Re: RFC: Add 32bit x86-64 support to binutils

["H. Peter Anvin" <hpa at zytor dot com>]

On 12/31/2010 02:03 AM, Jakub Jelinek wrote:
> On Thu, Dec 30, 2010 at 01:42:05PM -0800, H. Peter Anvin wrote:
>> On 12/30/2010 11:57 AM, Jakub Jelinek wrote:
>>>>
>>>> Would be nice if LFS would be mandatory on the new ABI, thus
>>>> off_t being 64bits.
>>>
>>> And avoid ambiguous cases that x86-64 ABI has, e.g. whether
>>> caller or callee is responsible for sign/zero extension of arguments, to
>>> avoid the need to sign/zero extend twice, etc.
>>>
>>
>> Ehwhat?  x86-64 is completely unambiguous on that point; the i386 one is
>> not.
> 
> It is not, sadly, see http://gcc.gnu.org/PR46942
> From what I can see the psABI doesn't talk about it, GCC usually sign/zero
> extends on both sides (exception is 32-bit arguments into 64-bit isn't
> apparently sign/zero extended on the caller side when doing tail calls),
> from what I gathered LLVM expects the caller to sign/zero extend (which is
> incompatible with GCC tail calls then), not sure about ICC, and kernel
> probably expects for security reasons that the callee sign/zero extends.
> 

This is weird... we had long discussions about this when the psABI was
originally written, and the decision was that any bits outside the
fundamental type was undefined -- callee extends (caller in the case of
a return value.)  Yet somehow that (and several other discussions) seem
to either never have made it into the document or otherwise have
disappeared somewhere in the process.

There seems to have been problems with closing the loop on a number of
things, and in some cases the compiler writers have gone off and
implemented something completely different from the written document,
yet failed to get the documentation updated to match reality (it took
many years until the definition of _Bool matched what the compilers
actually implemented.)

	-hpa

-- 
H. Peter Anvin, Intel Open Source Technology Center
I work for Intel.  I don't speak on their behalf.