On Fri, Oct 23, 2020 at 09:28:59PM +0000, David Laight wrote: > From: Segher Boessenkool > > Sent: 23 October 2020 19:27 > > On Fri, Oct 23, 2020 at 06:58:57PM +0100, Al Viro wrote: > > > On Fri, Oct 23, 2020 at 03:09:30PM +0200, David Hildenbrand wrote: > > > 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." > > > > Or the formal rule: > > > > C.9 If the argument is an Integral or Pointer Type, the size of the > > argument is less than or equal to 8 bytes and the NGRN is less > > than 8, the argument is copied to the least significant bits in > > x[NGRN]. The NGRN is incremented by one. The argument has now > > been allocated. > > So, in essence, if the value is in a 64bit register the calling > code is independent of the actual type of the formal parameter. > Clearly a value might need explicit widening. No, this says that if you pass a 32-bit integer in a 64-bit register, then the top 32 bits of that register hold an undefined value. > I've found a copy of the 64 bit arm instruction set. > Unfortunately it is alpha sorted and repetitive so shows none > of the symmetry and makes things difficult to find. All of this is ABI, not ISA. Look at the AAPCS64 pointed to above. > But, contrary to what someone suggested most register writes > (eg from arithmetic) seem to zero/extend the high bits. Everything that writes a "w" does, yes. But that has nothing to do with the parameter passing rules, that is ABI. It just means that very often a 32-bit integer will be passed zero-extended in a 64-bit register, but that is just luck (or not, it makes finding bugs harder ;-) ) Segher
