Possible performance issue with gfortran? denormalized numbers

[Toon Moene]

On 02/01/2016 12:55 PM, Jose Miguel Reynolds Barredo wrote:

> Hi everyone,
>
> I was developing a tridiagonal block solver and I found a performance
> issue that is ingriguing me: doing operations with numbers in the
> denormalized range is around ten times slower than regular numbers
> (https://en.wikipedia.org/wiki/Denormal_number). As an example, I
> builded a very simple code:

The reason operations with denormal numbers are ten times slower has - 
unfortunately, as otherwise we could do something about it - nothing to 
do with the compiler or the run time library (of *any* language).

Denormal number operations are handled by the operating system, because 
it is too costly to allocate silicon to handle them on the CPU. So when 
the CPU detects a denormal number, it traps. This trap is caught by the 
OS, which dispatches the computation to a routine written for the 
purpose. The trap and the software implementation of the handling of the 
operation involving a denormal are costly, as you observed.

There is nothing the compiler writers (*any* compiler writers, not just 
GCC's) can do about this.

Kind regards,

-- 
Toon Moene - e-mail: toon@moene.org - phone: +31 346 214290
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