Re: ABI incompatibility between libffi and LLVM-generated callees

[Andrew Haley <aph at redhat dot com>]

On 01/30/2013 07:30 AM, Mark H Weaver wrote:
> Hello all,
> 
> I'm a developer of Guile 2.0, which uses libffi, and we've received
> multiple bug reports of test failures on OS X related to libffi.  We
> recently discovered the root cause of these failures.
> 
> Functions compiled using LLVM for the x86_64 architecture assume that
> callers will sign-extend integer arguments less than 64-bits that are
> passed in registers.  Indeed, both GCC and LLVM generate code that does
> this sign-extension in callers, so it seems that their assumption is
> reasonable.
> 
> The only problem is that libffi does *not* sign-extend integer arguments
> passed in registers on x86_64.  Instead it zero-extends them, even if
> they are signed.  This works when calling GCC-compiled code, because GCC
> performs the sign-extension in the callee as well as the caller, but
> LLVM-compiled code is more strict.
> 
> Having read section 3.2.3 ("Parameter Passing") of the SysV x86_64 ABI,
> which is admittedly somewhat vague on this issue, it is far from clear
> to me that the LLVM behavior is a bug.

Isn't it?  LLVM is supposed to be adding an int8_t and an int64_t, but
it adds two int64_ts.

I suspect that it is indeed an LLVM bug, but I've asked the psABI
authors to comment.

> It seems to me that callers
> should sign-extend to be on the safe side.
> 
> I believe this is a case of GCC following Postel's Law of being
> permissive in what one accepts, whereas LLVM is more strict in its
> requirements.
> 
> However you might choose to interpret the ABI requirements, the fact is
> that LLVM-compiled libraries are widely deployed in Mac OS X, and
> libffi is currently unable to properly call such libraries properly.
> 
> For more details on this, see:
> 
>   http://debbugs.gnu.org/cgi/bugreport.cgi?bug=13342#26
>   http://debbugs.gnu.org/cgi/bugreport.cgi?bug=13342#32
> 
> Comment #26 shows the difference in assembly code generated by GCC and
> LLVM for functions that accept int8_t arguments.
> 
> Also see the recent report of libffi test failures on OS X:
> 
>   http://sourceware.org/ml/libffi-discuss/2012/msg00162.html
> 
> I suggest that libffi should be changed to sign-extend.
> What do you think?

I don't suppose it would hurt.

Andrew.