We need to have seg_mapping_table aligned on its natural alignement (ptr size). When it's not aligned,

aside from the perf problem, we also have a functional problem when an address that's not on a heap
segment is passed - we could read an intermediate value which would cause an AV. If the address is
on a heap segment it means we are guaranteed to read a ptr size atomically. This only affects functions
like IsHeapPointer which can be run when EE is not suspended.

Author: Maoni0

src/gc/gc.cpp

@@ -3377,6 +3377,13 @@ size_t size_seg_mapping_table_of (uint8_t* from, uint8_t* end)
     return sizeof (seg_mapping)*((end - from) / gc_heap::min_segment_size);
 }
 
+// for seg_mapping_table we want it to start from a pointer sized address.
+inline
+size_t align_for_seg_mapping_table (size_t size)
+{
+    return ((size + (sizeof (uint8_t*) - 1)) &~ (sizeof (uint8_t*) - 1));
+}
+
 inline
 size_t seg_mapping_word_of (uint8_t* add)
 {
@@ -6910,6 +6917,10 @@ uint32_t* gc_heap::make_card_table (uint8_t* start, uint8_t* end)
 
 #ifdef GROWABLE_SEG_MAPPING_TABLE
     size_t st = size_seg_mapping_table_of (g_lowest_address, g_highest_address);
+    size_t st_table_offset = sizeof(card_table_info) + cs + bs + cb + wws;
+    size_t st_table_offset_aligned = align_for_seg_mapping_table (st_table_offset);
+
+    st += (st_table_offset_aligned - st_table_offset);
 #else //GROWABLE_SEG_MAPPING_TABLE
     size_t st = 0;
 #endif //GROWABLE_SEG_MAPPING_TABLE
@@ -6958,7 +6969,7 @@ uint32_t* gc_heap::make_card_table (uint8_t* start, uint8_t* end)
 #endif // FEATURE_USE_SOFTWARE_WRITE_WATCH_FOR_GC_HEAP
 
 #ifdef GROWABLE_SEG_MAPPING_TABLE
-    seg_mapping_table = (seg_mapping*)((uint8_t*)card_table_brick_table (ct) + bs + cb + wws);
+    seg_mapping_table = (seg_mapping*)(mem + st_table_offset_aligned);
     seg_mapping_table = (seg_mapping*)((uint8_t*)seg_mapping_table - 
                                         size_seg_mapping_table_of (0, (align_lower_segment (g_lowest_address))));
 #endif //GROWABLE_SEG_MAPPING_TABLE
@@ -7098,6 +7109,9 @@ int gc_heap::grow_brick_card_tables (uint8_t* start,
 
 #ifdef GROWABLE_SEG_MAPPING_TABLE
         size_t st = size_seg_mapping_table_of (saved_g_lowest_address, saved_g_highest_address);
+        size_t st_table_offset = sizeof(card_table_info) + cs + bs + cb + wws;
+        size_t st_table_offset_aligned = align_for_seg_mapping_table (st_table_offset);
+        st += (st_table_offset_aligned - st_table_offset);
 #else //GROWABLE_SEG_MAPPING_TABLE
         size_t st = 0;
 #endif //GROWABLE_SEG_MAPPING_TABLE
@@ -7120,7 +7134,7 @@ int gc_heap::grow_brick_card_tables (uint8_t* start,
         dprintf (GC_TABLE_LOG, ("Table alloc for %Id bytes: [%Ix, %Ix[",
                                  alloc_size, (size_t)mem, (size_t)((uint8_t*)mem+alloc_size)));
 
-        {   
+        {
             // mark array will be committed separately (per segment).
             size_t commit_size = alloc_size - ms;
 
@@ -7160,7 +7174,7 @@ int gc_heap::grow_brick_card_tables (uint8_t* start,
 
 #ifdef GROWABLE_SEG_MAPPING_TABLE
         {
-            seg_mapping* new_seg_mapping_table = (seg_mapping*)((uint8_t*)card_table_brick_table (ct) + bs + cb + wws);
+            seg_mapping* new_seg_mapping_table = (seg_mapping*)(mem + st_table_offset_aligned);
             new_seg_mapping_table = (seg_mapping*)((uint8_t*)new_seg_mapping_table -
                                               size_seg_mapping_table_of (0, (align_lower_segment (saved_g_lowest_address))));
             memcpy(&new_seg_mapping_table[seg_mapping_word_of(g_lowest_address)],