4221 on new codegen
marlowsd at gmail.com
Thu Feb 3 10:05:04 CET 2011
I wonder if the fuel is also being used by "essential" transformations,
like the CPS pass?
On 03/02/2011 09:01, Simon Peyton-Jones wrote:
> Correct. The Cmm optimiser is supposed to make correctness preserving transformations. The idea of the "fuel" is that you can binary chop your way to a situation where
> Fuel = 0-143 Program works
> Fuel = 144 Program crashes
> Then look at the single transformation that introduces the crash.
> Well that's the intent anyway!
> | -----Original Message-----
> | From: ezyang [mailto:ezyang at MIT.EDU]
> | Sent: 02 February 2011 23:12
> | To: Simon Marlow; Simon Peyton-Jones
> | Cc: glasgow-haskell-users
> | Subject: Re: 4221 on new codegen
> | Simon Peyton Jones, I have a question about optimization fuel and GHC panics.
> | When I vary the fuel using -dopt-fuel, I get the following varying behavior:
> | ...
> | -dopt-fuel=144 = normal segfault (late in the program)
> | -dopt-fuel=143 = segfaults ~immediately
> | -dopt-fuel=142 = normal segfault
> | -dopt-fuel=141 = fails an assert in file compiler/cmm/CmmBuildInfoTables.hs,
> | line 128
> | -dopt-fuel=140 = ditto
> | -dopt-fuel=139 = resulting executable prints 'start' and then doesn't do
> | anything
> | ...
> | My impression was that these optimizations should not affect program
> | behavior,
> | in which case the first thing I should figure out is why -dopt-fuel results
> | in
> | the programming terminating after it prints 'start'. However, I'm not sure if
> | this is a red herring. Am I on the right track?
> | Cheers,
> | Edward
> | Quoting Simon Marlow<marlowsd at gmail.com>:
> |> 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
> |>> functions.
> |> 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.
> |> Cheers,
> |> Simon
> |>> Cheers,
> |>> Edward
> |>> 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:
> |>>>> c1jj:
> |>>>> _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
> |>>>> c1jh:
> |>>>> _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.
> |>>> Cheers,
> |>>> Simon
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