From 1a6ea4ffe64b0e3d93d1f5451637ab5cfd0ad555 Mon Sep 17 00:00:00 2001
From: Kwarde <kevin@kwarde.com>
Date: Sun, 6 Jul 2025 11:59:49 +0200
Subject: [PATCH] Optimize strcpy(): 1- Conditional moves (I keep forgetting
 about them) 2- Better setting of RCX (better accounting for EOS)

---
 src/string.asm | 23 ++++++++++-------------
 1 file changed, 10 insertions(+), 13 deletions(-)

diff --git a/src/string.asm b/src/string.asm
index 011f110..0987865 100644
--- a/src/string.asm
+++ b/src/string.asm
@@ -27,26 +27,23 @@ strlen:
 ; Return value: Pointer to dest
 ; Used registers:
 ;	rax*	(ret) Pointer to dest[]
-;	rdi	(arg) Pointer to dest[]
-;	rsi	(arg) Pointer to src[]
-;	rdx	(arg) Max amount of characters to copy / size of dest[] <!>Must never be greater than size of dest[]!
+;	rdi*	(arg) Pointer to dest[]
+;	rsi*	(arg) Pointer to src[]
+;	rdx	(arg) Size of dest[]
 ;	rcx*	Counter for movsb
 ;	r8*	Backup for rdi
 strcpy:
 	mov	r8, rdi
 	mov	rdi, rsi
 	call	strlen
-	mov	rcx, rdx
-	cmp	rcx, rax
-	je	.decMaxLen ;rcx-1 for EOS
-	ja	.setMaxLen ;set rcx to strlen(src)
-	jmp	.copy
-	.decMaxLen:
+	inc	rax
+	cmp	rax, rdx
+	cmova	rcx, rdx
+	cmovbe	rcx, rax
+	cmp	rcx, rdx
+	jb	.copy
 	dec	rcx
-	jmp	.copy
-	.setMaxLen:
-	mov	rcx, rax
-	.copy:
+	.copy
 	mov	rdi, r8
 	rep	movsb
 	mov	byte [rdi], EOS