relax() in relax.hpp can return false even if predecessor map is changed

[Alex Hagen-Zanker <ahh34@…>]

The relax function in relax.hpp checks whether the distance of an edge really is reduced after relaxation and only returns true if that is the case. (the alternative possibility is that the expected reduction in distance does not take place due to floating point precision).

However, the function updates the predecessor map in any case. It is therefore possible that the predecessor map is changed while the relax function returns false. I suppose this is a bug, even though I did not cause me any trouble.

[Alex Hagen-Zanker <ahh34@…>]

patch

[Jeremiah Willcock]

(In [80639]) Fixed relax logic to not write predecessor map unless distance was actually changed in memory (although this case will never be hit unless registers have extra precision compared to memory); fixes #7226