GHC FFI Return Type Bug

[Sigbjorn Finne]

Here's another data point on this topic - I asked around inside
of MS, and the consensus is that the contents of the unused portions
of EAX is undefined upon return (upper 3 bytes for byte-sized return
values, upper 2 for 16-bit values). This is invariant of x86 calling
convention on Win32 (cdecl,stdcall,fastcall). Still no known doc/spec
which spells this out.

==> GHC's NCG and C backends need to know the sizes of the
unboxed Ints and Words involved in CCalls to generate correct
code.

--sigbjorn

----- Original Message -----
From: "Sigbjorn Finne" <sof@galconn.com>
To: "Julian Seward (Intl Vendor)" <v-julsew@microsoft.com>
Cc: <partain@dcs.gla.ac.uk>; "Ashley Yakeley" <ashley@semantic.org>; "GHC
List" <glasgow-haskell-users@haskell.org>
Sent: Tuesday, August 07, 2001 16:00
Subject: Re: GHC FFI Return Type Bug


>
> "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.
>
>
>
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