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