[llvm-dev] ORC JIT Weekly #19 -- Relocatable object level mocking with llvm-jitlink.

Stefan Gränitz via llvm-dev llvm-dev at lists.llvm.org
Sun Aug 16 13:32:09 PDT 2020

Hi Lang

Thanks for explaining the new mocking and testing feature in the
llvm-jitlink tool in such great detail! Indeed, it sounds like an
interesting by-product of your jitlink efforts.

"a lot of non-trivial surface area" is a very polite description of the
C++ symbols disaster :)


On 10/08/2020 08:26, Lang Hames wrote:
> Hi All,
> There was no update last week -- I'm still trying to get back into a
> regular schedule.
> Open-source changes since the last update were:
> (1) Some bug fixes for JITLink MachO / arm64 support
> (PAGE21/PAGEOFF12 now handle addends correctly).
> (2) llvm-jitlink now supports loading archives as well as relocatable
> objects.
> (3) llvm-jitlink now supports basic object-file level mocking and testing.
> That last one is unusual so I want to expand on it. The llvm-jitlink
> tool acquired a couple of new options to support this feature:
> "--phony-externals" which resolves all otherwise unresolved symbols to
> null instead of erroring, and "-harness" which allows a set of objects
> to be designated as a test harness, with the regular object files
> implicitly treated as objects to be tested. If the -harness option is
> passed llvm-jitlink installs a JITLink plugin to make some
> modifications to the usual linker rules:
> -- All symbols referenced by the harness object(s) are promoted to
> default visibility so that the harness can directly access static
> symbols inside the test object(s)
> -- All symbols defined by the harness object(s) override (rather than
> clashing with) symbols defined in the test object(s).
> With these modifications in place we can selectively test functions in
> an object file by mocking their callees. For example, suppose we have
> an object file, test_code.o, compiled from the following C source
> (which we do not have access to):
> void irrelevant_function() { irrelevant_external(); }
> int function_to_mock(int X) {
>   return /* some function of X */;
> }
> static void function_to_test() {
>   ...
>   int Y = function_to_mock();
>   printf("Y is %i\n", Y);
> }
> We would like to know how function_to_test behaves when we change the
> behavior of function_to_mock. We can do this by writing a test harness:
> void function_to_test();
> int function_to_mock(int X) {
>   printf("used mock utility function\n");
>   return 42;
> }
> int main(int argc, char *argv[]) {
>   function_to_test():
>   return 0; 
> }
> This harness code could not be linked against test_code.o under normal
> circumstances: function_to_test is static and will fail to resolve
> from the harness code, function_to_mock is a duplicate definition, and
> irrelevant_external is undefined. Our new llvm-jitlink options take
> care of these issues however, and running the test looks like:
> % clang -c -o test_code_harness.o test_code_harness.c
> % llvm-jitlink -phony-externals test_code.o -harness test_code_harness.o
> used mock utility function
> Y is 42
> This utility may be of interest to people who want to perform some
> very late testing on build products to verify that compiled code
> behaves as expected. On basic C test cases it works reasonably well.
> C++ test cases are another matter: Any code involving classes tends to
> have a lot of non-trivial surface area (e.g. vtables) that would
> require great care to mock. Perhaps we need a new tool to tackle that
> problem.
> In the meantime, if nothing else this provides a neat example of how
> JITLink plugins can be used to modify the default linker behavior.
> Regards,
> Lang. 


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