(j3.2006) IEEE and the GPU world

[John Reid]

Bill Long wrote:
> 
> The IEEE feature in F03 is becoming the topic of discussion with the 
> advent of the Chinese GPU system hitting #2 on the Top 500 list, and 
> also users getting access to the GPU's in their laptops via schemes like 
> OpenCL.
> 
> While GPU's support the hardware bit patterns for IEEE floating point 
> format (more or less a requirement for them to be useful at all for 
> floating point in a shared data environment),  they do not support 
> things like exception flags or interrupts (halting), mainly for 
> performance reasons.
> 
> The question is what values should users expect from routines like 
> IEEE_SUPPORT_HALTING and IEEE_SUPPORT_FLAG?    The standard is worded as 
> if there is a single "processor".  But the common configuration will be, 
> at least at the beginning, of a combination of a "normal" processor 
> (x86_64, for example) for which these inquiry functions would return 
> true, and a GPU for which false is the right answer.   Does the presence 
> of the GPU code pollute the entire program (functions return false no 
> matter where called in the program), or do they return true if called in 
> part of the code that is compiled into instructions for the normal 
> processor, and false if called from  the parts of the code compiled for 
> the GPU?   Note that the programmer would not necessarily know which 
> parts of the program end up executing on which hardware type.  Insights 
> from the designers of this feature would be particularly interesting.

I think the standard is pretty clear. 14.1:p2 says

"If IEEE_EXCEPTIONS or IEEE_ARITHMETIC is accessible in a scoping unit, the 
exceptions IEEE_OVERFLOW and IEEE_DIVIDE_BY_ZERO are supported in the scoping 
unit for all kinds of real and complex IEEE fl oating-point data. Which other 
exceptions are supported can be determined by the inquiry function 
IEEE_SUPPORT_FLAG (14.11.27); whether control of halting is supported can be 
determined by the inquiry function IEEE_SUPPORT_HALTING. ..."

and 14.1:p3 says

"If a scoping unit does not access IEEE_FEATURES, IEEE_EXCEPTIONS, or 
IEEE_ARITHMETIC, the level of support is processor dependent, and need not 
include support for any exceptions. If a  flag is signaling on entry to
such a scoping unit, the processor ensures that it is signaling on exit. If a 
flag is quiet on entry to such a scoping unit, whether it is signaling on exit 
is processor dependent."

Cheers,

John.