I don't follow the point of your first four paragraphs. I specifically said the code was numerically stable on 128 bits - why are you introducing those irreleva

I don't follow the point of your first four paragraphs. I specifically said the code was numerically stable on 128 bits - why are you introducing those irrelevant facts, like how C compiler details can make some algorithms unstable? Really it comes down to what you mean by "another environment" and what your case is for access to the source code. Let me give a simpler example. I wrote code which expects ILP32. It's stable under ILP32. However, it produces different and non-deterministic answers under LP64. That is "another environment." Does the numerical instability under a different environment cast doubt on its validity in the original environment? Why is it a strong sign that there may be subtle errors?