Re: [PATCH] seq_file: delete small-value optimization

Re: [PATCH] seq_file: delete small-value optimization

[Alexey Dobriyan]

> Joe Perches wrote:
> 
> On Tue, 2017-05-16 at 23:42 +0300, Alexey Dobriyan wrote:
> > num_to_str() optimizes printing small integers [0..9], so the same
> > check higher in callchain is unnecessary.
> 
> Doesn't the optimization exists for the frequent use of 0
> in seq output?  
> 
> These seq_put_decimal calls are now slightly more expensive.

That additional CALL instruction is hardly measurable so you're adding
branch to skip branch in the next function.

Re: [PATCH] seq_file: delete small-value optimization

[Joe Perches]

On Wed, 2017-05-17 at 13:33 +0300, Alexey Dobriyan wrote:
> > Joe Perches wrote:
> > 
> > On Tue, 2017-05-16 at 23:42 +0300, Alexey Dobriyan wrote:
> > > num_to_str() optimizes printing small integers [0..9], so the same
> > > check higher in callchain is unnecessary.
> > 
> > Doesn't the optimization exists for the frequent use of 0
> > in seq output?  
> > 
> > These seq_put_decimal calls are now slightly more expensive.
> 
> That additional CALL instruction is hardly measurable so you're adding
> branch to skip branch in the next function.

It's not the call instruction.

num_to_str pushes the value first to stack
and then sets up a loop to copy those chars
to buffer.

The current code immediately pushes to buffer.

It's a fair amount of overhead.

Have you measured it?

Re: [PATCH] seq_file: delete small-value optimization

[Joe Perches]

On Tue, 2017-05-16 at 23:42 +0300, Alexey Dobriyan wrote:
> num_to_str() optimizes printing small integers [0..9], so the same
> check higher in callchain is unnecessary.

Doesn't the optimization exists for the frequent use of 0
in seq output?  

These seq_put_decimal calls are now slightly more expensive.

[PATCH] seq_file: delete small-value optimization

[Alexey Dobriyan]

num_to_str() optimizes printing small integers [0..9], so the same
check higher in callchain is unnecessary.

Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
---

 fs/seq_file.c |   10 ----------
 1 file changed, 10 deletions(-)

--- a/fs/seq_file.c
+++ b/fs/seq_file.c
@@ -694,11 +694,6 @@ void seq_put_decimal_ull(struct seq_file *m, const char *delimiter,
 	if (m->count + 1 >= m->size)
 		goto overflow;
 
-	if (num < 10) {
-		m->buf[m->count++] = num + '0';
-		return;
-	}
-
 	len = num_to_str(m->buf + m->count, m->size - m->count, num);
 	if (!len)
 		goto overflow;
@@ -733,11 +728,6 @@ void seq_put_decimal_ll(struct seq_file *m, const char *delimiter, long long num
 		num = -num;
 	}
 
-	if (num < 10) {
-		m->buf[m->count++] = num + '0';
-		return;
-	}
-
 	len = num_to_str(m->buf + m->count, m->size - m->count, num);
 	if (!len)
 		goto overflow;