New codegen failing test-cases

[Edward Z. Yang]

With further poking, I think the new codegen is actually tickling
an existing bug in the native code generator optimizations, since
the cmmz output looks ok:

      cSH:
          _sQR::I32 = I32[_sRi::I32 + 3];   // CmmAssign
          _sQS::I32 = I32[_sRi::I32 + 7];   // CmmAssign
          _sQT::I32 = I32[_sRi::I32 + 11];   // CmmAssign
          _sQU::I32 = I32[_sRi::I32 + 15];   // CmmAssign
          (_sR1::I32) = call "ccall" arg hints:  [`signed',
                                                  PtrHint,]  result hints:  [] (_sQR::I32 & (-4))(_sQS::I32, _sQT::I32, _sQU::I32);   // CmmUnsafeForeignCall

And the only change is that in the original code generator,
the assignment to _sQR is elided.

         _cSn::I32 = I32[R1 + 7];
         _cSp::I32 = I32[R1 + 11];
         _cSr::I32 = I32[R1 + 15];
         (_sR1::I32,) = foreign "ccall"
           I32[R1 + 3]((_cSn::I32, `signed'), (_cSp::I32, PtrHint),
                       (_cSr::I32,))[_unsafe_call_];

I further verified that there was no problem if I used -fvia-C.
On closer inspection, the fact that _sQR is referenced nowhere
in this dump should have raised alarms (I think the register
allocater happened to assign it to the same register as
_sRi, which is why the assembly looked vaguely plausible.)

I'm still not sure where a fix might lie, but if I take another
crack at it tomorrow I will probably figure it out.

Cheers,
Edward

Excerpts from Edward Z. Yang's message of Wed Jan 12 17:10:11 -0500 2011:
> I appear to have tracked down the bug for ffi021:  the new
> code generator doesn't appear to clear the tag bit for the
> pointer to heap before:
> 
>         // outOfLine should follow:
>         (_sR1::I32,) = foreign "ccall"
>           _sQR::I32((I32[_sRi::I32 + 7], `signed'),
>                     (I32[_sRi::I32 + 11], PtrHint),
>                     (I32[_sRi::I32 + 15],))[_unsafe_call_];
>         // emitReturn: Sequel: Assign
>         ;
> 
> (gdb) disas
> Dump of assembler code for function sRi_info:
> => 0x0804aa6c <+0>:     mov    %esi,%eax
>    0x0804aa6e <+2>:     lea    0x0(%ebp),%ecx
>    0x0804aa71 <+5>:     cmp    0x54(%ebx),%ecx
>    0x0804aa74 <+8>:     jb     0x804aab3 <sRi_info+71>
>    0x0804aa76 <+10>:    add    $0x4,%ebp
>    0x0804aa79 <+13>:    add    $0x8,%edi
>    0x0804aa7c <+16>:    cmp    0x5c(%ebx),%edi
>    0x0804aa7f <+19>:    ja     0x804aaa4 <sRi_info+56>
>    0x0804aa81 <+21>:    pushl  0xf(%eax)
>    0x0804aa84 <+24>:    pushl  0xb(%eax)
>    0x0804aa87 <+27>:    pushl  0x7(%eax)
>    0x0804aa8a <+30>:    call   *%eax
> 
> The pushes to the stack properly untag eax, but then we just
> call the tagged pointer, which seems pretty wrong to me. Here is
> the old C--:
> 
>         (_sR1::I32,) = foreign "ccall"
>           I32[R1 + 3]((_cSc::I32, `signed'), (_cSe::I32, PtrHint),
>                       (_cSg::I32,))[_unsafe_call_];
> 
> Unfortunately, I can't figure out where this +3 is supposed to
> be happening, so I don't have a patch. Some guidance here would
> be appreciated.
> 
> Cheers,
> Edward