Implement Write Barrier and dsize for FFI::StructLayout

And FFI::StructLayout::Field

Write barrier protected objects are allowed to be promoted to the old generation,
which means they only get marked on major GC.

The downside is that the RB_BJ_WRITE macro MUST be used to set references,
otherwise the referenced object may be garbaged collected.

This commit also implement a `dsize` function so that these instance
report a more relevant size in various memory profilers.

I had to get rid of the `memset(0)` in `struct_layout_mark` has with Write Barriers
it's not guaranteed that the layout will be marked before `fieldName` is moved.

I don't think it was a good fix anyway, it's better to mark these VALUE so the GC pin
them. I think we could go back to an `st_table` and mark only the keys with `rb_mark_set`.
But I didn't want to go down this rabbit hole.

1 files changed, 2 insertions, 1 deletions

diff --git a/ext/ffi_c/Struct.h b/ext/ffi_c/Struct.h
index 1cba8bb..8e9491b 100644
--- a/ext/ffi_c/Struct.h
+++ b/ext/ffi_c/Struct.h
@@ -78,11 +78,12 @@ extern "C" {
         * This avoids full ruby hash lookups for repeated lookups.
         */
         #define FIELD_CACHE_LOOKUP(this, sym) ( &(this)->cache_row[((sym) >> 8) & 0xff] )
+        #define FIELD_CACHE_ROWS 0x100
 
         struct field_cache_entry {
           VALUE fieldName;
           StructField *field;
-        } cache_row[0x100];
+        } cache_row[FIELD_CACHE_ROWS];
 
         /** The number of reference tracking fields in this struct */
         int referenceFieldCount;