Optimize base-10 conversion for RV32I libc

To improve performance on RV32I, introduce __str_base10(), a base-10
conversion routine that avoids both division and multiplication by
using bit shifts and additions. This is especially beneficial for
targets lacking hardware support for RV32M.

itoa() and vsprintf() now use this implementation for decimal output.
A helper __str_base10_signed() is also added for signed integers.

Author: visitorckw

lib/libc.c

@@ -462,6 +462,56 @@ int32_t strtol(const char *s, char **end, int32_t base)
     return value;
 }
 
+/* Base-10 string conversion without division or multiplication */
+static char* __str_base10(uint32_t value, char* buffer, int* length)
+{
+    if (value == 0) {
+        buffer[0] = '0';
+        *length = 1;
+        return buffer;
+    }
+
+    char temp[12]; /* Max digits for 32-bit: 4,294,967,295 (10 digits) + sign + null */
+    int pos = 0;
+
+    while (value > 0) {
+        uint32_t q, r, t;
+
+        q = (value >> 1) + (value >> 2);
+        q += (q >> 4);
+        q += (q >> 8);
+        q += (q >> 16);
+        q >>= 3;
+        r = value - (((q << 2) + q) << 1);
+        t = ((r + 6) >> 4);
+        q += t;
+        r -= (((t << 2) + t) << 1);
+
+        temp[pos++] = '0' + r;
+        value = q;
+    }
+
+    /* Reverse digits into output buffer */
+    *length = pos;
+    for (int i = 0; i < pos; i++) {
+        buffer[i] = temp[pos - 1 - i];
+    }
+
+    return buffer;
+}
+
+/* Handle signed integers */
+static char* __str_base10_signed(int32_t value, char* buffer, int* length) {
+    if (value < 0) {
+        buffer[0] = '-';
+        __str_base10((uint32_t)(-value), buffer + 1, length);
+        (*length)++;
+        return buffer;
+    } else {
+        return __str_base10((uint32_t)value, buffer, length);
+    }
+}
+
 /* Converts string @s to an integer. */
 int32_t atoi(const char *s)
 {
@@ -501,6 +551,7 @@ void itoa(int32_t i, char *s, int32_t base)
     char *p = s;
     char *q = s;
     uint32_t h;
+    int32_t len;
 
     if (base == 16) { /* Hexadecimal conversion */
         h = (uint32_t) i;
@@ -519,7 +570,9 @@ void itoa(int32_t i, char *s, int32_t base)
             (*q > '9') ? (*p = *q + 39) : (*p = *q);
             *q = c;
         }
-    } else { /* Decimal or other bases */
+    } else if (base == 10) { /* Decimal conversion */
+        __str_base10_signed(i, s, &len);
+    } else { /* Other bases */
         if (i >= 0) {
             do {
                 *q++ = '0' + (i % base);
@@ -822,6 +875,8 @@ static int vsprintf(char **buf, const char *fmt, va_list args)
         i = 0;
         if (num == 0)
             tmp[i++] = '0';
+        else if (base == 10)
+            __str_base10(num, tmp, &i);
         else {
             while (num != 0)
                 tmp[i++] = digits[divide(&num, base)];