Fullwidth text conversion without libc

fullwidth-freestanding.c

#include <sys/syscall.h>

#define EOF (-1)

static unsigned char buf[3 * 8192];

long
read(void)
{
	long ret;
	__asm__ volatile (
		"syscall"
		: "=a"(ret)
		: "a"(SYS_read), "D"(0), "S"(buf), "d"(1)
		: "rcx", "r11", "memory"
	);
	return ret;
}

long
write(long len)
{
	long ret;
	__asm__ volatile (
		"syscall"
		: "=a"(ret)
		: "a"(SYS_write), "D"(1), "S"(buf), "d"(len)
		: "rcx", "r11", "memory"
	);
	return ret;
}

void
fullwidth(long len)
{
	unsigned char *x = buf + len;
	unsigned char *y = buf + sizeof(buf);
	do {
		--x;
		unsigned char d = *x & 0x1f;
		if ((*x | 0x20) > 0x60 && d < 26) {
			*(--y) = 0xa0 + d;
			*(--y) = 0xbc;
			*(--y) = 0xef;
		} else {
			*(--y) = *x;
		}
	} while (buf < x);

	long written = 0;
	do {
		long writelen = sizeof(buf) - (y - buf);
		__asm__ volatile (
			"syscall"
			: "=a"(written)
			: "a"(SYS_write), "D"(1), "S"(y), "d"(writelen)
			: "rcx", "r11", "memory"
		);
		y += written;
	} while (written == -1);
}

void
_start(void)
{
	long len;
	while ((len = read())) {
		if (len == -1)
			continue;
		fullwidth(len);
	}

	/* Exit */
	__asm__ volatile (
		"syscall"
		:
		: "a"(SYS_exit), "D"(0)
	);
	__builtin_unreachable();
}

Makefile

CFLAGS = -ffreestanding -fno-stack-protector
LDFLAGS = -nostdlib -static

fullwidth-freestanding: fullwidth-freestanding.o
fullwidth-freestanding.o: fullwidth-freestanding.c