(j3.2006) ieee_set_halting_mode

[Bill Long]

John Reid wrote:
> Bill Long wrote:

>>> I "sense" that this question arises because different people have 
>>> different ideas of what PURE means.  It means different things in 
>>> different C.S.-like contexts, in particular Fortran PURE procedures 
>>> are not "pure functions" in at least one of the traditional CS senses 
>>> as they are permitted to read global state.
>>
>> That's fine. But ieee_set_halting_mode performs a WRITE of global state. 
> 
> I don't see where the standard says this.
> 

The standard does not say how to implement the function. I'm just 
pointing out what actually happens in the real world to implement the 
function.

 >  With 10**6 images, would you really work with a single register?

Each image has its own ieee status.  The "global" state is only relative 
to the local image - global in the same sense that a variable in a 
common block or module is global.  Which is also the meaning of 'global' 
in the Note in the standard.

> 
> See 14.6 for the rule on halting mode: "In a procedure other than 
> IEEE_SET_HALTING_MODE or IEEE_SET_STATUS, the processor shall not change
> the halting mode on entry, and on return shall ensure that the halting mode is 
> the same as it was on entry." It is purposely vague on when halting actually 
> occurs: "Halting is not precise and may occur any time after the exception has 
> occurred."
> 

Thanks.  This is the critical quote that answers my original question. 
  This requires that you have to make a second write to a global state 
before returning from  the procedure where the call was made.   So a 
call at the beginning of the main program is fine - it lasts through the 
program.  Calls inside a subprogram would require a read and two writes 
of the trap mask for a conforming implementation.   The performance hit 
for that is quite large. So I would not expect to see this used in a 
subprogram, which does match what I've seen so far.

Cheers,
Bill




-- 
Bill Long                                           longb at cray.com
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