Hi, Michael Matz wrote:
> On Wed, 9 Dec 2009, Andrew Haley wrote: > >>> The intent of H.J.'s proposal is to require bits <7:1> == 0 in all cases >>> (and higher bits as don't cares, the same way a char is passed), as >>> opposed to the current text which requires <63:1> == 0 when passed as >>> registers or on the stack (and <7:1> == 0 when stored in a memory >>> object.) Furthermore, the current psABI text is inconsistent for >>> arguments are return values; this is a bug in the wordsmithing of the >>> text rather than intentional, if I remember the original discussions >>> correctly. >> Surely Postel's Law applies: >> >> Be conservative in what you do; be liberal in what you accept from others. >> >> So, return values should be zero-extended to the full word, but we shouldn't >> assume that parameters will be. > > We also try to be efficient (meaning not doing booleanization at producer > _and_ consumer side), that's why we define an psABI at all. So, while > generally a good principle, it shouldn't be applied blindly :) Yeah, but maximizing efficiency, while generally a good principle, shouldn't be applied blindly either. :-) AIUI, the proposal on the table is to change the psABI in a way that removes a guarantee, i.e. in the opposite direction from the robustness principle. So, at least in theory, we might break things. And, I remember from a good deal of libffi hacking that I had to do last time, this is a nonzero risk. Andrew.
