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Faster bytecode immediate argument fetching

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* 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.
This commit is contained in:
Mattias Engdegård 2022-03-03 13:57:26 +01:00
parent fe65db05f4
commit 2fb98486e1
1 changed files with 1 additions and 1 deletions
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src/bytecode.c
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@ -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. */