[libc] Add an option to make `libc` only build the `libc-hdrgen` tool

[jhuber6]

Summary

The libc-hdergen tool is required for cross-builds, however some cases
can cause issues when configuring this build. This patch adds an
ovveride option LIBC_HDRGEN_ONLY to allow us to retain the old
(incorrect) behaviour where libc would not build with any other
runtimes enabled.

[michaelrj]

libc/CMakeLists.txt

32

nit: having some parentheses here would help clarify what the actual condition is.

[jhuber6]

Add

[tra]

Is libc-hdrgen the only target that's supposed to be built? I find it puzzling that LIBC_HDRGEN_ONLY enables/disables the same early return which is normally controlled by the runtime/project level knob. I mean -- it appears to enable way more than just libc-hdrgen.

[jhuber6]

In D151155#4362483, @tra wrote:

Is libc-hdrgen the only target that's supposed to be built? I find it puzzling that LIBC_HDRGEN_ONLY enables/disables the same early return which is normally controlled by the runtime/project level knob. I mean -- it appears to enable way more than just libc-hdrgen.

When building with runtimes, we use the projects portion to just build the libc-hdrgen tool. So here the check is that if we have libc in the runtimes, but we're currently not doing a runtime build, then exit out early. This patch adds an extra way to assert that if we're building independently.

[sivachandra]

libc/CMakeLists.txt

32

This is OK but add an option which is OFF by default. Its not clear to me that this how we have to the libc build. We are stretching the limits of what is formally supported because of what happens to work.

[jhuber6]

Add option.

libc/CMakeLists.txt

if(NOT LIBC_HDRGEN_EXE)
     # We need to set up hdrgen first since other targets depend on it.
     add_subdirectory(utils/LibcTableGenUtil)
     add_subdirectory(utils/HdrGen)
   else()
     message(STATUS "Will use ${LIBC_HDRGEN_EXE} for libc header generation.")
   endif()
 endif()
 
-if("libc" IN_LIST LLVM_ENABLE_RUNTIMES AND NOT LLVM_RUNTIMES_BUILD)
+option(LIBC_HDRGEN_ONLY "Only build the 'libc-hdrgen' executable" OFF)
+if(("libc" IN_LIST LLVM_ENABLE_RUNTIMES AND NOT LLVM_RUNTIMES_BUILD) OR 
+   LIBC_HDRGEN_ONLY)
   # When libc is build as part of the runtimes/bootstrap build's CMake run, we
   # only need to build the host tools to build the libc. So, we just do enough
   # to build libc-hdrgen and return.
   return()
 endif()
 
 option(LIBC_CMAKE_VERBOSE_LOGGING
        "Log details warnings and notifications during CMake configuration." OFF)