Commit b39a7340 authored by Denis Vlasenko's avatar Denis Vlasenko Committed by Linus Torvalds
Browse files

vsprintf.c: optimizing, part 1 (easy and obvious stuff) 



* There is no point in having full "0...9a...z" constant vector,
  if we use only "0...9a...f" (and "x" for "0x").

* Post-decrement usually needs a few more instructions, so use
  pre decrement instead where makes sense:
-       while (i < precision--) {
+       while (i <= --precision) {

* if base != 10 (=> base 8 or 16), we can avoid using division
  in a loop and use mask/shift, obtaining much faster conversion.
  (More complex optimization for base 10 case is in the second patch).

Overall, size vsprintf.o shows ~80 bytes smaller text section
with this patch applied.

Signed-off-by: default avatarDouglas W Jones <jones@cs.uiowa.edu>
Signed-off-by: default avatarDenys Vlasenko <vda.linux@googlemail.com>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent 4b4e5a14

lib/vsprintf.c

+45 −28
Original line number Diff line number Diff line
@@ -145,10 +145,12 @@ static int skip_atoi(const char **s) 


static char *number(char *buf, char *end, unsigned long long num, int base, int size, int precision, int type)

{

	char c,sign,tmp[66];

	char sign,tmp[66];

	const char *digits;

	static const char small_digits[] = "0123456789abcdefghijklmnopqrstuvwxyz";

	static const char large_digits[] = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ";

	/* we are called with base 8, 10 or 16, only, thus don't need "g..."  */

	static const char small_digits[] = "0123456789abcdefx"; /* "ghijklmnopqrstuvwxyz"; */

	static const char large_digits[] = "0123456789ABCDEFX"; /* "GHIJKLMNOPQRSTUVWXYZ"; */

	int need_pfx = ((type & SPECIAL) && base != 10);

	int i;



	digits = (type & LARGE) ? large_digits : small_digits;
@@ -156,7 +158,6 @@ static char * number(char * buf, char * end, unsigned long long num, int base, i 
		type &= ~ZEROPAD;

	if (base < 2 || base > 36)

		return NULL;

	c = (type & ZEROPAD) ? '0' : ' ';

	sign = 0;

	if (type & SIGN) {

		if ((signed long long) num < 0) {
@@ -171,64 +172,80 @@ static char * number(char * buf, char * end, unsigned long long num, int base, i 
			size--;

		}

	}

	if (type & SPECIAL) {

	if (need_pfx) {

		size--;

		if (base == 16)

			size -= 2;

		else if (base == 8)

			size--;

	}



	/* generate full string in tmp[], in reverse order */

	i = 0;

	if (num == 0)

		tmp[i++] = '0';

	else while (num != 0)

		tmp[i++] = digits[do_div(num,base)];

	else if (base != 10) { /* 8 or 16 */

		int mask = base - 1;

		int shift = 3;

		if (base == 16) shift = 4;

		do {

			tmp[i++] = digits[((unsigned char)num) & mask];

			num >>= shift;

		} while (num);

	} else do { /* generic code, works for any base */

		tmp[i++] = digits[do_div(num,10 /*base*/)];

	} while (num);



	/* printing 100 using %2d gives "100", not "00" */

	if (i > precision)

		precision = i;

	/* leading space padding */

	size -= precision;

	if (!(type & (ZEROPAD+LEFT))) {

		while(size-->0) {

		while(--size >= 0) {

			if (buf < end)

				*buf = ' ';

			++buf;

		}

	}

	/* sign */

	if (sign) {

		if (buf < end)

			*buf = sign;

		++buf;

	}

	if (type & SPECIAL) {

		if (base==8) {

			if (buf < end)

				*buf = '0';

			++buf;

		} else if (base==16) {

	/* "0x" / "0" prefix */

	if (need_pfx) {

		if (buf < end)

			*buf = '0';

		++buf;

		if (base == 16) {

			if (buf < end)

				*buf = digits[33];

				*buf = digits[16]; /* for arbitrary base: digits[33]; */

			++buf;

		}

	}

	/* zero or space padding */

	if (!(type & LEFT)) {

		while (size-- > 0) {

		char c = (type & ZEROPAD) ? '0' : ' ';

		while (--size >= 0) {

			if (buf < end)

				*buf = c;

			++buf;

		}

	}

	while (i < precision--) {

	/* hmm even more zero padding? */

	while (i <= --precision) {

		if (buf < end)

			*buf = '0';

		++buf;

	}

	while (i-- > 0) {

	/* actual digits of result */

	while (--i >= 0) {

		if (buf < end)

			*buf = tmp[i];

		++buf;

	}

	while (size-- > 0) {

	/* trailing space padding */

	while (--size >= 0) {

		if (buf < end)

			*buf = ' ';

		++buf;
@@ -276,7 +293,7 @@ int vsnprintf(char *buf, size_t size, const char *fmt, va_list args) 
	   used for unknown buffer sizes. */

	if (unlikely((int) size < 0)) {

		/* There can be only one.. */

		static int warn = 1;

		static char warn = 1;

		WARN_ON(warn);

		warn = 0;

		return 0;

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