c++: Fix tinst_level::to_list [PR112968]

With valgrind checking, there are various errors reported on some C++26
libstdc++ tests, like:
==2009913== Conditional jump or move depends on uninitialised value(s)
==2009913==    at 0x914C59: gt_ggc_mx_lang_tree_node(void*) (gt-cp-tree.h:107)
==2009913==    by 0x8AB7A5: gt_ggc_mx_tinst_level(void*) (gt-cp-pt.h:32)
==2009913==    by 0xB89B25: ggc_mark_root_tab(ggc_root_tab const*) (ggc-common.cc:75)
==2009913==    by 0xB89DF4: ggc_mark_roots() (ggc-common.cc:104)
==2009913==    by 0x9D6311: ggc_collect(ggc_collect) (ggc-page.cc:2227)
==2009913==    by 0xDB70F6: execute_one_pass(opt_pass*) (passes.cc:2738)
==2009913==    by 0xDB721F: execute_pass_list_1(opt_pass*) (passes.cc:2755)
==2009913==    by 0xDB7258: execute_pass_list(function*, opt_pass*) (passes.cc:2766)
==2009913==    by 0xA55525: cgraph_node::analyze() (cgraphunit.cc:695)
==2009913==    by 0xA57CC7: analyze_functions(bool) (cgraphunit.cc:1248)
==2009913==    by 0xA5890D: symbol_table::finalize_compilation_unit() (cgraphunit.cc:2555)
==2009913==    by 0xEB02A1: compile_file() (toplev.cc:473)

I think the problem is in the tinst_level::to_list optimization from 2018.
That function returns a TREE_LIST with TREE_PURPOSE/TREE_VALUE filled in.
Either it freshly allocates using build_tree_list (NULL, NULL); + stores
TREE_PURPOSE/TREE_VALUE, that case is fine (the whole tree_list object
is zeros, except for TREE_CODE set to TREE_LIST and TREE_PURPOSE/TREE_VALUE
modified later; the above also means in particular TREE_TYPE of it is NULL
and TREE_CHAIN is NULL and both are accessible/initialized even in valgrind
annotations.
Or it grabs a TREE_LIST node from a freelist.
If defined(ENABLE_GC_CHECKING), the object is still all zeros except
for TREE_CODE/TREE_PURPOSE/TREE_VALUE like in the fresh allocation case
(but unlike the build_tree_list case in the valgrind annotations
TREE_TYPE and TREE_CHAIN are marked as uninitialized).
If !defined(ENABLE_GC_CHECKING), I believe the actual memory content
is that everything but TREE_CODE/TREE_PURPOSE/TREE_VALUE/TREE_CHAIN is
zeros and TREE_CHAIN is something random (whatever next entry is in the
freelist, nothing overwrote it) and from valgrind POV again,
TREE_TYPE and TREE_CHAIN are marked as uninitialized.

When using the other freelist instantiations (pending_template and
tinst_level) I believe everything is correct, from valgrind POV it marks
the whole pending_template or tinst_level as uninitialized, but the
caller initializes it all).

One way to fix this would be let tinst_level::to_list not store just
  TREE_PURPOSE (ret) = tldcl;
  TREE_VALUE (ret) = targs;
but also
  TREE_TYPE (ret) = NULL_TREE;
  TREE_CHAIN (ret) = NULL_TREE;
Though, that seems like wasted effort in the build_tree_list case to me.

So, the following patch instead does that TREE_CHAIN = NULL_TREE store only
in the case where it isn't already done (and likewise for TREE_TYPE just to
be sure) and marks both TREE_CHAIN and TREE_TYPE as initialized (the latter
is at that spot, the former is because we never really touch TREE_TYPE of a
TREE_LIST anywhere and so the NULL gets stored into the freelist and
restored from there (except for ENABLE_GC_CHECKING where it is poisoned
and then cleared again).

2023-12-14  Jakub Jelinek  <jakub@redhat.com>

	PR c++/112968
	* pt.cc (freelist<tree_node>::reinit): Make whole obj->common
	defined for valgrind annotations rather than just obj->base,
	and do it even for ENABLE_GC_CHECKING.  If not ENABLE_GC_CHECKING,
	clear TREE_CHAIN (obj) and TREE_TYPE (obj).

1 files changed, 4 insertions, 2 deletions

diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
index b6a450c..810d33c 100644
--- a/gcc/cp/pt.cc
+++ b/gcc/cp/pt.cc
@@ -9525,7 +9525,7 @@ template <>
 inline void
 freelist<tree_node>::reinit (tree obj ATTRIBUTE_UNUSED)
 {
-  tree_base *b ATTRIBUTE_UNUSED = &obj->base;
+  tree_common *c ATTRIBUTE_UNUSED = &obj->common;
 
 #ifdef ENABLE_GC_CHECKING
   gcc_checking_assert (TREE_CODE (obj) == TREE_LIST);
@@ -9540,8 +9540,10 @@ freelist<tree_node>::reinit (tree obj ATTRIBUTE_UNUSED)
 #ifdef ENABLE_GC_CHECKING
   TREE_SET_CODE (obj, TREE_LIST);
 #else
-  VALGRIND_DISCARD (VALGRIND_MAKE_MEM_DEFINED (b, sizeof (*b)));
+  TREE_CHAIN (obj) = NULL_TREE;
+  TREE_TYPE (obj) = NULL_TREE;
 #endif
+  VALGRIND_DISCARD (VALGRIND_MAKE_MEM_DEFINED (c, sizeof (*c)));
 }
 
 /* Point to the first object in the TREE_LIST freelist.  */