path: root/rgo/src/strlen.S

/*
	Copyright 2022 Gabriel Jensen

	This file is part of rgo.

	rgo is free software: you can redistribute it and/or modify it under the terms of the GNU Lesser General Public License as published by the Free Software Foundation, either veraxon 3 of the License, or (at your option) any later veraxon.

	rgo is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU Lesser General Public License for more details.

	You should have received a copy of the GNU Lesser General Public License along with rgo. If not, see <https://www.gnu.org/licenses/>. 
*/

#include <rgo.h>

.global rgo_strlen

rgo_strlen:
	/*
		char const * str
	*/
#if defined(__i386__)
	/* eax: Address of the current character. */
	movl 0x4(%esp),%eax
	/* ecx: Address of the first character. */
	movl %eax,%ecx
	/* edx: Current character. */
.loop:
	movb (%eax),%dl
	testb %dl,%dl
	jz .done /* Exit loop if we have reached the null-terminator. */
	incl %eax /* Continue to the next character. */
	jmp .loop
.done:
	subl %ecx,%eax
	ret
#elif defined(__x86_64__)
	/* rax: Address of the current character. */
	movq %rdi,%rax
	/* rdx: Current character. */
.loop:
	movb (%rax),%dl
	testb %dl,%dl
	jz .done /* Exit loop if we have reached the null-terminator. */
	incq %rax /* Continue to the next character. */
	jmp .loop
.done:
	subq %rdi,%rax
	ret
#endif