Hi,
this patch removes second char array from tree_def union and makes it
!TYPELESS_STORAGE.  Now all accesses to anything in tree no longer have alias
set 0, but they all have alias set 1 :)
This is because the way we handle unions. However it still increases TBAA
effectivity by about 12%. From:

Alias oracle query stats:
  refs_may_alias_p: 65066258 disambiguations, 74846942 queries
  ref_maybe_used_by_call_p: 152444 disambiguations, 65966862 queries
  call_may_clobber_ref_p: 22546 disambiguations, 28559 queries
  nonoverlapping_component_refs_p: 0 disambiguations, 36816 queries
  nonoverlapping_refs_since_match_p: 27230 disambiguations, 58300 must 
overlaps, 86300 queries
  aliasing_component_refs_p: 66090 disambiguations, 2048800 queries
  TBAA oracle: 25578632 disambiguations 59483650 queries
               12219919 are in alias set 0
               10534575 queries asked about the same object
               125 queries asked about the same alias set
               0 access volatile
               9491563 are dependent in the DAG
               1658836 are aritificially in conflict with void *

Modref stats:
  modref use: 14421 disambiguations, 48129 queries
  modref clobber: 1528229 disambiguations, 1926907 queries
  3881547 tbaa queries (2.014392 per modref query)
  565057 base compares (0.293246 per modref query)

PTA query stats:
  pt_solution_includes: 947491 disambiguations, 13119151 queries
  pt_solutions_intersect: 1043695 disambiguations, 13221495 queries

To:

Alias oracle query stats:
  refs_may_alias_p: 66455561 disambiguations, 75202803 queries
  ref_maybe_used_by_call_p: 155301 disambiguations, 67370278 queries
  call_may_clobber_ref_p: 22550 disambiguations, 28587 queries
  nonoverlapping_component_refs_p: 0 disambiguations, 37058 queries
  nonoverlapping_refs_since_match_p: 28126 disambiguations, 59906 must 
overlaps, 88990 queries
  aliasing_component_refs_p: 66375 disambiguations, 2440039 queries
  TBAA oracle: 28800751 disambiguations 64328055 queries
               8053661 are in alias set 0
               11181983 queries asked about the same object
               125 queries asked about the same alias set
               0 access volatile
               13905691 are dependent in the DAG
               2385844 are aritificially in conflict with void *

Modref stats:
  modref use: 16781 disambiguations, 52031 queries
  modref clobber: 1745589 disambiguations, 2149518 queries
  4192266 tbaa queries (1.950328 per modref query)
  559148 base compares (0.260127 per modref query)

PTA query stats:
  pt_solution_includes: 906487 disambiguations, 13105994 queries
  pt_solutions_intersect: 1041144 disambiguations, 13659726 queries

Bootstrapped/regtested x86_64-linux, OK?

        * tree.c (build_string): Update.
        * tree-core.h (tree_fixed_cst): Avoid typeless storage.

diff --git a/gcc/tree-core.h b/gcc/tree-core.h
index c9280a8d3b1..63dbb5b8eab 100644
--- a/gcc/tree-core.h
+++ b/gcc/tree-core.h
@@ -1401,7 +1401,8 @@ struct GTY(()) tree_fixed_cst {
 struct GTY(()) tree_string {
   struct tree_typed typed;
   int length;
-  char str[1];
+  /* Avoid char array that would make whole type to be typeless storage.  */
+  struct {char c;} str[1];
 };
 
 struct GTY(()) tree_complex {
diff --git a/gcc/tree.c b/gcc/tree.c
index 81f867ddded..84115630184 100644
--- a/gcc/tree.c
+++ b/gcc/tree.c
@@ -2273,7 +2273,7 @@ build_string (unsigned len, const char *str /*= NULL */)
     memcpy (s->string.str, str, len);
   else
     memset (s->string.str, 0, len);
-  s->string.str[len] = '\0';
+  s->string.str[len].c = '\0';
 
   return s;
 }

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