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Can > different compiler versions move them around? I always thought they can. > If not, I can also use them in memfd code since they make the code > neater. It is specified by the SYSV ABI, compilers are not free to change it. https://gitlab.com/x86-psABIs/x86-64-ABI/-/blob/master/x86-64-ABI/low-level-sys-info.tex?ref_type=heads#L337 I didn't check ARM64 but would be shocked to find it is not specified. Jason