[Info-vax] decrementing & for loops in C

[Bill Gunshannon]

In article <176uZD2KcidF-pn2-o5JGhldCleMl at rikki.tavi.co.uk>,
	"Bob Eager" <rde42 at spamcop.net> writes:
> On Fri, 9 Jan 2009 17:35:56 UTC, billg999 at cs.uofs.edu (Bill Gunshannon) 
> wrote:
> 
>> Well, let's be clear on this.  By "exactly as written" I mean just what it
>> says.  Not "as the writer intended", but "exactly as written".  That allows
>> for optimizing out code that would never be executed because I wrote the 
>> code wrong, like the original example.  It may not be what I wanted, but
>> it is what I wrote.  But, the system should not be changing the order of
>> operations for any code I wrote because it has absolutely no way of
>> determining that there was not a particular reason for doing it in that
>> order.
> 
> Lots of compilers re-order operations. So do lots of CPUs; the Pentium 
> does it, and I suspect some VAXes do, as well (although this kind of 
> thing is relatively modern).
> 
>> "Let's see.  The programmer wants me to check for weight on the wheel
>>  struts before raising the landing gear.  but I think it would be much
>>  more efficient to raise the gear first."   :-)
>>  
>> What, you say that's just silly.  How about this?
>>  
>> {
>>    int thrust_reversers, power_level;
>>  
>> { thrust_reversers = 10; power_level = 20; }
>> }
>>  
>> Does the order of these two (apparently) equal operations make a difference?
> 
> If they are assignments to simple variables, no. If they were function 
> calls, then the compiler wouldn't know, and wouldn't re-order them. 
> Likewise for access to device registers.

And, how does the compiler know that the memory address of "power_level"
is a device register as opposed to a memory location?  An address is an
address.  There is no way that I can think of that the compiler could
know anything about the hardware the programis going to eventually be
executed on.  Unless it can read the comments.  :-)

> 
> Compilers are generally allowed to re-order stuff in blocks of code 
> (i.e. those without branches) as long as execution of none of that block
> generates side effects (which is basically what I just said).

In device control applications, something this is the primary use of
Ada, how can the compiler know what the possible side effects of an
operation are?  That's my point.  Not that there aren't safe and unsafe
things that can be done.  Just that there is no compiler I would be
willing to trust to make that decision for me.  The mistakes programmers
make are bad enough (anybody remember the Therac?) but the mistakes a
machine is capable of are the stuff nightmares are made of.
 
bill

-- 
Bill Gunshannon          |  de-moc-ra-cy (di mok' ra see) n.  Three wolves
billg999 at cs.scranton.edu |  and a sheep voting on what's for dinner.
University of Scranton   |
Scranton, Pennsylvania   |         #include <std.disclaimer.h>