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compile - Catching and debugging numerical errors in compiled functions


I have a compiled function (that was created using the second method of this question). Basically it's using a compiled closure. The problem is that for some parameter values it does not evaluate. Here is a small example:


(*just some function that has a singularity at x=5*)
f = Compile[{{x, _Real, 0}, {a, _Real, 0}}, Sin[a*x*Pi/180]/(x - 5)
, RuntimeOptions -> {"Speed", "EvaluateSymbolically" -> False}];


(*The wrapper just increments the x values by diff. Aa is the
variable that is supposed to be inlined in the closure*)
Block[{Aa},
With[{wrapper =
Compile[{{x, _Real, 0}, {diff, _Real, 0}},
Evaluate@f[x + diff, Aa]]},
min = Compile[{{xs, _Real, 1}, {Aa, _Real, 0}, {diff, _Real, 0}},
First@Sort[wrapper[#, diff] & /@ xs]
, CompilationOptions -> {"InlineCompiledFunctions" -> True,

"InlineExternalDefinitions" -> True,
"ExpressionOptimization" -> True},
RuntimeOptions -> {"Speed"}](*compile*)
]
];

(*The warning is harmeless*)
(*CompiledFunction::cfsa:Argument diff+x at position 1 should be a machine-size real number. >>*)

(*The function is compiled*)

Needs["CompiledFunctionTools`"]
CompilePrint[min]


3 arguments
9 Integer registers
11 Real registers
3 Tensor registers
Underflow checking off
Overflow checking off

Integer overflow checking off
RuntimeAttributes -> {}

T(R1)0 = A1
R0 = A2
R1 = A3
I7 = 0
I6 = -5
I3 = 1
R6 = 3.141592653589793

I5 = 180
Result = R7

1 I2 = Length[ T(R1)0]
2 I8 = I7
3 T(R1)1 = Table[ I2]
4 I4 = I7
5 goto 20
6 R3 = GetElement[ T(R1)0, I4]
7 R2 = R1

8 R5 = R2 + R3
9 R4 = R0
10 R8 = I5
11 R9 = Reciprocal[ R8]
12 R8 = R6 * R9
13 R9 = R4 * R5 * R8
14 R8 = Sin[ R9]
15 R9 = I6
16 R10 = R5 + R9
17 R9 = Reciprocal[ R10]

18 R8 = R8 * R9
19 Element[ T(R1)1, I8] = R8
20 if[ ++ I4 < I2] goto 6
21 T(R1)2 = Sort[ T(R1)1]]
22 R7 = Part[ T(R1)2, I3]
23 Return

But then here are the use cases:


(*everything works OK even though the singularity is in the arguments*)
min[{1,5,5},1,0]

(*=> -0.0043631*)

But for some arguments it does not:


(*But if all the arguments are singularites: BLAM!*)
min[{5,5,5},1,0]

(*=> CompiledFunction[{x,a},Sin[(a x \[Pi])/180]/(x-5),-CompiledCode-][5,Aa] *)


   CompiledFunction::cfne: Numerical error encountered; proceeding with uncompiled evaluation. >>

CompiledFunction::cfse: Compiled expression Aa should be a machine-size real number. >>
CompiledFunction::cfex: Could not complete external evaluation at instruction 2; proceeding with uncompiled evaluation. >>

Questions:



  • Why is this working if some of the arguments give division by zero, but does not evaluate if all of the arguments are incorrect?

  • How can one debug such problems? There is nothing obviously wrong at instruction 2 in the compiled output...

  • How to guard against and/or catch such errors in compiled code?




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