Re: [PATCH] lib: Avoid redundant sizeof checking in __bitmap_weight() calculation.

[Rakib Mullick]

On Tue, Nov 14, 2017 at 1:23 PM, Rasmus Villemoes
 wrote:
>
> hint: sizeof() very rarely evaluates to zero... So this is the same as
> "is32 = 1". So the patch as-is is broken (and may explain the 1-byte
> delta in vmlinux). But even if this condition is fixed, the patch
> doesn't change anything, since the sizeof() evaluation is done at
> compile-time, regardless of whether it happens inside the inlined
> hweight_long or outside. So it is certainly not worth it to duplicate
> the loop.
>
Right, no need to duplicate the loop. Checking the objdump output, it
didn't changed anything at all. Fixed condition nullifies the vmlinux
size advantage also. Drop it.

Thanks,
Rakib.

Re: [PATCH] lib: Avoid redundant sizeof checking in __bitmap_weight() calculation.

[Rakib Mullick]

On Tue, Nov 14, 2017 at 1:23 PM, Rasmus Villemoes
 wrote:
>
> hint: sizeof() very rarely evaluates to zero... So this is the same as
> "is32 = 1". So the patch as-is is broken (and may explain the 1-byte
> delta in vmlinux). But even if this condition is fixed, the patch
> doesn't change anything, since the sizeof() evaluation is done at
> compile-time, regardless of whether it happens inside the inlined
> hweight_long or outside. So it is certainly not worth it to duplicate
> the loop.
>
Right, no need to duplicate the loop. Checking the objdump output, it
didn't changed anything at all. Fixed condition nullifies the vmlinux
size advantage also. Drop it.

Thanks,
Rakib.

Re: [PATCH] lib: Avoid redundant sizeof checking in __bitmap_weight() calculation.

[Rasmus Villemoes]

On 14 November 2017 at 07:57, Rakib Mullick  wrote:
> Currently, during __bitmap_weight() calculation hweight_long() is used.
> Inside a hweight_long() a check has been made to figure out whether a
> hweight32() or hweight64() version to use.
>
> diff --git a/lib/bitmap.c b/lib/bitmap.c
> index d8f0c09..552096f 100644
> --- a/lib/bitmap.c
> +++ b/lib/bitmap.c
> @@ -241,10 +241,15 @@ EXPORT_SYMBOL(__bitmap_subset);
>  int __bitmap_weight(const unsigned long *bitmap, unsigned int bits)
>  {
> unsigned int k, lim = bits/BITS_PER_LONG;
> -   int w = 0;
> -
> -   for (k = 0; k < lim; k++)
> -   w += hweight_long(bitmap[k]);
> +   int w = 0, is32 = sizeof(bitmap[0]) ? 1 : 0;
> +

hint: sizeof() very rarely evaluates to zero... So this is the same as
"is32 = 1". So the patch as-is is broken (and may explain the 1-byte
delta in vmlinux). But even if this condition is fixed, the patch
doesn't change anything, since the sizeof() evaluation is done at
compile-time, regardless of whether it happens inside the inlined
hweight_long or outside. So it is certainly not worth it to duplicate
the loop.

Rasmus

Re: [PATCH] lib: Avoid redundant sizeof checking in __bitmap_weight() calculation.

[Rasmus Villemoes]

On 14 November 2017 at 07:57, Rakib Mullick  wrote:
> Currently, during __bitmap_weight() calculation hweight_long() is used.
> Inside a hweight_long() a check has been made to figure out whether a
> hweight32() or hweight64() version to use.
>
> diff --git a/lib/bitmap.c b/lib/bitmap.c
> index d8f0c09..552096f 100644
> --- a/lib/bitmap.c
> +++ b/lib/bitmap.c
> @@ -241,10 +241,15 @@ EXPORT_SYMBOL(__bitmap_subset);
>  int __bitmap_weight(const unsigned long *bitmap, unsigned int bits)
>  {
> unsigned int k, lim = bits/BITS_PER_LONG;
> -   int w = 0;
> -
> -   for (k = 0; k < lim; k++)
> -   w += hweight_long(bitmap[k]);
> +   int w = 0, is32 = sizeof(bitmap[0]) ? 1 : 0;
> +

hint: sizeof() very rarely evaluates to zero... So this is the same as
"is32 = 1". So the patch as-is is broken (and may explain the 1-byte
delta in vmlinux). But even if this condition is fixed, the patch
doesn't change anything, since the sizeof() evaluation is done at
compile-time, regardless of whether it happens inside the inlined
hweight_long or outside. So it is certainly not worth it to duplicate
the loop.

