0  follow_copies_to_constant (var=<ssa_name 0x7fffefa46bd0>) at
../../gcc/tree-scalar-evolution.c:1548
#1  0x00000000010ce55e in analyze_scalar_evolution_1 (loop=0x7fffefb99550,
var=<ssa_name 0x7fffefa46bd0>)
    at ../../gcc/tree-scalar-evolution.c:2036
#2  0x00000000010ce7ff in analyze_scalar_evolution (loop=0x7fffefb99550,
var=<ssa_name 0x7fffefa46bd0>) at ../../gcc/tree-scalar-evolution.c:2114
#3  0x00000000010ce8be in analyze_scalar_evolution_in_loop
(wrto_loop=0x7fffefb99550, use_loop=0x7fffefb99550, version=<ssa_name
0x7fffefa46bd0>, 
    folded_casts=0x7fffffffd6bf) at ../../gcc/tree-scalar-evolution.c:2210
#4  0x00000000010d151e in simple_iv_with_niters (wrto_loop=0x7fffefb99550,
use_loop=0x7fffefb99550, op=<ssa_name 0x7fffefa46bd0>, 
    iv=0x7fffffffd7f0, iv_niters=0x7fffffffd7c8, allow_nonconstant_step=false)
at ../../gcc/tree-scalar-evolution.c:3326
#5  0x0000000001183ef6 in number_of_iterations_exit_assumptions
(loop=0x7fffefb99550, exit=<edge 0x7fffefb9cea0 (17 -> 10)>, 
    niter=0x7fffffffd8d0, at_stmt=0x7fffffffd8a8, every_iteration=true) at
../../gcc/tree-ssa-loop-niter.c:2358
#6  0x000000000118437a in number_of_iterations_exit (loop=0x7fffefb99550,
exit=<edge 0x7fffefb9cea0 (17 -> 10)>, niter=0x7fffffffd8d0, 
    warn=false, every_iteration=true) at ../../gcc/tree-ssa-loop-niter.c:2442
#7  0x0000000001184470 in find_loop_niter (loop=0x7fffefb99550,
exit=0x7fffffffda28) at ../../gcc/tree-ssa-loop-niter.c:2474
#8  0x00000000011556fb in canonicalize_loop_induction_variables
(loop=0x7fffefb99550, create_iv=false, ul=UL_NO_GROWTH, try_eval=false, 
    allow_peel=true) at ../../gcc/tree-ssa-loop-ivcanon.c:1198
#9  0x000000000115609d in tree_unroll_loops_completely_1
(may_increase_size=false, unroll_outer=true, father_bbs=0x2ac25e0,
loop=0x7fffefb99550)
    at ../../gcc/tree-ssa-loop-ivcanon.c:1410
#10 0x0000000001155faf in tree_unroll_loops_completely_1
(may_increase_size=false, unroll_outer=true, father_bbs=0x2ac25e0,
loop=0x7fffefb99000)
    at ../../gcc/tree-ssa-loop-ivcanon.c:1379
#11 0x0000000001156180 in tree_unroll_loops_completely
(may_increase_size=false, unroll_outer=true) at
../../gcc/tree-ssa-loop-ivcanon.c:1450

A few possible fixes:
1) just apply an upper bound on how many times we loop:
--- gcc/tree-scalar-evolution.c.jj      2018-01-03 10:19:54.735533890 +0100
+++ gcc/tree-scalar-evolution.c 2018-01-29 20:04:33.198563895 +0100
@@ -1540,9 +1540,12 @@ static tree
 follow_copies_to_constant (tree var)
 {
   tree res = var;
+  int depth = 0;
   while (TREE_CODE (res) == SSA_NAME)
     {
       gimple *def = SSA_NAME_DEF_STMT (res);
+      if (++depth == PARAM_VALUE (PARAM_MAX_SSA_NAME_QUERY_DEPTH))
+       break;
       if (gphi *phi = dyn_cast <gphi *> (def))
        {
          if (tree rhs = degenerate_phi_result (phi))
or give up if name_registered_for_update_p:
--- gcc/tree-scalar-evolution.c.jj      2018-01-03 10:19:54.735533890 +0100
+++ gcc/tree-scalar-evolution.c 2018-01-29 20:30:50.986363089 +0100
@@ -1540,7 +1540,8 @@ static tree
 follow_copies_to_constant (tree var)
 {
   tree res = var;
-  while (TREE_CODE (res) == SSA_NAME)
+  while (TREE_CODE (res) == SSA_NAME
+        && !name_registered_for_update_p (res))
     {
       gimple *def = SSA_NAME_DEF_STMT (res);
       if (gphi *phi = dyn_cast <gphi *> (def))

or a combination of both.  Thoughts on this?