Avoid some false matches in mark_maybe_pointer

This lets Emacs avoid marking some garbage as if it were in use.
On one test platform (RHEL 7.8, Intel Xeon Silver 4116) it
sped up ‘cd lisp; make compile-always’ by a bit over 1%.
* src/alloc.c (live_string_holding, live_cons_holding)
(live_symbol_holding, live_large_vector_holding)
(live_small_vector_holding):
Count only pointers that point to a struct component,
or are a tagged pointer to the start of the struct.
Exception: for non-bool-vector pseudovectors,
count any pointer past the header, since it’s too much
of a pain to write code for every pseudovector.
(live_vector_pointer): New function.

1 files changed, 70 insertions, 14 deletions

diff --git a/src/alloc.c b/src/alloc.c
index e057107f98c..5453c54d931 100644
--- a/src/alloc.c
+++ b/src/alloc.c
@@ -4457,9 +4457,17 @@ live_string_holding (struct mem_node *m, void *p)
      must not be on the free-list.  */
   if (0 <= offset && offset < sizeof b->strings)
     {
-      struct Lisp_String *s = p = cp -= offset % sizeof b->strings[0];
-      if (s->u.s.data)
-        return s;
+      ptrdiff_t off = offset % sizeof b->strings[0];
+      if (off == Lisp_String
+          || off == 0
+          || off == offsetof (struct Lisp_String, u.s.size_byte)
+          || off == offsetof (struct Lisp_String, u.s.intervals)
+          || off == offsetof (struct Lisp_String, u.s.data))
+        {
+          struct Lisp_String *s = p = cp -= off;
+          if (s->u.s.data)
+            return s;
+        }
     }
   return NULL;
 }
@@ -4489,9 +4497,15 @@ live_cons_holding (struct mem_node *m, void *p)
       && (b != cons_block
           || offset / sizeof b->conses[0] < cons_block_index))
     {
-      struct Lisp_Cons *s = p = cp -= offset % sizeof b->conses[0];
-      if (!deadp (s->u.s.car))
-        return s;
+      ptrdiff_t off = offset % sizeof b->conses[0];
+      if (off == Lisp_Cons
+          || off == 0
+          || off == offsetof (struct Lisp_Cons, u.s.u.cdr))
+        {
+          struct Lisp_Cons *s = p = cp -= off;
+          if (!deadp (s->u.s.car))
+            return s;
+        }
     }
   return NULL;
 }
@@ -4522,9 +4536,23 @@ live_symbol_holding (struct mem_node *m, void *p)
       && (b != symbol_block
           || offset / sizeof b->symbols[0] < symbol_block_index))
     {
-      struct Lisp_Symbol *s = p = cp -= offset % sizeof b->symbols[0];
-      if (!deadp (s->u.s.function))
-        return s;
+      ptrdiff_t off = offset % sizeof b->symbols[0];
+      if (off == Lisp_Symbol
+
+          /* Use u's offset since '|| off == 0' would run afoul of gcc
+             -Wlogical-op, as Lisp_Symbol happens to be zero.  */
+          || off == offsetof (struct Lisp_Symbol, u)
+
+          || off == offsetof (struct Lisp_Symbol, u.s.name)
+          || off == offsetof (struct Lisp_Symbol, u.s.val)
+          || off == offsetof (struct Lisp_Symbol, u.s.function)
+          || off == offsetof (struct Lisp_Symbol, u.s.plist)
+          || off == offsetof (struct Lisp_Symbol, u.s.next))
+        {
+          struct Lisp_Symbol *s = p = cp -= off;
+          if (!deadp (s->u.s.function))
+            return s;
+        }
     }
   return NULL;
 }
@@ -4571,6 +4599,37 @@ live_float_p (struct mem_node *m, void *p)
   return live_float_holding (m, p) == p;
 }
 
+/* Return VECTOR if P points within it, NULL otherwise.  */
+
+static struct Lisp_Vector *
+live_vector_pointer (struct Lisp_Vector *vector, void *p)
+{
+  void *vvector = vector;
+  char *cvector = vvector;
+  char *cp = p;
+  ptrdiff_t offset = cp - cvector;
+  return ((offset == Lisp_Vectorlike
+           || offset == 0
+           || (sizeof vector->header <= offset
+               && offset < vector_nbytes (vector)
+               && (! (vector->header.size & PSEUDOVECTOR_FLAG)
+                   ? (offsetof (struct Lisp_Vector, contents) <= offset
+                      && (((offset - offsetof (struct Lisp_Vector, contents))
+                           % word_size)
+                          == 0))
+                   /* For non-bool-vector pseudovectors, treat any pointer
+                      past the header as valid since it's too much of a pain
+                      to write special-case code for every pseudovector.  */
+                   : (! PSEUDOVECTOR_TYPEP (&vector->header, PVEC_BOOL_VECTOR)
+                      || offset == offsetof (struct Lisp_Bool_Vector, size)
+                      || (offsetof (struct Lisp_Bool_Vector, data) <= offset
+                          && (((offset
+                                - offsetof (struct Lisp_Bool_Vector, data))
+                               % sizeof (bits_word))
+                              == 0))))))
+          ? vector : NULL);
+}
+
 /* If P is a pointer to a live, large vector-like object, return the object.
    Otherwise, return nil.
    M is a pointer to the mem_block for P.  */
@@ -4579,10 +4638,7 @@ static struct Lisp_Vector *
 live_large_vector_holding (struct mem_node *m, void *p)
 {
   eassert (m->type == MEM_TYPE_VECTORLIKE);
-  struct Lisp_Vector *vp = p;
-  struct Lisp_Vector *vector = large_vector_vec (m->start);
-  struct Lisp_Vector *next = ADVANCE (vector, vector_nbytes (vector));
-  return vector <= vp && vp < next ? vector : NULL;
+  return live_vector_pointer (large_vector_vec (m->start), p);
 }
 
 static bool
@@ -4612,7 +4668,7 @@ live_small_vector_holding (struct mem_node *m, void *p)
     {
       struct Lisp_Vector *next = ADVANCE (vector, vector_nbytes (vector));
       if (vp < next && !PSEUDOVECTOR_TYPEP (&vector->header, PVEC_FREE))
-        return vector;
+        return live_vector_pointer (vector, vp);
       vector = next;
     }
   return NULL;