## An example of when nauty, on two different platforms, gives different canonical labels for the same input graph? [closed]

Let $G$ be a graph. I've heard that, if we use nauty to canonically label $G$ on two different platforms, it's possible to obtain distinct labels. However, I've never actually seen this occur.

The nauty user guide (pdf) writes:

Beginning at version 2.1, the canonical labelling does not depend on the compiler, the system, or the word size.

This seems to imply that it's still possible to receive distinct canonical labels on different platforms. (This might also be somewhat out of context, since this sentence is written in the "dreadnaut" section.)

Question: What is an example of a situation where, on two different platforms, nauty would canonically label the same graph differently (if there are any)?

If it can still occur, is it a rare occurrence?

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 are you sure this is an ok question for MO? i thought such software related questions were not really entertained here...? – S. Sra May 29 at 3:29 Some obscure CPU/compiler/OS bug certainly will lead to different result. CPU bugs are not uncommon, check the errata. – joro May 29 at 5:16 This is not apprpropriate, as @Suvrit notes. I am sure @Brendan would be happy to enlighten you over email. – Igor Rivin May 29 at 12:40