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> A lot of code relies on FP operations being deterministic

A lot, but still quite the minority. People do run into situations where the same code with the same data produces slightly different results all the time because of changes in compiler, compiler settings, cpu arch, etc… They just don’t notice.

But, then they run into a situation where it matters and it’s a huge PITA to nail down.

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Yes this is exactly it. The gold standard is bit-4-bit matching when shifting code between systems. If you move your code to a new system and get different results --> is it due to different summing order, or something else wrong? By ruling out the former you're more likely to get the bit-4-bit match and thus avoid a lot of debugging effort -- or spend it solving a real bug.

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