Speed up replace-buffer-contents

* src/editfns.c (EXTRA_CONTEXT_FIELDS): Add a_unibyte and
b_unibyte members.
(rbc_quitcounter): New static variable.
(Freplace_buffer_contents): Initialize a_unibyte, b_unibyte, and
rbc_quitcounter.  Inhibit modification hooks if they were not
already inhibited.  Use rarely_quit to allow user to quit, to
avoid calling maybe_quit too frequently (which hurts performance).
Remove redundant assertions (which hurt performance too much).
Call signal_after_change and update_compositions after all the
changes are done.
(buffer_chars_equal): Remove redundant assertions (which hurt
performance).  Avoid using BUF_FETCH_CHAR_AS_MULTIBYTE, which
hurts performance by referencing Lisp symbols; instead, use
lower-level macros with explicit tests to select which macro to
use.  (Bug#31888)

src/editfns.c

@@ -3115,6 +3115,9 @@ determines whether case is significant or ignored.  */)
 #undef ELEMENT
 #undef EQUAL
 
+/* Counter used to rarely_quit in replace-buffer-contents.  */
+static unsigned short rbc_quitcounter;
+
 #define XVECREF_YVECREF_EQUAL(ctx, xoff, yoff)  \
   buffer_chars_equal ((ctx), (xoff), (yoff))
 
@@ -3124,6 +3127,9 @@ determines whether case is significant or ignored.  */)
   /* Buffers to compare.  */                    \
   struct buffer *buffer_a;                      \
   struct buffer *buffer_b;                      \
+  /* Whether each buffer is unibyte/plain-ASCII or not.  */ \
+  bool a_unibyte;				\
+  bool b_unibyte;				\
   /* Bit vectors recording for each character whether it was deleted
      or inserted.  */                           \
   unsigned char *deletions;                     \
@@ -3202,6 +3208,8 @@ differences between the two buffers.  */)
   struct context ctx = {
     .buffer_a = a,
     .buffer_b = b,
+    .a_unibyte = BUF_ZV (a) == BUF_ZV_BYTE (a),
+    .b_unibyte = BUF_ZV (b) == BUF_ZV_BYTE (b),
     .deletions = SAFE_ALLOCA (del_bytes),
     .insertions = SAFE_ALLOCA (ins_bytes),
     .fdiag = buffer + size_b + 1,
@@ -3218,10 +3226,25 @@ differences between the two buffers.  */)
      early.  */
   eassert (! early_abort);
 
+  rbc_quitcounter = 0;
+
   Fundo_boundary ();
+  bool modification_hooks_inhibited = false;
   ptrdiff_t count = SPECPDL_INDEX ();
   record_unwind_protect (save_excursion_restore, save_excursion_save ());
 
+  /* We are going to make a lot of small modifications, and having the
+     modification hooks called for each of them will slow us down.
+     Instead, we announce a single modification for the entire
+     modified region.  But don't do that if the caller inhibited
+     modification hooks, because then they don't want that.  */
+  if (!inhibit_modification_hooks)
+    {
+      prepare_to_modify_buffer (BEGV, ZV, NULL);
+      specbind (Qinhibit_modification_hooks, Qt);
+      modification_hooks_inhibited = true;
+    }
+
   ptrdiff_t i = size_a;
   ptrdiff_t j = size_b;
   /* Walk backwards through the lists of changes.  This was also
@@ -3230,15 +3253,13 @@ differences between the two buffers.  */)
   while (i >= 0 || j >= 0)
     {
       /* Allow the user to quit if this gets too slow.  */
-      maybe_quit ();
+      rarely_quit (++rbc_quitcounter);
 
       /* Check whether there is a change (insertion or deletion)
          before the current position.  */
       if ((i > 0 && bit_is_set (ctx.deletions, i - 1)) ||
           (j > 0 && bit_is_set (ctx.insertions, j - 1)))
 	{
-	  maybe_quit ();
-
           ptrdiff_t end_a = min_a + i;
           ptrdiff_t end_b = min_b + j;
           /* Find the beginning of the current change run.  */
@@ -3246,14 +3267,13 @@ differences between the two buffers.  */)
             --i;
 	  while (j > 0 && bit_is_set (ctx.insertions, j - 1))
             --j;
+
+	  rarely_quit (rbc_quitcounter++);
+
           ptrdiff_t beg_a = min_a + i;
           ptrdiff_t beg_b = min_b + j;
-          eassert (beg_a >= BEGV);
-          eassert (beg_b >= BUF_BEGV (b));
           eassert (beg_a <= end_a);
           eassert (beg_b <= end_b);
-          eassert (end_a <= ZV);
-          eassert (end_b <= BUF_ZV (b));
           eassert (beg_a < end_a || beg_b < end_b);
           if (beg_a < end_a)
             del_range (beg_a, end_a);
@@ -3269,6 +3289,13 @@ differences between the two buffers.  */)
     }
   unbind_to (count, Qnil);
   SAFE_FREE ();
+  rbc_quitcounter = 0;
+
+  if (modification_hooks_inhibited)
+    {
+      signal_after_change (BEGV, size_a, ZV - BEGV);
+      update_compositions (BEGV, ZV, CHECK_BORDER);
+    }
 
