Add option LLVM_NM to allow specifying the location of the llvm-nm tool.

[arlosi]

Summary

The new option works like the existing LLVM_TABLEGEN, and LLVM_CONFIG_PATH options.
Instead of building llvm-nm, the build uses the executable defined by LLVM_NM.

This is useful for cross-compilation scenarios where the host cannot run the cross-compiled
tool, and recursing into another cmake build is not an option (due to required DEFINE's, for example).

[smeenai]

LGTM

llvm/tools/llvm-shlib/CMakeLists.txt

167

I guess this works and is a no-op, but setting this variable to empty (or not setting it at all) should also work, I believe, and perhaps expresses the intent clearer (that there's no target for llvm-nm and you're just supplying a prebuilt one).

[arlosi]

Set llvm_nm_target to empty string as requested by smeenai.

[arlosi]

I do not have commit access. Could someone land this for me?

[smeenai]

Sure, I'll land it for you.

llvm/tools/llvm-shlib/CMakeLists.txt

else()
     # Write out the full lib names into file to be read by the python script.
     file(WRITE ${LIBSFILE} "${FILE_CONTENT}")
   endif()
 
   # Generate the exports file dynamically.
   set(GEN_SCRIPT ${CMAKE_CURRENT_SOURCE_DIR}/gen-msvc-exports.py)
 
   set(LLVM_EXPORTED_SYMBOL_FILE ${CMAKE_BINARY_DIR}/${CMAKE_CFG_INTDIR}/libllvm-c.exports)
-
+  if(NOT LLVM_NM)
     if(CMAKE_CROSSCOMPILING)
       build_native_tool(llvm-nm llvm_nm)
       set(llvm_nm_target "${llvm_nm}")
     else()
       set(llvm_nm $<TARGET_FILE:llvm-nm>)
       set(llvm_nm_target llvm-nm)
     endif()
+  else()
+    set(llvm_nm ${LLVM_NM})
+    set(llvm_nm_target "")
+  endif()
 
   add_custom_command(OUTPUT ${LLVM_EXPORTED_SYMBOL_FILE}
     COMMAND "${Python3_EXECUTABLE}" ${GEN_SCRIPT} --libsfile ${LIBSFILE} ${GEN_UNDERSCORE} --nm "${llvm_nm}" -o ${LLVM_EXPORTED_SYMBOL_FILE}
     DEPENDS ${LIB_NAMES} ${llvm_nm_target}
     COMMENT "Generating export list for LLVM-C"
     VERBATIM )
 
   # Finally link the target.
   add_llvm_library(LLVM-C SHARED INSTALL_WITH_TOOLCHAIN ${SOURCES} DEPENDS intrinsics_gen)
 
 endif()