[CMake] Add LLVM_BUILD_32_BITS to LLVMConfig.cmake

[john.brawn]

Summary

Previously out-of-tree passes could detect if LLVM was built with LLVM_BUILD_32_BITS by looking for -m32 in LLVM_DEFINITIONS, but as of r271871 it no longer appears there. Resolve this by instead emitting LLVM_BUILD_32_BITS in LLVMConfig so it can be checked for directly.

[chapuni]

I don't like this idea that exposes LLVM_BUILD_32_BITS. Didn't you inspect LLVM_C*_FLAGS?

Or, we could introduce the way to export options for code formats, for other targets.

I supposed LLVM_BUILD_32_BITS as an option for developers.

[john.brawn]

In D21434#460551, @chapuni wrote:

I don't like this idea that exposes LLVM_BUILD_32_BITS. Didn't you inspect LLVM_C*_FLAGS?

LLVM_C*_FLAGS don't exist in LLVMConfig.cmake. I see LLVM_BUILD_32_BITS as similar in a way to LLVM_ENABLE_EH and LLVM_ENABLE_RTTI - it's something where out-of-tree plugins need to be aware of how LLVM was built otherwise LLVM will give an error when trying to load the plugin, so it needs to be exposed somehow.

[john.brawn]

Ping.

[beanz]

This change is the best way to accomplish what needs to be done here for platforms that don't utilize fat binaries.

@chapuni, this option is only exposed to people using our shipped CMake modules, and they do need a way to know whether the modules they are importing are built for 32 or 64 bit targets.

[chapuni]

I won't object any more. Thank you.

llvm/trunk/cmake/modules/LLVMConfig.cmake.in

 set(LLVM_ENABLE_THREADS @LLVM_ENABLE_THREADS@)
 
 set(LLVM_ENABLE_ZLIB @LLVM_ENABLE_ZLIB@)
 
 set(LLVM_NATIVE_ARCH @LLVM_NATIVE_ARCH@)
 
 set(LLVM_ENABLE_PIC @LLVM_ENABLE_PIC@)
 
+set(LLVM_BUILD_32_BITS @LLVM_BUILD_32_BITS@)
+
 set(LLVM_ENABLE_PLUGINS @LLVM_ENABLE_PLUGINS@)
 set(LLVM_EXPORT_SYMBOLS_FOR_PLUGINS @LLVM_EXPORT_SYMBOLS_FOR_PLUGINS@)
 set(LLVM_PLUGIN_EXT @LLVM_PLUGIN_EXT@)
 
 set(LLVM_ON_UNIX @LLVM_ON_UNIX@)
 set(LLVM_ON_WIN32 @LLVM_ON_WIN32@)
 
 set(LLVM_LIBDIR_SUFFIX @LLVM_LIBDIR_SUFFIX@)