4221 on new codegen
marlowsd at gmail.com
Wed Feb 2 10:25:20 CET 2011
On 02/02/2011 00:29, Edward Z. Yang wrote:
> More Hoopling later, I see this segment in the rewrite function:
> middle m@(CmmUnsafeForeignCall _ fs _) live = return $
> case map spill (filter (flip elemRegSet (on_stack live)) fs) ++
> map reload (uniqSetToList (kill fs (in_regs live))) of
>  -> Nothing
> reloads -> Just $ mkMiddles (m : reloads)
> So, if I understand this code correctly, it unilaterally reloads
> /anything/ in the registers according to the analysis at that point.
> Well, I could see that resulting in the behavior below.
> It's not so clear to me what the correct rewrite is; according to
> Marlow's comment on IRC, we ought not to be spilling/reloading foreign
> calls yet, so maybe the whole bit should get excised? Otherwise, it seems
> to me that transfer function needs to accomodate unsafe foreign
Right, there's no need to spill/reload anything around an *unsafe*
foreign call in the Cmm code generator. The NCG's register allocator
will do any necessary spilling/reloading around foreign calls.
> Excerpts from Simon Marlow's message of Tue Feb 01 03:44:41 -0500 2011:
>> On 01/02/2011 00:01, Edward Z. Yang wrote:
>>> Current theory:
>>> _s1ep::I32 = I32[(slot<_s1ep::I32> + 4)]; // CmmAssign
>>> _s1fP::I32 = I32[(slot<_s1fP::I32> + 4)]; // CmmAssign
>>> // outOfLine should follow:
>>> _s1eq::F64 = F64[_s1fP::I32 + 3]; // CmmAssign
>>> I32[(young<c1jh> + 4)] = c1jh; // CmmStore
>>> foreign call "ccall" arg hints: [PtrHint,] result hints:  call_fn_blob(...) returns to c1jh args: ([_s1ep::I32,
>>> _s1eq::F64]) ress: ([_s1ev::F64]) with update frame 4; // CmmForeignCall
>>> _s1ev::F64 = F64[(slot<_s1ev::F64> + 8)]; // CmmAssign
>>> // emitReturn: Sequel: Assign
>>> _s1ev::F64 = _s1ev::F64; // CmmAssign
>>> F64[(slot<_s1ev::F64> + 8)] = _s1ev::F64; // CmmStore
>>> goto u1Ak; // CmmBranch
>>> Note the line immediately after c1jh, where we reload the ostensibly
>>> spilled _s1ev back into a register. Except that it was never spilled
>>> there in the first place, and we just clobbered the real value. Oops.
>>> Is this interpretation correct?
>> It sounds plausible, but I really have no idea. The code generator does
>> not have to generate spill/reloads around foreign calls, the register
>> allocator will do that.
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