[Bug 1001605] New: much faster and simpler(!) replacement for_simple_mktime() (in wallclock.inl)

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           Summary: much faster and simpler(!) replacement for
                    _simple_mktime() (in wallclock.inl)
           Product: eCos
           Version: CVS
          Platform: All
        OS/Version: All
            Status: UNCONFIRMED
          Severity: enhancement
          Priority: low
         Component: Wallclock
        AssignedTo: unassigned@bugs.ecos.sourceware.org
        ReportedBy: bernard.fouche@kuantic.com
                CC: ecos-bugs@ecos.sourceware.org
             Class: Advice Request


If _simple_mktime() is called for Dec 31st 2012, it will do:

  42 (2012-1970) additions of a value requiring 3 modulo's
+ 12 additions (of the month)
+ 1 substraction
+ 3 multiplications and 3 additions

That is a total of (42*3)+12+1+(3+3)=145 operations.

The proposed change makes _simple_mktime() to use:

- 1 substraction, 1 multiplication, 1 addition and 1 substraction to calculate
a 'base' second value.
- 1 comparison that can lead to 1 substraction
- 1 addition, 1 substraction, 1 division
- 2 substractions, 1 division
- 1 addition, 1 substraction, 1 division
- 1 addition
- 1 substraction
- 1 multiplication
- 1 addition
- 1 multiplication
- 1 addition
- 1 multiplication
- 1 addition

That is a total of:

(1+1+1+1)+(1+1)+(1+1+1)+(2+1)+(1+1+1)+1+1+1+1+1+1+1+1=23 operations.

This count is nearly always the same whatever the time/date processed, which is
not the case for the original code, which takes 3 modulo's more per invocation
each year, since this function is called by all RTC HW drivers.

I don't think eCos runs on MCU/CPU without HW division, but in such a case this
would mean that 2/3rd of the modulo's in the current code are also done in
software (for 100 and 400 values).

I attach an example of the proposed replacement code that compiles under Linux
and uses brute force to check all possible values: it takes about 6 minutes to
run in a recent computer.

If someone can review and accept this small code change, I can make a patch vs
ecos/packages/io/wallclock/current/include/wallclock/wallclock.inl

  Bernard

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