Re: 4221 on new codegen

I wonder if the fuel is also being used by "essential" transformations, like the CPS pass? Cheers, Simon 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!

Simon

| -----Original Message----- | From: ezyang [mailto:ezyang@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: | |> 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 |> |> | |