[RFC] Printing numbers in SI units

[Matthew Wilcox <willy@infradead.org>]

I was looking at hugetlbfs and it has several snippets of code like
this:

        string_get_size(huge_page_size(h), 1, STRING_UNITS_2, buf, 32);
        pr_warn("HugeTLB: allocating %u of page size %s failed node%d.  Only allocated %lu hugepages.\n",
                h->max_huge_pages_node[nid], buf, nid, i);

That's not terribly ergonomic, so I wondered if I could do better.
Unfortunately, I decided to do it using the SPECIAL flag which GCC
warns about.  But I've written the code now, so I'm sending it out in
case anybody has a better idea for how to incorporate it.


diff --git a/lib/test_printf.c b/lib/test_printf.c
index 69b6a5e177f2..69af4d24a814 100644
--- a/lib/test_printf.c
+++ b/lib/test_printf.c
@@ -178,6 +178,16 @@ test_number(void)
 	 * behaviour.
 	 */
 	test("00|0|0|0|0", "%.2d|%.1d|%.0d|%.*d|%1.0d", 0, 0, 0, 0, 0, 0);
+
+	/*
+	 * C23 does not define the effect of "alternative form".  Indeed
+	 * I think it actually defines it to be Undefined Behaviour which
+	 * apparently lets the compiler delete your entire source code.
+	 */
+	test("2KiB", "%#d", 2048);
+	test("2MiB", "%#d", 2048 * 1024);
+	test("1GiB", "%#d", 1024 * 1024 * 1024);
+	test("1000MiB", "%#d", 1024 * 1024 * 1000);
 }
 
 static void __init
diff --git a/lib/vsprintf.c b/lib/vsprintf.c
index 552738f14275..a702582c598c 100644
--- a/lib/vsprintf.c
+++ b/lib/vsprintf.c
@@ -370,6 +370,56 @@ char *put_dec(char *buf, unsigned long long n)
 
 #endif
 
+/*
+ * precision is the number of digits after the decimal place.  we limit
+ * it to two, because more would be complicated and unnecessary.
+ */
+static noinline_for_stack
+char *put_si(char *buf, unsigned long long n, int precision)
+{
+	char *name = "KMGTPEZB";
+	int unit = 0;
+	unsigned remainder = 0;
+
+	if (precision > 2)
+		precision = 2;
+
+	while (n >= 1024) {
+		remainder = n % 1024;
+		n /= 1024;
+		unit++;
+	}
+
+	remainder *= 1000;
+	remainder /= 1024;
+
+	/* Round up */
+	if (precision == 2)
+		remainder += 500;
+	else if (precision == 1)
+		remainder += 50;
+	else
+		remainder += 5;
+	if (remainder >= 1000) {
+		remainder -= 1000;
+		n += 1;
+	}
+
+	*buf++ = 'B';
+	if (unit > 0) {
+		*buf++ = 'i';
+		*buf++ = name[unit - 1];
+	}
+
+	if (precision > 0) {
+		*((u16 *)buf) = decpair[remainder / 10];
+		buf += precision;
+		*buf++ = '.';
+	}
+
+	return put_dec_trunc8(buf, n);
+}
+
 /*
  * Convert passed number to decimal string.
  * Returns the length of string.  On buffer overflow, returns 0.
@@ -507,6 +557,9 @@ char *number(char *buf, char *end, unsigned long long num,
 			tmp[i++] = (hex_asc_upper[((unsigned char)num) & mask] | locase);
 			num >>= shift;
 		} while (num);
+	} else if (spec.flags & SPECIAL) {
+		i = put_si(tmp, num, precision) - tmp;
+		precision = i;
 	} else { /* base 10 */
 		i = put_dec(tmp, num) - tmp;
 	}