Exercise 2.21 MIPS Coding and ASCII Strings

[30] <§§2.7, 2.8> Write a program in MIPS assembly language to convert an ASCII decimal string to an integer. Your program should expect register $a0 to hold the address of a null-terminated string containing some combination of the digits 0 through 9. Your program should compute the integer value equivalent to this string of digits, then place the number in register $v0.
Your program need not handle negative numbers. If a nondigit character appears anywhere in the string, your program should stop with the value -1 in register $v0.
For example, if register $a0 points to a sequence of three bytes 50, 52, 0 (the null-terminated string “24”), then when the program stops, register $v0 should contain the value 24

文字列→intプログラムをMIPS アセンブラで書きなさい。

面倒なのでCで書きます。

#include <stdio.h>
#include <stdlib.h>
#include <assert.h>
#include <limits.h>

int s2c(const char *s);
#define RET_ERR (-1)

const char *test_data[] = {"24", "0", "412", "3562", "1234567890", NULL};
const char *error_data[] = {"-24", "abc", "", "3562a", NULL};
//const char *ovflow_data[] = {"214748364", "2147483647", NULL};

int s2c(const char *s)
{
	const char *p;
	int v, i;
	char c;

	assert(s!=NULL);

	if(*s=='\0') {
		return RET_ERR;
	}

	v = 0;
	for(p=s;*p!='\0';p++){
		c = *p;
		if((c < '0') || ( c > '9')) {
			return RET_ERR;
		}
		i = c - '0';

		assert(i>=0);
		assert(i<=9);

		assert(v>=0);
		assert(v<(INT_MAX / 10));

		v *= 10;
		v += i;
	}
	return v;
}

int main(void)
{
	int i;
	int golden;
	int ans;
	int err;

	err = 0;

	for(i=0;test_data[i]!=NULL;i++){
		golden = atoi(test_data[i]);
		ans = s2c(test_data[i]);
		if(ans == golden) {
			printf("OK:%d\n", ans);
		} else {
			err++;
			printf("ERROR:golden = %d, ans = %d\n", golden, ans);
		}
	}

	for(i=0;error_data[i]!=NULL;i++){
		ans = s2c(error_data[i]);
		if(ans == RET_ERR) {
			printf("OK:%d\n", ans);
		} else {
			err++;
			printf("ERROR:data = \"%s\", ans = %d\n", error_data[i], ans);
		}
	}

	if(err!=0){
		printf("ERROR %d found\n", err);
	} else {
		printf("ALL DATA PASSED\n");
	}
	
	return 0;
}

assert外して、最適化かけたコードがこれです。

s2c:
	move	$3,$4
	lbu	$4,0($4)
	beq	$4,$0,.L13
	li	$2,-1			# 0xffffffffffffffff

	move	$5,$3
	move	$6,$0
	sll	$9,$6,2
.L12:
	addiu	$4,$4,-48
	addu	$8,$9,$6     
	andi	$7,$4,0x00ff
	sll	$2,$8,1      
	sltu	$3,$7,10
	addiu	$5,$5,1
	beq	$3,$0,.L10
	addu	$6,$2,$4

	lbu	$4,0($5)
	bne	$4,$0,.L12
	sll	$9,$6,2       

	move	$2,$6
.L13:
	j	$31
	nop

.L10:
	j	$31
	li	$2,-1			# 0xffffffffffffffff