crash on trivial program

[samuelpmish]

see: https://fwd.gymni.ch/qW2CKI

duplicated here for completeness:

#include <stdio.h>

int enzyme_dup;
int enzyme_out;
int enzyme_const;

template < typename return_type, typename ... T >
extern return_type __enzyme_fwddiff(void*, T ... );

double square(double x) { return x * x; }

template < typename function, typename T > 
auto jvp(function f, T arg, T darg) {
  using output_type = decltype(f(arg));
  return __enzyme_fwddiff<output_type>((void*)f, enzyme_dup, arg, darg); 
}

int main() {

    // calling fwddiff directly seems to work
    auto output1 = __enzyme_fwddiff<double>((void*)square, enzyme_dup, 1.0, 1.0);
    printf("%f\n", output1);

    // calling fwddiff with the same args in a function template crashes
    auto output2 = jvp(square, 1.0, 1.0);
    printf("%f\n", output2);

}

[wsmoses]

Added a PR to make a reasonable error message.

Regardless the issue here is that the function pointer isn't a constant by differentiation time. Increasing the optimization level remedies. Adding static can also help fix (perhaps at a lower opt level): You could also pass the function as a template param perhaps?

wmoses@beast:~/git/Enzyme/enzyme/build12 (enofn) $ /home/wmoses/llvms/llvm12/buildD/./bin/clang -g -O0 /home/wmoses/git/Enzyme/enzyme/test/Integration/ForwardMode/nofn.cpp -S -emit-llvm -o -   -Xclang -load -Xclang /home/wmoses/git/Enzyme/enzyme/build12/Enzyme/ClangEnzyme-12.so -o /dev/null
/home/wmoses/git/Enzyme/enzyme/test/Integration/ForwardMode/nofn.cpp:19:10: error: Enzyme: failed to find fn to differentiate  %call = call double @_Z16__enzyme_fwddiffIdJiddEET_PvDpT0_(i8* %1, i32 %2, double %3, double %4), !dbg !616 - found -   %1 = bitcast double (double)* %0 to i8*, !dbg !612
  return __enzyme_fwddiff<output_type>((void*)f, enzyme_dup, arg, darg); // expected-error {{Enzyme: failed to find fn to differentiate}}
         ^
1 error generated.
wmoses@beast:~/git/Enzyme/enzyme/build12 (enofn) $ /home/wmoses/llvms/llvm12/buildD/./bin/clang -g -O0 /home/wmoses/git/Enzyme/enzyme/test/Integration/ForwardMode/nofn.cpp -S -emit-llvm -o -   -Xclang -load -Xclang /home/wmoses/git/Enzyme/enzyme/build12/Enzyme/ClangEnzyme-12.so -o /dev/null -O1
/home/wmoses/git/Enzyme/enzyme/test/Integration/ForwardMode/nofn.cpp:19:10: error: Enzyme: failed to find fn to differentiate  %call = call double @_Z16__enzyme_fwddiffIdJiddEET_PvDpT0_(i8* %0, i32 %1, double %arg, double %darg), !dbg !618 - found -   %0 = bitcast double (double)* %f to i8*, !dbg !612
  return __enzyme_fwddiff<output_type>((void*)f, enzyme_dup, arg, darg); // expected-error {{Enzyme: failed to find fn to differentiate}}
         ^
1 error generated.
wmoses@beast:~/git/Enzyme/enzyme/build12 (enofn) $ /home/wmoses/llvms/llvm12/buildD/./bin/clang -g -O0 /home/wmoses/git/Enzyme/enzyme/test/Integration/ForwardMode/nofn.cpp -S -emit-llvm -o -   -Xclang -load -Xclang /home/wmoses/git/Enzyme/enzyme/build12/Enzyme/ClangEnzyme-12.so -o /dev/null -O2

[samuelpmish]

for reference, @wsmoses suggestion of moving the function to a template argument seems like a reasonable workaround:

#include <stdio.h>

int enzyme_dup;
int enzyme_out;
int enzyme_const;

template < typename return_type, typename ... T >
extern return_type __enzyme_fwddiff(void*, T ... );

double square(double x) { return x * x; }

template < auto function, typename T > 
auto jvp(T arg, T darg) {
  using output_type = decltype(function(arg));
  return __enzyme_fwddiff<output_type>((void*)function, enzyme_dup, arg, darg); 
}

int main() {

    // calling fwddiff directly seems to work
    auto output1 = __enzyme_fwddiff<double>((void*)square, enzyme_dup, 1.0, 1.0);
    printf("%f\n", output1);

    // calling fwddiff with the function as a template argument doesn't crash any more
    auto output2 = jvp<square>(1.0, 1.0);
    printf("%f\n", output2);

}

note: C++17 or later is required to use the auto template parameter

https://fwd.gymni.ch/q3DHL9