[gcc r13-2539] pch: Fix the reconstruction of adhoc data hash table

[gcc r13-2539] pch: Fix the reconstruction of adhoc data hash table

[Lewis Hyatt]

https://gcc.gnu.org/g:95c7d5899521a9e266c68cbcc92edfd2cde8694e

commit r13-2539-g95c7d5899521a9e266c68cbcc92edfd2cde8694e
Author: Lewis Hyatt <lhyatt@gmail.com>
Date:   Wed Sep 7 09:33:26 2022 -0400

    pch: Fix the reconstruction of adhoc data hash table
    
    The function rebuild_location_adhoc_htab() was meant to reconstruct the
    adhoc location hash map after restoring a line_maps instance from a
    PCH. However, the function has never performed as intended because it
    missed the last step of adding the data into the newly reconstructed hash
    map. This patch fixes that.
    
    It does not seem possible to construct a test case such that the current
    incorrect behavior is observable as a compiler issue. It would be
    observable, if it were possible for a precompiled header to contain an
    adhoc location with a non-zero custom data pointer. But currently, such
    data pointers are used only by the middle end to track inlining
    information, and this happens later, too late to show up in a PCH.
    
    I also noted that location_adhoc_data_update, which updates the hash map
    pointers in a different scenario, was relying on undefined pointer
    arithmetic behavior. I'm not aware of this having caused any issue in
    practice, but in this patch I have also changed it to use defined pointer
    operations instead.
    
    libcpp/ChangeLog:
    
            * line-map.cc (location_adhoc_data_update): Remove reliance on
            undefined behavior.
            (get_combined_adhoc_loc): Likewise.
            (rebuild_location_adhoc_htab): Fix issue where the htab was not
            properly updated.

Diff:
---
 libcpp/line-map.cc | 41 +++++++++++++++++++++++++++--------------
 1 file changed, 27 insertions(+), 14 deletions(-)

diff --git a/libcpp/line-map.cc b/libcpp/line-map.cc
index 62077c3857c..391f1d4bbc1 100644
--- a/libcpp/line-map.cc
+++ b/libcpp/line-map.cc
@@ -85,27 +85,38 @@ location_adhoc_data_eq (const void *l1, const void *l2)
 	  && lb1->data == lb2->data);
 }
 
-/* Update the hashtable when location_adhoc_data is reallocated.  */
+/* Update the hashtable when location_adhoc_data_map::data is reallocated.
+   The param is an array of two pointers, the previous value of the data
+   pointer, and then the new value.  The pointers stored in the hash map
+   are then rebased to be relative to the new data pointer instead of the
+   old one.  */
 
 static int
-location_adhoc_data_update (void **slot, void *data)
+location_adhoc_data_update (void **slot_v, void *param_v)
 {
-  *((char **) slot)
-    = (char *) ((uintptr_t) *((char **) slot) + *((ptrdiff_t *) data));
+  const auto slot = reinterpret_cast<location_adhoc_data **> (slot_v);
+  const auto param = static_cast<location_adhoc_data **> (param_v);
+  *slot = (*slot - param[0]) + param[1];
   return 1;
 }
 
-/* Rebuild the hash table from the location adhoc data.  */
+/* The adhoc data hash table is not part of the GGC infrastructure, so it was
+   not initialized when SET was reconstructed from PCH; take care of that by
+   rebuilding it from scratch.  */
 
 void
 rebuild_location_adhoc_htab (line_maps *set)
 {
-  unsigned i;
   set->location_adhoc_data_map.htab =
       htab_create (100, location_adhoc_data_hash, location_adhoc_data_eq, NULL);
-  for (i = 0; i < set->location_adhoc_data_map.curr_loc; i++)
-    htab_find_slot (set->location_adhoc_data_map.htab,
-		    set->location_adhoc_data_map.data + i, INSERT);
+  for (auto p = set->location_adhoc_data_map.data,
+	    end = p + set->location_adhoc_data_map.curr_loc;
+      p != end; ++p)
+    {
+      const auto slot = reinterpret_cast<location_adhoc_data **>
+	(htab_find_slot (set->location_adhoc_data_map.htab, p, INSERT));
+      *slot = p;
+    }
 }
 
 /* Helper function for get_combined_adhoc_loc.
@@ -211,8 +222,7 @@ get_combined_adhoc_loc (line_maps *set,
       if (set->location_adhoc_data_map.curr_loc >=
 	  set->location_adhoc_data_map.allocated)
 	{
-	  char *orig_data = (char *) set->location_adhoc_data_map.data;
-	  ptrdiff_t offset;
+	  const auto orig_data = set->location_adhoc_data_map.data;
 	  /* Cast away extern "C" from the type of xrealloc.  */
 	  line_map_realloc reallocator = (set->reallocator
 					  ? set->reallocator
@@ -226,10 +236,13 @@ get_combined_adhoc_loc (line_maps *set,
 	      reallocator (set->location_adhoc_data_map.data,
 			   set->location_adhoc_data_map.allocated
 			   * sizeof (struct location_adhoc_data));
-	  offset = (char *) (set->location_adhoc_data_map.data) - orig_data;
 	  if (set->location_adhoc_data_map.allocated > 128)
-	    htab_traverse (set->location_adhoc_data_map.htab,
-			   location_adhoc_data_update, &offset);
+	    {
+	      location_adhoc_data *param[2]
+		= {orig_data, set->location_adhoc_data_map.data};
+	      htab_traverse (set->location_adhoc_data_map.htab,
+			     location_adhoc_data_update, param);
+	    }
 	}
       *slot = set->location_adhoc_data_map.data
 	      + set->location_adhoc_data_map.curr_loc;