From: Jan Engelhardt
> Sent: 10 June 2022 09:32
> 
> 
> On Friday 2022-06-10 10:17, David Laight wrote:
> >>
> >> Calling a "printf" style function is already insanely expensive. :-) I
> >> understand that it's not okay blithely to increase runtime resources
> >> simply because it's already slow, but in this case it's worthwhile.
> >
> >Yep, IMHO definitely should be fixed.
> >It is even possible that using "%s" is faster because the printf
> >code doesn't have to scan the string for format effectors.
> 
> I see no special handling; the vsnprintf function just loops
> over fmt as usual and I see no special casing of fmt by
> e.g. strcmp(fmt, "%s") == 0 to take a shortcut.

Consider the difference between:
	printf("fubar");
and
	printf("%s", "fubar");
In the former all of "fubar" is checked for '%'.
In the latter only the length of "fubar" has to be counted.

FWIW the patch description should probably by:
	use "%s" when formatting a single string.
(or something to that effect).

	David

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On Friday 2022-06-10 11:14, David Laight wrote:
>> >Yep, IMHO definitely should be fixed.
>> >It is even possible that using "%s" is faster because the printf
>> >code doesn't have to scan the string for format effectors.
>> 
>> I see no special handling; the vsnprintf function just loops
>> over fmt as usual and I see no special casing of fmt by
>> e.g. strcmp(fmt, "%s") == 0 to take a shortcut.
>
>Consider the difference between:
>	printf("fubar");
>and
>	printf("%s", "fubar");
>In the former all of "fubar" is checked for '%'.
>In the latter only the length of "fubar" has to be counted.

To check the length of "fubar", printf first needs to know that there
even is an argument to be pulled from the stack, which it does by
evaluating the format string.

So, in fairness, it's more like:

 >> In the latter, all of "%s" is checked for '%'.