Re: Bug in x86-64 psABI or in gcc?

[Michael Matz <matz at suse dot de>]

Hi,

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 :)


Ciao,
Michael.