GHC FFI Return Type Bug

[Sigbjorn Finne]

"Julian Seward (Intl Vendor)" <v-julsew@microsoft.com> writes:
> 
> | > char fooble ( ... )
> | > {
> | >    return 'z';
> | > }
> | > 
> | > on an x86, 'z' will be returned at the lowest 8 bits in %eax. What I
> 
> | > don't know is, is the C compiler obliged to clear the upper 24 bits
> of 
> | > %eax, or does that onus fall on the callee?
> | 
> | Yes, the callee is required to narrow <expr> in 'return 
> | <expr>;' to fit that of the specified return type -- see 9.8 
> | of Harbison and Steele.
> | 
...
> So we don't need to worry about doing the masking on the 
> Haskell side, right?
> 

Yes, that's been my understanding. But, as Carl Witty rightly points
out, a distinction can be made between narrowing down the result
value and the transmission of the narrowed result back to the caller.

Carl's example doesn't fully convince me though, since the masking
done in the code for "g" is patently redundant ("f" does mask out
the upper 3 bytes of %eax upon return). However, the example given
in the original bug report can be made to emit code that doesn't
mask %eax  (via "gcc-2.96 -O" on an i686-pc-linux). Another
data point is that MSVC also emits code that masks.

So, indications are that you actually do need to mask on an x86, but
I've got no supporting documentation. If anyone is able to dig up
more info and/or know someone that could answer this one decisively,
I'd interested to hear of their findings.

--sigbjorn

btw, the document specifying the x86 ABI, located via

    http://uwsg.iu.edu/hypermail/linux/kernel/0011.1/0596.html

, doesn't cover this as far as I've been able to tell.