System V Application Binary Interface 0.99.5

[Andrew Haley]

Hi,

On 01/30/2013 02:18 PM, Michael Matz wrote:

> On Wed, 30 Jan 2013, Andrew Haley wrote:
> 
>> I'm looking at Section 3.2.3, Parameter Passing.
>> http://artfiles.org/kernel.org/pub/scm/devel/binutils/hjl/x86-64-psabi.git/
>>
>> I still cannot tell whether parameters should or should not be sign- or
>> zero-extended when they are moved into registers at a call.  I'm guessing
>> not.
> 
> It's intentionally unspecified.

Aha!  It would have been nice if the psABI said so explicitly.  Quite
a few people have spent time trying to find this information.

>> Which is it?  This is important for interworking.
> 
> How?  You aren't allowed to access the bits outside the specified argument 
> type (which must match on caller and callee side), so you can't observe 
> them, so it's not required to specify their content.

OK, thanks.  It's clear now.

The problem is that LLVM assumes that values are extended at a call.  GCC
does that, but libffi doesn't.  So, calls via libffi to LLVM don't work
correctly.

Andrew.