Rasmus

[PATCH] lib: Avoid redundant sizeof checking in __bitmap_weight() calculation.

[Rakib Mullick]

Currently, during __bitmap_weight() calculation hweight_long() is used.
Inside a hweight_long() a check has been made to figure out whether a
hweight32() or hweight64() version to use.

However, it's unnecessary to do it in case of __bitmap_weight() calculation
inside the loop. We can detect whether to use hweight32() or hweight64()
upfront and call respective function directly. It also reduces the
vmlinux size.

Before the patch:
   textdata bss dec hex filename
129013327798930 1454181635242078219c05e vmlinux

After the patch:
   textdata bss dec hex filename
129013317798930 1454181635242077219c05d vmlinux

Signed-off-by: Rakib Mullick 
Cc: Andrew Morton 
Cc: Rasmus Villemoes 
Cc: Matthew Wilcox 
Cc: Yury Norov 
Cc: Mauro Carvalho Chehab 
 
---
Patch was created against torvald's tree (commit 43ff2f4db9d0f764).

 lib/bitmap.c | 13 +
 1 file changed, 9 insertions(+), 4 deletions(-)

diff --git a/lib/bitmap.c b/lib/bitmap.c
index d8f0c09..552096f 100644
--- a/lib/bitmap.c
+++ b/lib/bitmap.c
@@ -241,10 +241,15 @@ EXPORT_SYMBOL(__bitmap_subset);
 int __bitmap_weight(const unsigned long *bitmap, unsigned int bits)
 {
unsigned int k, lim = bits/BITS_PER_LONG;
-   int w = 0;
-
-   for (k = 0; k < lim; k++)
-   w += hweight_long(bitmap[k]);
+   int w = 0, is32 = sizeof(bitmap[0]) ? 1 : 0;
+
+   if (is32) {
+   for (k = 0; k < lim; k++)
+   w += hweight32(bitmap[k]);
+   } else {
+   for (k = 0; k < lim; k++)
+   w += hweight64(bitmap[k]);
+   }
 
if (bits % BITS_PER_LONG)
w += hweight_long(bitmap[k] & BITMAP_LAST_WORD_MASK(bits));
-- 
2.9.3

[PATCH] lib: Avoid redundant sizeof checking in __bitmap_weight() calculation.

[Rakib Mullick]

Currently, during __bitmap_weight() calculation hweight_long() is used.
Inside a hweight_long() a check has been made to figure out whether a
hweight32() or hweight64() version to use.

However, it's unnecessary to do it in case of __bitmap_weight() calculation
inside the loop. We can detect whether to use hweight32() or hweight64()
upfront and call respective function directly. It also reduces the
vmlinux size.

Before the patch:
   textdata bss dec hex filename
129013327798930 1454181635242078219c05e vmlinux

After the patch:
   textdata bss dec hex filename
129013317798930 1454181635242077219c05d vmlinux

Signed-off-by: Rakib Mullick 
Cc: Andrew Morton 
Cc: Rasmus Villemoes 
Cc: Matthew Wilcox 
Cc: Yury Norov 
Cc: Mauro Carvalho Chehab 
 
---
Patch was created against torvald's tree (commit 43ff2f4db9d0f764).

 lib/bitmap.c | 13 +
 1 file changed, 9 insertions(+), 4 deletions(-)

diff --git a/lib/bitmap.c b/lib/bitmap.c
index d8f0c09..552096f 100644
--- a/lib/bitmap.c
+++ b/lib/bitmap.c
@@ -241,10 +241,15 @@ EXPORT_SYMBOL(__bitmap_subset);
 int __bitmap_weight(const unsigned long *bitmap, unsigned int bits)
 {
unsigned int k, lim = bits/BITS_PER_LONG;
-   int w = 0;
-
-   for (k = 0; k < lim; k++)
-   w += hweight_long(bitmap[k]);
+   int w = 0, is32 = sizeof(bitmap[0]) ? 1 : 0;
+
+   if (is32) {
+   for (k = 0; k < lim; k++)
+   w += hweight32(bitmap[k]);
+   } else {
+   for (k = 0; k < lim; k++)
+   w += hweight64(bitmap[k]);
+   }
 
if (bits % BITS_PER_LONG)
w += hweight_long(bitmap[k] & BITMAP_LAST_WORD_MASK(bits));
-- 
2.9.3