Semantics of statement reordering relevant to Ada.Real_Time

[Eric Hughes <eric.eh9@gmail.com>]

On Mar 29, 7:51 am, kongra <kon...@gmail.com> wrote:
>    Start_T := Ada.Real_Time.Clock;
>    for K in 1 .. 10 loop
>       for I in 1 .. 10_000_000 loop
>          Val := Val + I;
>       end loop;
>    end loop;
>    End_T   := Ada.Real_Time.Clock;

I have a question about this piece of the program.  Because the middle
uses only variables 'K', 'I', and 'Val', and the side-effects of
modifying these variables are entirely local, and because there's no
inter-dependency of these variables upon either the first or last
statements, aren't the following reorderings legal for code
generation?  To wit:

    Long_Statement ;
    Start_T := Ada.Real_Time.Clock ;
    End_T   := Ada.Real_Time.Clock ;

    Start_T := Ada.Real_Time.Clock ;
    End_T   := Ada.Real_Time.Clock ;
    Long_Statement ;

Irrelevant to the quirks mentioned earlier, isn't it legal operation
for the interval to come out zero?  It seems that for known-correct
operation for all possible compilers you'd need to manually serialize
these operations in the order desired.

Eric