be-code-6800.c: gen_fast_div() does not calculate correct value when dividend is negative

[zu2]

The following program returns -1 and 0. 0 is correct. Division always truncates toward 0 (C99). But shifting truncates toward negative infinity, so the values ​​may differ for negative numbers.

Before the right shift (ASRA/RORB), the program must pre-add the remainder that will be truncated. This issue may occur on other architectures as well.

/*
 *	Signed integer divide by 2
 */

void
print(int a)
{
	char	*d = (char *)(0xFEFC);

	d[0] = a>>8;
	d[1] = a&0xff;
}

static unsigned x = 12;

int main(int argc, char *argv[])
{
	int a=-1;
 	int b=2;

	print(a/2);
	print(a/b);

	return 0;
}

--- be-code-6800.c.old	2024-10-11 01:47:18
+++ be-code-6800.c	2024-10-11 01:45:50
@@ -1058,6 +1058,8 @@
 
 unsigned gen_fast_div(unsigned n, unsigned s, unsigned u)
 {
+	int m = n-1;
+
 	u &= UNSIGNED;
 	if (s != 2)
 		return 0;
@@ -1076,6 +1078,8 @@
 			n >>= 1;
 		}
 	} else {
+		printf("\taddb #%u\n", m & 0xFF);
+		printf("\tadca #%u\n", m >> 8);
 		while(n > 1) {
 			printf("\tasra\n\trorb\n");
 			n >>= 1;