[Bug tree-optimization/82320] [8 Regression] Compile time hog w/ -O

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82320

--- Comment #2 from Richard Biener <rguenth at gcc dot gnu.org> ---
Starting iteration 37161
Value numbering md_11 stmt = md_11 = PHI <md_21(D)(2), md_10(5)>
Setting value number of md_11 to md_21(D) (changed)
Value numbering md_10 stmt = md_10 = PHI <lq_24(D)(7), md_9(4)>
Setting value number of md_10 to lq_24(D) (changed)
Value numbering md_9 stmt = md_9 = PHI <md_10(5), md_11(8)>
Setting value number of md_9 to md_21(D) (changed)
Starting iteration 37162
Value numbering md_11 stmt = md_11 = PHI <md_21(D)(2), md_10(5)>
Setting value number of md_11 to lq_24(D) (changed)
Value numbering md_10 stmt = md_10 = PHI <lq_24(D)(7), md_9(4)>
Setting value number of md_10 to md_21(D) (changed)
Value numbering md_9 stmt = md_9 = PHI <md_10(5), md_11(8)>
Setting value number of md_9 to lq_24(D) (changed)
Starting iteration 37163
Value numbering md_11 stmt = md_11 = PHI <md_21(D)(2), md_10(5)>
Setting value number of md_11 to md_21(D) (changed)
Value numbering md_10 stmt = md_10 = PHI <lq_24(D)(7), md_9(4)>
Setting value number of md_10 to lq_24(D) (changed)
Value numbering md_9 stmt = md_9 = PHI <md_10(5), md_11(8)>
Setting value number of md_9 to md_21(D) (changed)
...

So we iterate between the different leaders for "undefined".  I suppose not
considering lq_24(D) -> md_21(D) a change in values would fix this.

Index: gcc/tree-ssa-sccvn.c
===================================================================
--- gcc/tree-ssa-sccvn.c        (revision 253149)
+++ gcc/tree-ssa-sccvn.c        (working copy)
@@ -3355,6 +3355,12 @@ set_ssa_val_to (tree from, tree to)

   if (currval != to
       && !operand_equal_p (currval, to, 0)
+      /* Different undefined SSA names are not actually different.  See
+         PR82320 for a testcase were we'd otherwise not terminate iteration. 
*/
+      && !(TREE_CODE (currval) == SSA_NAME
+          && TREE_CODE (to) == SSA_NAME
+          && ssa_undefined_value_p (currval, false)
+          && ssa_undefined_value_p (to, false))
       /* ???  For addresses involving volatile objects or types
operand_equal_p
          does not reliably detect ADDR_EXPRs as equal.  We know we are only
         getting invariant gimple addresses here, so can use