I don't think pointer tagging (in and of itself, when implemented correctly for a particular platform) is ever undefined behavior. Rather, it is implementation-defined behavior - the result of converting a pointer to an integer is not specified, and could in theory be anything, so you have to know how your compiler and platform behave.

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RossBencina 2 days ago | parent [-]

We pray that it is implementation defined. According to some [1] even arithmetic on intptr_t is UB, let alone bit operations.

[1] https://stackoverflow.com/a/64863331/2013747

	▲	2 days ago \| parent \| next [-]
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	▲	gpderetta a day ago \| parent \| prev [-]
		Eh, that's about strictly conforming programs. In practice the vast majority of programs are going to use platform facilities and extensions (POSIX for example) while still being conforming.