[gcc/devel/c++-coroutines] c++: Fix another PCH hash_map issue [PR96901]

[Iain D Sandoe]

https://gcc.gnu.org/g:ba6730bd18371a3dff1e37d2c2ee27233285b597

commit ba6730bd18371a3dff1e37d2c2ee27233285b597
Author: Jakub Jelinek <jakub@redhat.com>
Date:   Thu Sep 3 21:53:40 2020 +0200

    c++: Fix another PCH hash_map issue [PR96901]
    
    The recent libstdc++ changes caused lots of libstdc++-v3 tests FAILs
    on i686-linux, all of them in the same spot during constexpr evaluation
    of a recursive _S_gcd call.
    The problem is yet another hash_map that used the default hasing of
    tree keys through pointer hashing which is preserved across PCH write/read.
    During PCH handling, the addresses of GC objects are changed, which means
    that the hash values of the keys in such hash tables change without those
    hash tables being rehashed.  Which in the fundef_copies_table case usually
    means we just don't find a copy of a FUNCTION_DECL body for recursive uses
    and start from scratch.  But when the hash table keeps growing, the "dead"
    elements in the hash table can sometimes reappear and break things.
    In particular what I saw under the debugger is when the fundef_copies_table
    hash map has been used on the outer _S_gcd call, it didn't find an entry for
    it, so returned a slot with *slot == NULL, which is treated as that the
    function itself is used directly (i.e. no recursion), but that addition of
    a hash table slot caused the recursive _S_gcd call to actually find
    something in the hash table, unfortunately not the new *slot == NULL spot,
    but a different one from the pre-PCH streaming which contained the returned
    toplevel (non-recursive) call entry for it, which means that for the
    recursive _S_gcd call we actually used the same trees as for the outer ones
    rather than a copy of those, which breaks constexpr evaluation.
    
    2020-09-03  Jakub Jelinek  <jakub@redhat.com>
    
            PR c++/96901
            * tree.h (struct decl_tree_traits): New type.
            (decl_tree_map): New typedef.
    
            * constexpr.c (fundef_copies_table): Change type from
            hash_map<tree, tree> * to decl_tree_map *.

Diff:
---
 gcc/cp/constexpr.c | 2 +-
 gcc/tree.h         | 5 +++++
 2 files changed, 6 insertions(+), 1 deletion(-)

diff --git a/gcc/cp/constexpr.c b/gcc/cp/constexpr.c
index 2a5051c7b43..72fbdab06c5 100644
--- a/gcc/cp/constexpr.c
+++ b/gcc/cp/constexpr.c
@@ -1203,7 +1203,7 @@ maybe_initialize_constexpr_call_table (void)
 
    This is not GC-deletable to avoid GC affecting UID generation.  */
 
-static GTY(()) hash_map<tree, tree> *fundef_copies_table;
+static GTY(()) decl_tree_map *fundef_copies_table;
 
 /* Reuse a copy or create a new unshared copy of the function FUN.
    Return this copy.  We use a TREE_LIST whose PURPOSE is body, VALUE
diff --git a/gcc/tree.h b/gcc/tree.h
index b0ef14b6cd9..9ec24a3008b 100644
--- a/gcc/tree.h
+++ b/gcc/tree.h
@@ -5453,6 +5453,11 @@ struct type_tree_cache_traits
   : simple_cache_map_traits<tree_type_hash, tree> { };
 typedef hash_map<tree,tree,type_tree_cache_traits> type_tree_cache_map;
 
+/* Similarly to decl_tree_cache_map, but without caching.  */
+struct decl_tree_traits
+  : simple_hashmap_traits<tree_decl_hash, tree> { };
+typedef hash_map<tree,tree,decl_tree_traits> decl_tree_map;
+
 /* Initialize the abstract argument list iterator object ITER with the
    arguments from CALL_EXPR node EXP.  */
 static inline void