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An unexpected optimisation can introduce a side channel (most commonly timing). This one would be safe, but "how do you tell a compiler which ones [not] to use" is a whole topic by itself.

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The C++ standard doesn't forbid introducing side channels, so the answer to the question is yes.

With all the UB, I wonder how did we manage to write any secure or safety-critical code at all.

In C++? We pretty much did not.

this isn't UB, and any other language can do this optimization as well

even the one you cult over

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