master 2fb98486e1 3/5: Faster bytecode immediate argument fetching

[Mattias Engdegård]

branch: master
commit 2fb98486e18f8a3275adc56d2740901ef5cb6e8b
Author: Mattias Engdegård <mattiase@acm.org>
Commit: Mattias Engdegård <mattiase@acm.org>

    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.
---
 src/bytecode.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/src/bytecode.c b/src/bytecode.c
index 96f1f90581..c5cc659012 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.  */