This is behavior is mostly just unspecified, at least for C++ (not sure about C).
I'm aware of some efforts to bring deterministic floating point operations into the C++ standard, but AFAIK there are no publicly available papers yet.
P3375R0 is public now [0], with a couple implementations available [1], [2].
Subexpression combining has more general implications that are usually worked around with gratuitous volatile abuse or magical incantations to construct compiler optimization barriers. Floating point is simply the most straightforward example where it leads to an observable change in behavior.
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This is behavior is mostly just unspecified, at least for C++ (not sure about C).
I'm aware of some efforts to bring deterministic floating point operations into the C++ standard, but AFAIK there are no publicly available papers yet.
P3375R0 is public now [0], with a couple implementations available [1], [2].
Subexpression combining has more general implications that are usually worked around with gratuitous volatile abuse or magical incantations to construct compiler optimization barriers. Floating point is simply the most straightforward example where it leads to an observable change in behavior.
[0] https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2024/p33...
[1] https://github.com/sixitbb/sixit-dmath
[2] https://github.com/J-Montgomery/rfloat/