[PATCH 1/2] lib/lcm.c: Ensure correct result whenever it fits

[PATCH 1/2] lib/lcm.c: Ensure correct result whenever it fits

[Rasmus Villemoes]

Ensure that lcm(a,b) returns the mathematically correct result,
provided it fits in an unsigned long. The current version returns
garbage if a*b overflows, even if the final result would fit.

Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
---
There are of course still plenty of cases where the return value is
wrong.

 lib/lcm.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/lib/lcm.c b/lib/lcm.c
index b9c8de4..01b3aa9 100644
--- a/lib/lcm.c
+++ b/lib/lcm.c
@@ -7,7 +7,7 @@
 unsigned long lcm(unsigned long a, unsigned long b)
 {
 	if (a && b)
-		return (a * b) / gcd(a, b);
+		return (a / gcd(a, b)) * b;
 	else if (b)
 		return b;
 
-- 
1.8.4.rc3.2.g61bff3f

[PATCH 2/2] lib/lcm.c: lcm(n,0)=lcm(0,n) is 0, not n

[Rasmus Villemoes]

Return the mathematically correct answer when an argument is 0.

Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
---
I don't think there is any instance of lcm(0,n) in the kernel, but at
least this reduces code size a little.

 lib/lcm.c | 6 ++----
 1 file changed, 2 insertions(+), 4 deletions(-)

diff --git a/lib/lcm.c b/lib/lcm.c
index 01b3aa9..51cc6b1 100644
--- a/lib/lcm.c
+++ b/lib/lcm.c
@@ -8,9 +8,7 @@ unsigned long lcm(unsigned long a, unsigned long b)
 {
 	if (a && b)
 		return (a / gcd(a, b)) * b;
-	else if (b)
-		return b;
-
-	return a;
+	else
+		return 0;
 }
 EXPORT_SYMBOL_GPL(lcm);
-- 
1.8.4.rc3.2.g61bff3f

Re: [PATCH 1/2] lib/lcm.c: Ensure correct result whenever it fits

[Rasmus Villemoes]

Rasmus Villemoes <linux@rasmusvillemoes.dk> writes:

Ping...

> Ensure that lcm(a,b) returns the mathematically correct result,
> provided it fits in an unsigned long. The current version returns
> garbage if a*b overflows, even if the final result would fit.
>
> Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
> ---
> There are of course still plenty of cases where the return value is
> wrong.
>
>  lib/lcm.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/lib/lcm.c b/lib/lcm.c
> index b9c8de4..01b3aa9 100644
> --- a/lib/lcm.c
> +++ b/lib/lcm.c
> @@ -7,7 +7,7 @@
>  unsigned long lcm(unsigned long a, unsigned long b)
>  {
>  	if (a && b)
> -		return (a * b) / gcd(a, b);
> +		return (a / gcd(a, b)) * b;
>  	else if (b)
>  		return b;

[PATCH 1/2] lib/lcm.c: Ensure correct result whenever it fits

[Rasmus Villemoes]

Ensure that lcm(a,b) returns the mathematically correct result,
provided it fits in an unsigned long. The current version returns
garbage if a*b overflows, even if the final result would fit.

Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
---
 lib/lcm.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/lib/lcm.c b/lib/lcm.c
index b9c8de461e9e..01b3aa922dda 100644
--- a/lib/lcm.c
+++ b/lib/lcm.c
@@ -7,7 +7,7 @@
 unsigned long lcm(unsigned long a, unsigned long b)
 {
 	if (a && b)
-		return (a * b) / gcd(a, b);
+		return (a / gcd(a, b)) * b;
 	else if (b)
 		return b;
 
-- 
2.1.3