Re: [PATCH] readkey: shrink table of known escape sequences in size

[Ahmad Fatoum <a.fatoum@pengutronix.de>]

Hello Sascha,

On 9/28/20 11:31 AM, Sascha Hauer wrote:
> On Mon, Sep 14, 2020 at 11:59:48AM +0200, Ahmad Fatoum wrote:
>> Instead of storing pointers to 4-byte strings, we could just store the
>> characters directly in the struct. Can save us up to 18 pointers worth
>> of space. Additionally, the nul byte need not be stored explicitly for
>> 3-byte strings, if we know those are the largest strings we have.
>>
>> The latter likely does not save us any space because of the usual
>> alignment rules, but it will allow us to support sequences one byte
>> bigger in future at no increase in size.
>>
>> Signed-off-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
>> ---
>>  lib/readkey.c | 8 +++++---
>>  1 file changed, 5 insertions(+), 3 deletions(-)
>>
>> diff --git a/lib/readkey.c b/lib/readkey.c
>> index c26e9d51aba9..551296de3eb6 100644
>> --- a/lib/readkey.c
>> +++ b/lib/readkey.c
>> @@ -20,8 +20,10 @@
>>  #include <linux/ctype.h>
>>  #include <readkey.h>
>>  
>> +#define MAX_ESC_LEN 3
>> +
>>  struct esc_cmds {
>> -	const char *seq;
>> +	const char seq[MAX_ESC_LEN];
> 
> I would have expected that when this array is initialized with a static
> initializer, the compiler would add a \0 at the end. Apparently this is
> not the case, initializing this 3 byte array with "[6~" is perfectly
> fine for the compiler.

Initializers are padded with zeroes if they are too short.

>> @@ -49,7 +51,7 @@ static const struct esc_cmds esccmds[] = {
>>  int read_key(void)
>>  {
>>  	unsigned char c;
>> -	unsigned char esc[5];
>> +	unsigned char esc[MAX_ESC_LEN + 2];
>>  	c = getchar();
>>  
>>  	if (c == 27) {
>> @@ -67,7 +69,7 @@ int read_key(void)
>>  		}
>>  		esc[i] = 0;
>>  		for (i = 0; i < ARRAY_SIZE(esccmds); i++){
>> -			if (!strcmp(esc, esccmds[i].seq))
>> +			if (!strncmp(esc, esccmds[i].seq, MAX_ESC_LEN))
>>  				return esccmds[i].val;
> 
> Anyway, I don't think we should play tricks with dropping string
> termination characters just to squeeze some bytes out of the binary.

I can define #define MAX_ESC_LEN 4 if you prefer that, but I consider
it superfluous.

Cheers,
Ahmad

> 
> Sascha
> 

-- 
Pengutronix e.K.                           |                             |
Steuerwalder Str. 21                       | http://www.pengutronix.de/  |
31137 Hildesheim, Germany                  | Phone: +49-5121-206917-0    |
Amtsgericht Hildesheim, HRA 2686           | Fax:   +49-5121-206917-5555 |

_______________________________________________
barebox mailing list
barebox@lists.infradead.org
http://lists.infradead.org/mailman/listinfo/barebox