(j3.2006) parameterized derived types

[Bill Long]

An example might help.   I could see a potential rounding issue with something like a component declaration


   real,dimension(INT( SQRT(25.0) * SQRT(121.0) )) :: X

but this is not unique to PDTs.  The type parameters themselves are always type integer.

Cheers,
Bill


On Dec 8, 2015, at 2:52 AM, Robert Corbett <robert.corbett at oracle.com> wrote:

> Constraints placed on components of derived types appear to be intended to 
> ensure that the values of expressions that appear in derived type definitions 
> shall not depend on any nonconstant values other than the values of the type 
> parameters.  One way in which the constraints fail to do so is that the 
> expressions can include floating-point operations whose value depend on the 
> rounding mode.  The rounding mode might be considered a hidden variable, but the 
> standard does not say that.  Does any implementation of parameterized derived 
> types take the possibility of changing rounding modes into account?
> 
> I considered changing the rounding mode to the processor-dependent initial 
> rounding mode during elaboration of a type specifier with parameters, but the 
> standard does not appear to allow that.
> 
> Ensuring that floating-point expressions produce the same values under a single 
> rounding mode is difficult but possible given the backends I need to support.  
> By default, the optimizers reorder floating-point expressions to produce more 
> efficient code.  Forcing them not to do so requires giving up many optimization 
> that are harmless.  The x87 environments are particularly difficult to handle.
> 
> Bob Corbett
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Bill Long                                                                       longb at cray.com
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