On Fri, Oct 23, 2020 at 03:09:30PM +0200, David Hildenbrand wrote: > Now, I am not a compiler expert, but as I already cited, at least on > x86-64 clang expects that the high bits were cleared by the caller - in > contrast to gcc. I suspect it's the same on arm64, but again, I am no > compiler expert. > > If what I said and cites for x86-64 is correct, if the function expects > an "unsigned int", it will happily use 64bit operations without further > checks where valid when assuming high bits are zero. That's why even > converting everything to "unsigned int" as proposed by me won't work on > clang - it assumes high bits are zero (as indicated by Nick). > > As I am neither a compiler experts (did I mention that already? ;) ) nor > an arm64 experts, I can't tell if this is a compiler BUG or not. On arm64 when callee expects a 32bit argument, the caller is *not* responsible for clearing the upper half of 64bit register used to pass the value - it only needs to store the actual value into the lower half. The callee must consider the contents of the upper half of that register as undefined. See AAPCS64 (e.g. https://github.com/ARM-software/abi-aa/blob/master/aapcs64/aapcs64.rst#parameter-passing-rules ); AFAICS, the relevant bit is "Unlike in the 32-bit AAPCS, named integral values must be narrowed by the callee rather than the caller."
