[PATCH] staging: wfx: make a const array static, makes object smaller

[PATCH] staging: wfx: make a const array static, makes object smaller

[Colin King]

From: Colin Ian King <colin.king@canonical.com>

Don't populate const array filter_ies on the stack but instead
make it static. Makes the object code smaller by 261 bytes.

Before:
   text	   data	    bss	    dec	    hex	filename
  21674	   3166	    448	  25288	   62c8	drivers/staging/wfx/sta.o

After:
   text	   data	    bss	    dec	    hex	filename
  21349	   3230	    448	  25027	   61c3	drivers/staging/wfx/sta.o

(gcc version 10.2.0)

Signed-off-by: Colin Ian King <colin.king@canonical.com>
---
 drivers/staging/wfx/sta.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/staging/wfx/sta.c b/drivers/staging/wfx/sta.c
index 2320a81eae0b..196779a1b89a 100644
--- a/drivers/staging/wfx/sta.c
+++ b/drivers/staging/wfx/sta.c
@@ -63,7 +63,7 @@ void wfx_suspend_hot_dev(struct wfx_dev *wdev, enum sta_notify_cmd cmd)
 
 static void wfx_filter_beacon(struct wfx_vif *wvif, bool filter_beacon)
 {
-	const struct hif_ie_table_entry filter_ies[] = {
+	static const struct hif_ie_table_entry filter_ies[] = {
 		{
 			.ie_id        = WLAN_EID_VENDOR_SPECIFIC,
 			.has_changed  = 1,
-- 
2.27.0

Re: [PATCH] staging: wfx: make a const array static, makes object smaller

[Joe Perches]

On Fri, 2020-10-16 at 23:33 +0100, Colin King wrote:
> From: Colin Ian King <colin.king@canonical.com>
> 
> Don't populate const array filter_ies on the stack but instead
> make it static. Makes the object code smaller by 261 bytes.
> 
> Before:
>    text	   data	    bss	    dec	    hex	filename
>   21674	   3166	    448	  25288	   62c8	drivers/staging/wfx/sta.o
> 
> After:
>    text	   data	    bss	    dec	    hex	filename
>   21349	   3230	    448	  25027	   61c3	drivers/staging/wfx/sta.o

Thanks.

It's odd to me it's so large a change as it's only
24 bytes of initialization. (3 entries, each 8 bytes)

This line in the same function:

		hif_set_beacon_filter_table(wvif, 3, filter_ies);

might as well use ARRAY_SIZE(filter_ies) instead of 3

RE: [PATCH] staging: wfx: make a const array static, makes object smaller

[David Laight]

From: Joe Perches
> Sent: 17 October 2020 01:12
> 
> On Fri, 2020-10-16 at 23:33 +0100, Colin King wrote:
> > From: Colin Ian King <colin.king@canonical.com>
> >
> > Don't populate const array filter_ies on the stack but instead
> > make it static. Makes the object code smaller by 261 bytes.
> >
> > Before:
> >    text	   data	    bss	    dec	    hex	filename
> >   21674	   3166	    448	  25288	   62c8	drivers/staging/wfx/sta.o
> >
> > After:
> >    text	   data	    bss	    dec	    hex	filename
> >   21349	   3230	    448	  25027	   61c3	drivers/staging/wfx/sta.o
> 
> Thanks.
> 
> It's odd to me it's so large a change as it's only
> 24 bytes of initialization. (3 entries, each 8 bytes)

Perhaps the 'stack protector' crap?

Interestingly, loading the data from the 'readonly' section
is probably a data cache miss.
Which might end up being slower than the extra code to
update the on-stack data.
The extra code might get prefetched...

	David
 

-
Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
Registration No: 1397386 (Wales)

Re: [PATCH] staging: wfx: make a const array static, makes object smaller

[Jérôme Pouiller]

On Monday 19 October 2020 10:09:19 CEST David Laight wrote:
> From: Joe Perches
> > Sent: 17 October 2020 01:12
> >
> > On Fri, 2020-10-16 at 23:33 +0100, Colin King wrote:
> > > From: Colin Ian King <colin.king@canonical.com>
> > >
> > > Don't populate const array filter_ies on the stack but instead
> > > make it static. Makes the object code smaller by 261 bytes.
> > >
> > > Before:
> > >    text        data     bss     dec     hex filename
> > >   21674        3166     448   25288    62c8 drivers/staging/wfx/sta.o
> > >
> > > After:
> > >    text        data     bss     dec     hex filename
> > >   21349        3230     448   25027    61c3 drivers/staging/wfx/sta.o
> >
> > Thanks.
> >
> > It's odd to me it's so large a change as it's only
> > 24 bytes of initialization. (3 entries, each 8 bytes)
> 
> Perhaps the 'stack protector' crap?
> 
> Interestingly, loading the data from the 'readonly' section
> is probably a data cache miss.
> Which might end up being slower than the extra code to
> update the on-stack data.
> The extra code might get prefetched...

I had never realized the difference between "const" and "static const" in
this case.

With my gcc fro arm, the output of "objdump -h sta.o" gives:

Before:
  0 .text         000019fc  00000000  00000000  00000034  2**2
                  CONTENTS, ALLOC, LOAD, READONLY, CODE
  [...]
  7 .rodata       00000015  00000000  00000000  00001e78  2**2
                  CONTENTS, ALLOC, LOAD, READONLY, DATA

After:
  0 .text         00001974  00000000  00000000  00000034  2**2
                  CONTENTS, ALLOC, LOAD, READONLY, CODE
  [...]
  7 .rodata       0000002d  00000000  00000000  00001dd4  2**2
                  CONTENTS, ALLOC, LOAD, READONLY, DATA

The difference of .rodata is exactly what is expected (24 bytes) and we
save 115 bytes of code.

Reviewed-by: Jérôme Pouiller <jerome.pouiller@silabs.com>


-- 
Jérôme Pouiller

Re: [PATCH] staging: wfx: make a const array static, makes object smaller

[Christophe JAILLET]

Le 17/10/2020 à 02:11, Joe Perches a écrit :
> On Fri, 2020-10-16 at 23:33 +0100, Colin King wrote:
>> From: Colin Ian King <colin.king@canonical.com>
>>
>> Don't populate const array filter_ies on the stack but instead
>> make it static. Makes the object code smaller by 261 bytes.
>>
>> Before:
>>     text	   data	    bss	    dec	    hex	filename
>>    21674	   3166	    448	  25288	   62c8	drivers/staging/wfx/sta.o
>>
>> After:
>>     text	   data	    bss	    dec	    hex	filename
>>    21349	   3230	    448	  25027	   61c3	drivers/staging/wfx/sta.o
> 
> Thanks.
> 
> It's odd to me it's so large a change as it's only
> 24 bytes of initialization. (3 entries, each 8 bytes)
> 

The function looks small.
Maybe it is inlined by gcc in each of the 3 callers?

CJ

> This line in the same function:
> 
> 		hif_set_beacon_filter_table(wvif, 3, filter_ies);
> 
> might as well use ARRAY_SIZE(filter_ies) instead of 3
> 
> 
>