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GraphBLAS / graphblas-api-c

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BB-33: Clarify behaviour of Nan and Inf #19

Closed mcmillan03 closed 3 years ago

mcmillan03 commented 3 years ago

How do the operators behave with Nan and Inf? what is 'max(x,NaN)'?

See 'fmax' in ANSI C11, which is the "omit nan" behavior, so that 'max(x,NaN)=x' for all 'x'. That is faster since it allows MAX to terminate earlier.

See also 'max' in MATLAB. MATLAB also has a way of specifying that 'max(x,NaN)=NaN'.

tgmattso commented 3 years ago

I am nervous about specifying behavior that is happening deep within the compiler. I would like to say that max(x,NaN) returns the value that the compiler used to build the GraphBLAS library would return. Comparing against a NaN is problematic and had better be rare. I don't want to define a behavior that slows down execution for cases without a NaN just to make sure we cover all exceptional cases.

DrTimothyAldenDavis commented 3 years ago

The ANSI C11 specification states that fmax, fmin, fmaxf, and fminf have the "omit NaN" behavior, which is the fastest option anway. However, if a library uses comparisons to compute MIN or MAX monoids, instead of fmin, fmax, etc, it might not get this behavior.

I use the ANSI C11 spec, with "omit NaN".

DrTimothyAldenDavis commented 3 years ago

We can close this issue and leave it up to the implementation.