From: Mattias Engdegård <mattiase@acm.org>
Date: Thu, 3 Mar 2022 12:57:26 +0000 (+0100)
Subject: Faster bytecode immediate argument fetching
X-Git-Tag: emacs-29.0.90~1931^2~1182
X-Git-Url: http://git.eshelyaron.com/gitweb/?a=commitdiff_plain;h=2fb98486e18f8a3275adc56d2740901ef5cb6e8b;p=emacs.git

Faster bytecode immediate argument fetching

* src/bytecode.c (FETCH2):
Use `|` instead of `+` to combine the bytes forming a 16-bit immediate
argument so that GCC (prior to version 12) recognises the idiom and
generates a 16-bit load.  This applies for little-endian machines with
cheap unaligned accesses such as x86[-64], arm64 and power64le.

This 1-character change results in a measurable speed gain on many
kinds of Lisp code, as 16-bit immediates are used by all jump
instructions.

Clang performs this optimisation for both `+` and `|` from version 10.
---

diff --git a/src/bytecode.c b/src/bytecode.c
index 96f1f905812..c5cc6590121 100644
--- a/src/bytecode.c
+++ b/src/bytecode.c
@@ -291,7 +291,7 @@ enum byte_code_op
 /* Fetch two bytes from the bytecode stream and make a 16-bit number
    out of them.  */
 
-#define FETCH2 (op = FETCH, op + (FETCH << 8))
+#define FETCH2 (op = FETCH, op | (FETCH << 8))
 
 /* Push X onto the execution stack.  The expression X should not
    contain TOP, to avoid competing side effects.  */