Mark H Weaver - ABI incompatibility between libffi and LLVM-generated callees (original) (raw)

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• From: Mark H Weaver
• To: libffi-discuss at sourceware dot org
• Date: Wed, 30 Jan 2013 02:30:05 -0500
• Subject: ABI incompatibility between libffi and LLVM-generated callees

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. 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?

Regards,
  Mark

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    * From: Andrew Haley