Building Enzyme CMake - Undefined symbol: main

[jpmedinagl]

Hello,

My group is trying to build Enzyme. We chose an older version of LLVM (https://github.com/llvm/llvm-project/releases/tag/llvmorg-17.0.6) because the newer versions were giving us problems.
We built LLVM using:

cmake -G Ninja ../llvm -DLLVM_TARGETS_TO_BUILD="host" -DLLVM_ENABLE_ASSERTIONS=ON -DLLVM_ENABLE_PROJECTS="clang;lld"  -DLLVM_ENABLE_PLUGINS=ON -DCMAKE_BUILD_TYPE=Release -DLLVM_BUILD_LLVM_DYLIB=ON  -DLLVM_ENABLE_TERMINFO=OFF -DBUILD_SHARED_LIBS=ON
ninja

Which was successful.

Then we built Enzyme using:

cmake -G Ninja .. -DLLVM_DIR=/path/to/llvm/lib/cmake/llvm
ninja

Which was successful.

To test Enzyme we're using the example test.c file from: https://enzyme.mit.edu/getting_started/UsingEnzyme/#differentiating-cc
We're using the same cmake file as: https://github.com/EnzymeAD/Enzyme/blob/main/enzyme/test/test_find_package/CMakeLists.txt.
We decided to remove the ClangEnzymeFlags as we wanted to focus on LLDEnzyme.

We used the command

cmake -DEnzyme_DIR=/u/jpmedina/Enzyme/enzyme/build

to build the project which was successful.

However, when running make we get the following errors.

found dir /u/jpmedina/Enzyme/enzyme/build
found 1
imported targets LLVMEnzyme-17;ClangEnzyme-17;ClangEnzymeFlags;LLDEnzyme-17;LLDEnzymeFlags;LLDEnzymeAssumeUnknownNoFree;LLDEnzymeLooseTypeFlags;LLDEnzymePrintTypeFlags;LLDEnzymePrintFlags;LLDEnzymeNoStrictAliasingFlags
-- Configuring done (0.0s)
-- Generating done (0.1s)
-- Build files have been written to: /u/jpmedina/test
[ 50%] Building C object CMakeFiles/enzyme_test_2.dir/main.c.o
[100%] Linking C executable enzyme_test_2
ld.lld: error: undefined symbol: main
>>> referenced by /usr/lib/gcc/x86_64-linux-gnu/9/../../../x86_64-linux-gnu/Scrt1.o:(_start)
collect2: error: ld returned 1 exit status
make[2]: *** [CMakeFiles/enzyme_test_2.dir/build.make:97: enzyme_test_2] Error 1
make[1]: *** [CMakeFiles/Makefile2:83: CMakeFiles/enzyme_test_2.dir/all] Error 2
make: *** [Makefile:91: all] Error 2

We believe that this is likely due to a cmake issue due to linking but we're not sure what this error means.

Any help is appreciated.

[wsmoses]

Hm that looks like the linker didn't find your main function for some reason. Mind sharing the whole code which fails?

[jpmedinagl]

We were originally building the test files outside of the Enzyme library. However we're currently trying inside the Enzyme/enzyme/test/test_find_package folder. Trying to build the enzyme_test_2 executable (we removed the enzyme_test_1 executable due to errors with Clang). We just want to test (and compile) the main.c source file to see if __enzyme_autodiff works.

In that folder, we tried building the executable with

cmake -DEnzyme_DIR=/u/jpmedina/Enzyme/enzyme/build .

This was successful

But then when we type make
we get the same error again

[ 50%] Building C object CMakeFiles/enzyme_test_2.dir/main.c.o
[100%] Linking C executable enzyme_test_2
ld.lld: error: undefined symbol: main
>>> referenced by /usr/lib/gcc/x86_64-linux-gnu/9/../../../x86_64-linux-gnu/Scrt1.o:(_start)
collect2: error: ld returned 1 exit status
make[2]: *** [CMakeFiles/enzyme_test_2.dir/build.make:97: enzyme_test_2] Error 1
make[1]: *** [CMakeFiles/Makefile2:83: CMakeFiles/enzyme_test_2.dir/all] Error 2
make: *** [Makefile:91: all] Error 2

For reference:
The code in /Enzyme/enzyme/test/test_find_package/CMakeLists.txt

cmake_minimum_required(VERSION 3.16)

project(enzyme_consumer LANGUAGES C)

find_package(Enzyme REQUIRED CONFIG NO_DEFAULT_PATH)
message("found dir ${Enzyme_DIR}")
message("found ${Enzyme_FOUND}")
get_property(importTargetsAfter DIRECTORY "${CMAKE_SOURCE_DIR}" PROPERTY IMPORTED_TARGETS)
message("imported targets ${importTargetsAfter}")

#add_subdirectory(${Enzyme_DIR} ${PROJECT_SOURCE_DIR}/build/enzyme)

#add_executable(enzyme_test_1 main.c)
#target_link_libraries(enzyme_test_1 PUBLIC ClangEnzymeFlags)

add_executable(enzyme_test_2 main.c)
target_compile_definitions(enzyme_test_2 PUBLIC -DNOVERSION)
target_link_libraries(enzyme_test_2 PUBLIC LLDEnzymeFlags)

The code for /Enzyme/enzyme/test/test_find_package/main.c

int printf(const char*, ...);

extern double __enzyme_autodiff(void*, double);

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

double dsquare(double x) {
  return __enzyme_autodiff((void*) square, x);
}

int main() {
  for(double i=1; i<5; i++) {
    printf("%f %f\n", square(i), dsquare(i));
  }
#ifndef NOVERSION
  printf("Enzyme version %d %d %d\n", ENZYME_VERSION_MAJOR, ENZYME_VERSION_MINOR, ENZYME_VERSION_PATCH);
#endif
}

[jpmedinagl]

@wsmoses

We realized that we were unintentionally using gcc instead of clang.

Would it be possible to recognize this and throw and error to the user?