Compiling simple parallel function gives inconsistent results and errors

Question

Im trying to compile the following function:

n := 5

fc = Compile[{{l,_Integer,1}},
    a=1;b=1;
    Do[
        If[l[[i]]==1,
            a=a+2b; b=b+i,
            b=a+b;  a=a+i],
        {i,n}];
    b,
    RuntimeAttributes -> {Listable},
    Parallelization -> True]

fc[Tuples[{0,1},n]]

However this gives the following errors:

CompiledFunction::pext: Instruction 1 in CompiledFunction[{10,11.2,5852},{{_Integer,1}},{{2,1,0},{2,0,1}},{{0,{2,0,2}},<<1>>,{1,{<<1>>}}},<<1>>,{{46,Function[{l},a=1],2,1,0,6,0,17},{46,Function[{l},b=1],2,1,0,6,0,17},{46,Function[{l},n],2,1,0,3,0,2},{40,50,3,0,2,2,0,1},<<10>>,{4,3,1,-8},{46,Function[{l},b],2,1,0,2,0,1},{1}},Function[{l},a=1;b=1;Do[If[l[[i]]==1,a=Plus[<<2>>];b=Plus[<<2>>],b=Plus[<<2>>];a=Plus[<<2>>]],{i,n}];b,Listable],Evaluate] calls ordinary code that can be evaluated on only one thread at a time.

and

CompiledFunction::cflist: Nontensor object generated; proceeding with uncompiled evaluation.

Additionally, the result is often wrong and inconsistent. It works correctly when Parallelization is turned off. Am I doing something wrong here or is there just something wrong with my setup?

Carl Woll · Accepted Answer · 2019-10-30 01:18:38Z

If you CompilePrint your compiled function, you'll see lots of calls to MainEvaluate:

Needs["CompiledFunctionTools`"]
CompilePrint[fc]

    1 argument
    1 Boolean register
    11 Integer registers
    1 Tensor register
    Underflow checking off
    Overflow checking off
    Integer overflow checking on
    RuntimeAttributes -> {Listable}

    T(I1)0 = A1
    I5 = 0
    I1 = 5
    I8 = 2
    I0 = 1
    Result = I2

 1    V17 = MainEvaluate[ Function[{l}, a = 1][ T(I1)0]]
 2    V17 = MainEvaluate[ Function[{l}, b = 1][ T(I1)0]]
 3    I4 = MainEvaluate[ Function[{l}, n][ T(I1)0]]
 4    I6 = I5
 5    goto 14
 6    I7 = Part[ T(I1)0, I6]
 7    B0 = I7 == I0
 8    if[ !B0] goto 12
 9    V17I6 = MainEvaluate[ Function[{l, iCompile$9}, Block[{i = iCompile$9}, {a = a + 2 b, i}]][ T(I1)0, I6]]
 10   V17I6 = MainEvaluate[ Function[{l, iCompile$10}, Block[{i = iCompile$10}, {b = b + i, i}]][ T(I1)0, I6]]
 11   goto 14
 12   V17I6 = MainEvaluate[ Function[{l, iCompile$11}, Block[{i = iCompile$11}, {b = a + b, i}]][ T(I1)0, I6]]
 13   V17I6 = MainEvaluate[ Function[{l, iCompile$12}, Block[{i = iCompile$12}, {a = a + i, i}]][ T(I1)0, I6]]
 14   if[ ++ I6 <= I4] goto 6
 15   I2 = MainEvaluate[ Function[{l}, b][ T(I1)0]]
 16   Return

To avoid these calls to MainEvaluate, you should modularize the variables a and b. Also, the global variable n needs to have a value. So:

Clear[fc]
fc[n_] := Compile[{{l,_Integer,1}},
    Module[{a=1, b=1},
        Do[
            If[l[[i]]==1,
                a = a + 2b; b = b + i,
                b = a + b; a = a + i
            ],
            {i,n}
        ];
        b
    ],
    RuntimeAttributes->{Listable},
    Parallelization->True
];

Now, fc[n] has no calls to MainEvaluate:

n := 5;
CompilePrint[fc[n]]

    1 argument
    1 Boolean register
    10 Integer registers
    1 Tensor register
    Underflow checking off
    Overflow checking off
    Integer overflow checking on
    RuntimeAttributes -> {Listable}

    T(I1)0 = A1
    I3 = 0
    I1 = 5
    I6 = 2
    I0 = 1
    Result = I4

1   I2 = I0
2   I4 = I0
3   I7 = I1
4   I5 = I3
5   goto 19
6   I8 = Part[ T(I1)0, I5]
7   B0 = I8 == I0
8   if[ !B0] goto 15
9   I8 = I6 * I4
10  I9 = I2 + I8
11  I2 = I9
12  I9 = I4 + I5
13  I4 = I9
14  goto 19
15  I9 = I2 + I4
16  I4 = I9
17  I9 = I2 + I5
18  I2 = I9
19  if[ ++ I5 <= I7] goto 6
20  Return

Check:

fc[n][Tuples[{0,1},n]]

{26, 20, 35, 17, 35, 24, 37, 16, 32, 24, 43, 19, 39, 26, 39, 16, 30, 23, 42, 19, 42, 28, 43, 17, 35, 26, 47, 20, 41, 27, 40, 16}

Thanks! I didn't know about CompilePrint, that's very useful. — Wessel de Weijer
– Wessel de Weijer, Commented Oct 31, 2019 at 15:55
I have a question related to this: what if I want to use custom compiled functions inside fc? I have the same error message which is apparently related to the calls to MainEvaluate when I call the custom functions. — Matteo
– Matteo, Commented Nov 9, 2022 at 19:27

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