retitle 13342 Clang, the FFI, and 8-bit signed integers
merge 13342 13386
thanks
Hello!
Mark H Weaver <m...@netris.org> skribis:
> l...@gnu.org (Ludovic Courtès) writes:
[...]
>> I looked at Section 3.2.3 (“Parameter Passing”) of the SysV ABI x86_64
>> PS but couldn’t find any evidence as to what the correct behavior is.
>
> I read the same section, and although it is not as clear as I'd prefer,
> my interpretation is that the caller is responsible for sign-extending
> signed chars to ints.
What exactly in that section makes you think so?
> Clang strictly requires callers to sign-extend, whereas GCC is tolerant
> of callers who fail to do so. IMO, both behaviors are permitted by the
> ABI.
>
> The problem is that libffi does *not* sign-extend arguments passed in
> registers when making calls, which is IMO a bug that has gone (mostly)
> unnoticed because of the tolerance and ubiquity of GCC.
Oh, I see.
>> However, on the caller side, both compilers emit the same code. This
>> program:
>>
>> #include <stdint.h>
>>
>> extern int64_t test_sum (int8_t a, int64_t b);
>>
>> int64_t
>> foo (void)
>> {
>> return test_sum (-1, 123132);
>> }
>>
>> leads to the following assembly with both compilers:
>>
>> foo: # @foo
>> .cfi_startproc
>> movl $-1, %edi
>> movl $123132, %esi # imm = 0x1E0FC
>> jmp test_sum # TAILCALL
>>
>> (And as we’ve seen, libffi does the same.)
>
> No, libffi does *not* do the same. Take a look at the relevant code:
>
> https://github.com/atgreen/libffi/blob/master/src/x86/ffi64.c#L488
Indeed!
[...]
> In summary, I think this is a bug in libffi.
>
> Note that it has already been reported that the libffi testsuite shows
> many failures on OS X Lion, and the failures appear to be related to
> this precise issue:
Note that it’s not just Darwin: I tested with Clang on GNU/Linux.
> http://sourceware.org/ml/libffi-discuss/2012/msg00162.html
>
> The libffi maintainer wrote "I'm going to chalk this up to compiler
> bugs", based on his observation that the tests worked properly when
> compiled with -O0. I think it's time to raise this issue again on the
> libffi-discuss mailing list.
Would you like to email them?
Anyway, thanks for the thorough investigation!
Ludo’.
