On Wed, Dec 9, 2009 at 7:51 AM, Andrew Haley <a...@redhat.com> wrote: > H.J. Lu wrote: >> On Wed, Dec 9, 2009 at 7:10 AM, Andrew Haley <a...@redhat.com> wrote: >>> H. Peter Anvin wrote: >>>> On 12/09/2009 06:56 AM, Michael Matz wrote: >>>>>> Aren't bits in the _Bool byte of"bar" specified by the psABI >>>>> Right now they are specified in the psABI, you suggested to remove that >>>>> specification. >>>>> >>>> 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. >> >> I guess you missed the discussion around July 2007: >> >> http://gcc.gnu.org/bugzilla/show_bug.cgi?id=42324#c5 > > No, I didn't miss it. We have had discussions about this before. > I still think Postel's Law applies, particularly as we have an > explicit guarantee in the psABI that > > --- > When a value of type _Bool is passed in a register or on the stack, > the upper 63 bits of the eightbyte shall be zero. > ---
Please read it again. It is only for parameter passing, not for function return. Since gcc 4.3, we return _Bool in %AL. -- H.J.
