[lldb] Let LLDB depend on intrinsics_gen to fix non-deterministic module build failures

[teemperor]

Summary

When building with modules we currently fail randomly to build LLDB's modules and get error messages like below:

In file included from <module-includes>:1:                                                                                                                                                    
In file included from llvm-project/llvm/include/llvm/IR/Argument.h:18:                                                                                                 
llvm-project/llvm/include/llvm/IR/Attributes.h:74:14: fatal error: 'llvm/IR/Attributes.inc' file not found                                                             
    #include "llvm/IR/Attributes.inc"                                                                                                                                                         
             ^~~~~~~~~~~~~~~~~~~~~~~~  
...
In file included from llvm-project/lldb/source/Plugins/ABI/SysV-arm/ABISysV_arm.cpp:9:                                                                                 
llvm-project/lldb/source/Plugins/ABI/SysV-arm/ABISysV_arm.h:12:10: fatal error: could not build module 'lldb_Wrapper'

The reason for this is that our source files can include Clang headers which in turn include files like Attributes.inc which
are generated by intrinsics_gen. However, it seems Clang can't express this dependency in its CMake code, so intrinsics_gen
is actually not guaranteed to be built before we start parsing LLDB source files and Clang headers. Clang worked around this
by letting all libraries depend on intrinsics_gen, which at least lets their builds pass.

As I haven't figured out how to solve these dependencies properly and I want to get the LLDB+Modules build green,
I suggest we copy Clang's hack until we figure out how to get Clang's dependencies right.

[JDevlieghere]

LGTM.

Out of curiosity, do you know why we don't see this on the CMake bot on GreenDragon, which is a modules build? Is this so infrequent that I might have missed it?

http://green.lab.llvm.org/green/view/LLDB/job/lldb-cmake/

[teemperor]

In D66208#1629378, @JDevlieghere wrote:

LGTM.

Out of curiosity, do you know why we don't see this on the CMake bot on GreenDragon, which is a modules build? Is this so infrequent that I might have missed it?

http://green.lab.llvm.org/green/view/LLDB/job/lldb-cmake/

Do the bots use a fresh build folder? Clang could find the generated headers from a previous build or there are headers with the same name in the possible include paths (even tough they are usually not included as the header that is generated would be before these other headers in the include path). That would be my explanation.

[JDevlieghere]

In D66208#1630327, @teemperor wrote:

In D66208#1629378, @JDevlieghere wrote:

LGTM.

Out of curiosity, do you know why we don't see this on the CMake bot on GreenDragon, which is a modules build? Is this so infrequent that I might have missed it?

http://green.lab.llvm.org/green/view/LLDB/job/lldb-cmake/

Do the bots use a fresh build folder? Clang could find the generated headers from a previous build or there are headers with the same name in the possible include paths (even tough they are usually not included as the header that is generated would be before these other headers in the include path). That would be my explanation.

The bot is incremental but the workspace is cleaned once in a while. The combination of both things happening only occasionally makes sense.

lldb/trunk/CMakeLists.txt

 if (WIN32)
   add_definitions(-D_ENABLE_EXTENDED_ALIGNED_STORAGE)
 endif()
 
 if (NOT LLDB_DISABLE_PYTHON)
   add_subdirectory(scripts)
 endif ()
 
+# We need the headers generated by instrinsics_gen before we can compile
+# any source file in LLDB as the imported Clang modules might include
+# some of these generated headers. This approach is copied from Clang's main
+# CMakeLists.txt, so it should kept in sync the code in Clang which was added
+# in llvm-svn 308844.
+if(LLVM_ENABLE_MODULES AND NOT LLDB_BUILT_STANDALONE)
+  list(APPEND LLVM_COMMON_DEPENDS intrinsics_gen)
+endif()
+
 if(CMAKE_CROSSCOMPILING AND LLDB_BUILT_STANDALONE)
   set(LLVM_USE_HOST_TOOLS ON)
   include(CrossCompile)
   if (NOT NATIVE_LLVM_DIR OR NOT NATIVE_Clang_DIR)
     message(FATAL_ERROR
       "Crosscompiling standalone requires the variables NATIVE_{CLANG,LLVM}_DIR
       for building the native lldb-tblgen used during the build process.")
   endif()