   return Qnil;
 }
@@ -3296,39 +3323,45 @@ bit_is_set (const unsigned char *a, ptrdiff_t i)
 /* Return true if the characters at position POS_A of buffer
    CTX->buffer_a and at position POS_B of buffer CTX->buffer_b are
    equal.  POS_A and POS_B are zero-based.  Text properties are
-   ignored.  */
+   ignored.
+
+   Implementation note: this function is called inside the inner-most
+   loops of compareseq, so it absolutely must be optimized for speed,
+   every last bit of it.  E.g., each additional use of BEGV or such
+   likes will slow down replace-buffer-contents by dozens of percents,
+   because builtin_lisp_symbol will be called one more time in the
+   innermost loop.  */
 
 static bool
 buffer_chars_equal (struct context *ctx,
                     ptrdiff_t pos_a, ptrdiff_t pos_b)
 {
-  eassert (pos_a >= 0);
   pos_a += BUF_BEGV (ctx->buffer_a);
-  eassert (pos_a >= BUF_BEGV (ctx->buffer_a));
-  eassert (pos_a < BUF_ZV (ctx->buffer_a));
-
-  eassert (pos_b >= 0);
   pos_b += BUF_BEGV (ctx->buffer_b);
-  eassert (pos_b >= BUF_BEGV (ctx->buffer_b));
-  eassert (pos_b < BUF_ZV (ctx->buffer_b));
-
-  bool a_unibyte = BUF_ZV (ctx->buffer_a) == BUF_ZV_BYTE (ctx->buffer_a);
-  bool b_unibyte = BUF_ZV (ctx->buffer_b) == BUF_ZV_BYTE (ctx->buffer_b);
 
   /* Allow the user to escape out of a slow compareseq call.  */
-  maybe_quit ();
+  rarely_quit (++rbc_quitcounter);
 
   ptrdiff_t bpos_a =
-    a_unibyte ? pos_a : buf_charpos_to_bytepos (ctx->buffer_a, pos_a);
+    ctx->a_unibyte ? pos_a : buf_charpos_to_bytepos (ctx->buffer_a, pos_a);
   ptrdiff_t bpos_b =
-    b_unibyte ? pos_b : buf_charpos_to_bytepos (ctx->buffer_b, pos_b);
+    ctx->b_unibyte ? pos_b : buf_charpos_to_bytepos (ctx->buffer_b, pos_b);
 
-  if (a_unibyte && b_unibyte)
+  /* We make the below a series of specific test to avoid using
+     BUF_FETCH_CHAR_AS_MULTIBYTE, which references Lisp symbols, and
+     is therefore significantly slower (see the note in the commentary
+     to this function).  */
+  if (ctx->a_unibyte && ctx->b_unibyte)
     return BUF_FETCH_BYTE (ctx->buffer_a, bpos_a)
       == BUF_FETCH_BYTE (ctx->buffer_b, bpos_b);
-
-  return BUF_FETCH_CHAR_AS_MULTIBYTE (ctx->buffer_a, bpos_a)
-    == BUF_FETCH_CHAR_AS_MULTIBYTE (ctx->buffer_b, bpos_b);
+  if (ctx->a_unibyte && !ctx->b_unibyte)
+    return UNIBYTE_TO_CHAR (BUF_FETCH_BYTE (ctx->buffer_a, bpos_a))
+      == BUF_FETCH_MULTIBYTE_CHAR (ctx->buffer_b, bpos_b);
+  if (!ctx->a_unibyte && ctx->b_unibyte)
+    return BUF_FETCH_MULTIBYTE_CHAR (ctx->buffer_a, bpos_a)
+      == UNIBYTE_TO_CHAR (BUF_FETCH_BYTE (ctx->buffer_b, bpos_b));
+  return BUF_FETCH_MULTIBYTE_CHAR (ctx->buffer_a, bpos_a)
+    == BUF_FETCH_MULTIBYTE_CHAR (ctx->buffer_b, bpos_b);
 }