Improving EQ

[Pip Cet via "Emacs development discussions." <emacs-devel@gnu.org>]

I looked at the "new" code generated for our EQ macro, and decided that
a fix was in order.  I'm therefore sending a first proposal to explain
what I think should be done, and some numbers.

This patch:
* moves the "slow path" of EQ into a NO_INLINE function
* exits early if the arguments to EQ are actually BASE_EQ
* returns quickly (after a single memory access which cannot be avoided
until we fix our tagging scheme to distinguish exotic objects from
ordinary ones) when symbols_with_pos_enabled isn't true.

The effect on the code size of the stripped emacs binary is small, but
significant: 8906336 bytes instead of 8955488 bytes on this machine.
(The effect on the code size of the emacs binary with debugging
information is much larger, reducing it from 32182000 bytes to 31125832
bytes on this system.)  There is no effect on the size of the .pdmp
file, which is expected.

What's missing here is a benchmark, but unless there's a really nasty
surprise when that happens, I'm quite confident that we can improve the
code here.

The proposed code doesn't use __builtin_expect anymore.

I've deliberately written slow_eq so it returns the same value as EQ,
even if the slow code path is disabled.

Pip

commit 2c807f7320bcb9654e0f148d64c92053b1a47b42 (HEAD -> faster-eq)
Author: Pip Cet <pipcet@protonmail.com>
Date:   Wed Dec 11 22:31:07 2024 +0000

    Change EQ to move slow code path into a separate function
    
    * src/data.c (slow_eq): New function.
    * src/lisp.h (EQ): Call it.

diff --git a/src/data.c b/src/data.c
index 66cf34c1e60..5ee383d2f48 100644
--- a/src/data.c
+++ b/src/data.c
@@ -162,6 +162,15 @@ circular_list (Lisp_Object list)
 \f
 /* Data type predicates.  */
 
+/* NO_INLINE to avoid excessive code growth when LTO is in use.  */
+NO_INLINE bool slow_eq (Lisp_Object x, Lisp_Object y)
+{
+  return BASE_EQ ((symbols_with_pos_enabled && SYMBOL_WITH_POS_P (x) ?
+		   XSYMBOL_WITH_POS_SYM (x) : x),
+		  (symbols_with_pos_enabled && SYMBOL_WITH_POS_P (y) ?
+		   XSYMBOL_WITH_POS_SYM (y) : y));
+}
+
 DEFUN ("eq", Feq, Seq, 2, 2, 0,
        doc: /* Return t if the two args are the same Lisp object.  */
        attributes: const)
diff --git a/src/lisp.h b/src/lisp.h
index 832a1755c04..64d4835a499 100644
--- a/src/lisp.h
+++ b/src/lisp.h
@@ -618,6 +618,7 @@ #define ENUM_BF(TYPE) enum TYPE
 extern Lisp_Object default_value (Lisp_Object symbol);
 extern void defalias (Lisp_Object symbol, Lisp_Object definition);
 extern char *fixnum_to_string (EMACS_INT number, char *buffer, char *end);
+extern bool slow_eq (Lisp_Object x, Lisp_Object y);
 
 
 /* Defined in emacs.c.  */
@@ -1353,10 +1354,12 @@ make_fixed_natnum (EMACS_INT n)
 INLINE bool
 EQ (Lisp_Object x, Lisp_Object y)
 {
-  return BASE_EQ ((__builtin_expect (symbols_with_pos_enabled, false)
-		   && SYMBOL_WITH_POS_P (x) ? XSYMBOL_WITH_POS_SYM (x) : x),
-		  (__builtin_expect (symbols_with_pos_enabled, false)
-		   && SYMBOL_WITH_POS_P (y) ? XSYMBOL_WITH_POS_SYM (y) : y));
+  if (BASE_EQ (x, y))
+    return true;
+  else if (!symbols_with_pos_enabled)
+    return false;
+  else
+    return slow_eq (x, y);
 }
 
 INLINE intmax_t