[U-Boot] [PATCH v3] Avoid use of divides in print_size

[U-Boot] [PATCH v3] Avoid use of divides in print_size

[Nick Thompson]

Modification of print_size to avoid use of divides and especially
long long divides. Keep the binary scale factor in terms of bit
shifts instead. This should be faster, since the previous code
gave the compiler no clues that the divides where always powers
of two, preventing optimisation.

Signed-off-by: Nick Thompson <nick.thompson@ge.com>
---
Since v1:
Removed misplaced brackets that resulted in round-up value of
5 rather than the intended 0.5. Now passes Timur Tabi's test.

Since v2:
Fix overflow in calculation of 0.5 rounding value for numbers
with engineered factional parts representing values >4GiB

 lib/display_options.c |   14 ++++++++------
 1 files changed, 8 insertions(+), 6 deletions(-)

diff --git a/lib/display_options.c b/lib/display_options.c
index 86df05d..a711425 100644
--- a/lib/display_options.c
+++ b/lib/display_options.c
@@ -46,13 +46,14 @@ int display_options (void)
 void print_size(unsigned long long size, const char *s)
 {
 	unsigned long m = 0, n;
+	unsigned long long f;
 	static const char names[] = {'E', 'P', 'T', 'G', 'M', 'K'};
-	unsigned long long d = 1ULL << (10 * ARRAY_SIZE(names));
+	unsigned long d = 10 * ARRAY_SIZE(names);
 	char c = 0;
 	unsigned int i;
 
-	for (i = 0; i < ARRAY_SIZE(names); i++, d >>= 10) {
-		if (size >= d) {
+	for (i = 0; i < ARRAY_SIZE(names); i++, d -= 10) {
+		if (size >> d) {
 			c = names[i];
 			break;
 		}
@@ -63,11 +64,12 @@ void print_size(unsigned long long size, const char *s)
 		return;
 	}
 
-	n = size / d;
+	n = size >> d;
+	f = size & ((1ULL << d) - 1);
 
 	/* If there's a remainder, deal with it */
-	if(size % d) {
-		m = (10 * (size - (n * d)) + (d / 2) ) / d;
+	if (f) {
+		m = (10ULL * f + (1ULL << (d - 1))) >> d;
 
 		if (m >= 10) {
 			m -= 10;
-- 
1.7.0.4

[U-Boot] [PATCH v3] Avoid use of divides in print_size

[Timur Tabi]

On Tue, May 11, 2010 at 5:29 AM, Nick Thompson <nick.thompson@ge.com> wrote:
> Modification of print_size to avoid use of divides and especially
> long long divides. Keep the binary scale factor in terms of bit
> shifts instead. This should be faster, since the previous code
> gave the compiler no clues that the divides where always powers
> of two, preventing optimisation.
>
> Signed-off-by: Nick Thompson <nick.thompson@ge.com>
> ---

Acked-by: Timur Tabi <timur@freescale.com>

I don't know if this solves the problem on ARM, but it works on PowerPC.

-- 
Timur Tabi
Linux kernel developer at Freescale