scratch/igc 55670e01d0a: Give string data objects a header

[Pip Cet]

branch: scratch/igc
commit 55670e01d0ac94f8eda1563f1d2388b63b26c564
Author: Pip Cet <pipcet@protonmail.com>
Commit: Pip Cet <pipcet@protonmail.com>

    Give string data objects a header
    
    This excludes C strings, which MPS ignores as they're not in
    MPS-managed memory.
    
    * src/igc.c (fix_string): Account for string data header.
    (alloc_string_data): Allocate as struct Lisp_String_Data.
    * src/pdumper.c (dump_cold_string): Assume the string data is preceded
    by a header, which we dump.
---
 src/igc.c     | 12 ++++++++----
 src/pdumper.c | 12 ++++++++++--
 2 files changed, 18 insertions(+), 6 deletions(-)

diff --git a/src/igc.c b/src/igc.c
index 7cdd665770a..6f7aabf2523 100644
--- a/src/igc.c
+++ b/src/igc.c
@@ -1544,7 +1544,10 @@ fix_string (mps_ss_t ss, struct Lisp_String *s)
 {
   MPS_SCAN_BEGIN (ss)
   {
-    IGC_FIX12_RAW (ss, &s->u.s.data);
+    struct Lisp_String_Data *ptr =
+      (void *) (s->u.s.data - sizeof (*ptr));
+    IGC_FIX12_RAW (ss, &ptr);
+    s->u.s.data = ptr->data;
     IGC_FIX12_RAW (ss, &s->u.s.intervals);
   }
   MPS_SCAN_END (ss);
@@ -3814,9 +3817,10 @@ igc_make_float (double val)
 static unsigned char *
 alloc_string_data (size_t nbytes, bool clear)
 {
-  unsigned char *data = alloc (nbytes + 1, IGC_OBJ_STRING_DATA);
-  data[nbytes] = 0;
-  return data;
+  struct Lisp_String_Data *data =
+    alloc (sizeof (*data) + nbytes + 1, IGC_OBJ_STRING_DATA);
+  data->data[nbytes] = 0;
+  return data->data;
 }
 
 void *
diff --git a/src/pdumper.c b/src/pdumper.c
index 1d96220348a..be3d6f1c6a4 100644
--- a/src/pdumper.c
+++ b/src/pdumper.c
@@ -3538,14 +3538,22 @@ dump_cold_string (struct dump_context *ctx, Lisp_Object 
string)
   eassert (total_size > 0);
 
 # ifdef HAVE_MPS
+  struct Lisp_String_Data *data = (struct Lisp_String_Data *)
+    (XSTRING (string)->u.s.data - sizeof (*data));
   dump_align_output (ctx, DUMP_ALIGNMENT);
-  dump_igc_start_obj (ctx, IGC_OBJ_STRING_DATA, XSTRING (string)->u.s.data);
-# endif
+  dump_igc_start_obj (ctx, IGC_OBJ_STRING_DATA, data);
+  dump_remember_fixup_ptr_raw
+    (ctx,
+     string_offset + dump_offsetof (struct Lisp_String, u.s.data),
+     ctx->offset + sizeof (*data));
+  dump_write (ctx, data, sizeof (*data) + total_size);
+# else
   dump_remember_fixup_ptr_raw
     (ctx,
      string_offset + dump_offsetof (struct Lisp_String, u.s.data),
      ctx->offset);
   dump_write (ctx, XSTRING (string)->u.s.data, total_size);
+# endif
 # ifdef HAVE_MPS
   dump_igc_finish_obj (ctx);
 # endif