[gcc r14-4327] ggc: do not wipe out unrelated data via gt_ggc_rtab [PR111505]

[Sergei Trofimovich]

https://gcc.gnu.org/g:7525707c5f3edb46958c4fdfbe30de5ddfa8923a

commit r14-4327-g7525707c5f3edb46958c4fdfbe30de5ddfa8923a
Author: Sergei Trofimovich <siarheit@google.com>
Date:   Thu Sep 28 20:20:31 2023 +0100

    ggc: do not wipe out unrelated data via gt_ggc_rtab [PR111505]
    
    There are 3 GC root tables:
    
       gt_ggc_rtab
       gt_ggc_deletable_rtab
       gt_pch_scalar_rtab
    
    `deletable` and `scalar` tables are both simple: each element always
    contains a pointer to the beginning of the object and it's size is the
    full object.
    
    `rtab` is different: it's `base` is a pointer in the middle of the
    struct and `stride` points to the next GC pointer in the array.
    
    Before the change there were 2 problems:
    
    1. We memset()ed not just pointers but data around them.
    2. We wen out of bounds of the last object described by gt_ggc_rtab
       and triggered bootstrap failures in profile and asan bootstraps.
    
    After the change we handle only pointers themselves like the rest of
    ggc-common.cc code.
    
    gcc/
            PR middle-end/111505
            * ggc-common.cc (ggc_zero_out_root_pointers, ggc_common_finalize):
            Add new helper. Use helper instead of memset() to wipe out pointers.

Diff:
---
 gcc/ggc-common.cc | 15 +++++++++++++--
 1 file changed, 13 insertions(+), 2 deletions(-)

diff --git a/gcc/ggc-common.cc b/gcc/ggc-common.cc
index 95803fa95a1..39e2581affd 100644
--- a/gcc/ggc-common.cc
+++ b/gcc/ggc-common.cc
@@ -75,6 +75,18 @@ ggc_mark_root_tab (const_ggc_root_tab_t rt)
       (*rt->cb) (*(void **) ((char *)rt->base + rt->stride * i));
 }
 
+/* Zero out all the roots in the table RT.  */
+
+static void
+ggc_zero_rtab_roots (const_ggc_root_tab_t rt)
+{
+  size_t i;
+
+  for ( ; rt->base != NULL; rt++)
+    for (i = 0; i < rt->nelt; i++)
+      (*(void **) ((char *)rt->base + rt->stride * i)) = (void*)0;
+}
+
 /* Iterate through all registered roots and mark each element.  */
 
 void
@@ -1307,8 +1319,7 @@ ggc_common_finalize ()
       memset (rti->base, 0, rti->stride * rti->nelt);
 
   for (rt = gt_ggc_rtab; *rt; rt++)
-    for (rti = *rt; rti->base != NULL; rti++)
-      memset (rti->base, 0, rti->stride * rti->nelt);
+    ggc_zero_rtab_roots (*rt);
 
   for (rt = gt_pch_scalar_rtab; *rt; rt++)
     for (rti = *rt; rti->base != NULL; rti++)