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

[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:
   text	   data	    bss	    dec	    hex	filename
12901332	7798930	14541816	35242078	219c05e	vmlinux

    After the patch:
   text	   data	    bss	    dec	    hex	filename
12901331	7798930	14541816	35242077	219c05d	vmlinux

Signed-off-by: Rakib Mullick <rakib.mullick@gmail.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Cc: Matthew Wilcox <mawilcox@microsoft.com>
Cc: Yury Norov <ynorov@caviumnetworks.com>
Cc: Mauro Carvalho Chehab <mchehab@kernel.org>
 
---
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

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

[Rasmus Villemoes]

On 14 November 2017 at 07:57, Rakib Mullick <rakib.mullick@gmail.com> 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.

[Rakib Mullick]

On Tue, Nov 14, 2017 at 1:23 PM, Rasmus Villemoes
<linux@rasmusvillemoes.dk> 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.