> Non-determinism occurs in programs running on intel with floats when threads are interrupted and the math coprocessor flushed

That's ridiculous. No OS in his right mind would flush FPU regs to 64 bits only, because that would break many things, most obviously "real" 80 bit FP which is still a thing and the only reason x87 instructions still work. It would even break plain equality comparisons making all FP useless.

For 64 bit FP most compilers prefer SSE rather than x87 instructions these days.

https://bugs.php.net/bug.php?id=53632

Never, for sure.