[flang] Fix DYLIB builds

[awarzynski]

Summary

https://reviews.llvm.org/D120568 broke builds that set
both LLVM_BUILD_LLVM_DYLIB and LLVM_LINK_LLVM_DYLIB. This patch
fixes that.

The build failure was caused by the fact that some LLVM libraries (which
are also LLVM components) were listed directly as link-time dependencies
instead of using LINK_COMPONENTS in CMake files. This lead to a linker
invocation like this (simplified version to demonstrate the problem):

ld lib/libLLVM.so lib/libLLVMAnalysis.a lib/libLLVMTarget.a

That's problematic and unnecessary because libLLVM.so incorporates
libLLVMAnalysis and libLLVMTarget. A correct invocation would look
like this (LLVM_LINK_LLVM_DYLIB _is not_ set):

ld  lib/libLLVMAnalysis.a lib/libLLVMTarget.a

or this (LLVM_LINK_LLVM_DYLIB _is_ set):

ld lib/libLLVM.so

[DavidSpickett]

So LINK_LIBS is what needs to be named in the linker command and LINK_COMPONENTS is what must be somewhere in the libraries in the link command, but could be part of another one?

[DavidSpickett]

Hence why we got an option registered twice because the registration ran once in some library and again in another than also contained the option registration.

[awarzynski]

Yes and yes :)

Items under LINK_LIBS are libraries, which are simply added to the linker invocation. The semantics are identical to target_link_libraries().

Items under LINK_COMPONENTS are components rather than libraries. A component will be translated into one or more libraries and this translation depends on whether various other flags, e.g. LLVM_LINK_LLVM_DYLIB, are used. IIUC, when this flag is set then all components are translated into libLLVM (see here). You can also experiment with llvm-config (e.g. bin/llvm-config --libs Target) and see what you get depending on the build flags used.

Basically, one should not mix LINK_LIBS and LINK_COMPONENTS because that may lead to the same library being linked twice (and then various CL options being registered twice).

[DavidSpickett]

LGTM

flang/lib/Frontend/CMakeLists.txt

add_flang_library(flangFrontend
   LINK_LIBS
   FortranParser
   FortranSemantics
   FortranEvaluate
   FortranCommon
   FortranLower
   clangBasic
   clangDriver
-  LLVMAnalysis
-  LLVMTarget
   FIRDialect
   FIRSupport
   FIRBuilder
   FIRCodeGen
   FIRTransforms
   MLIRTransforms
   MLIRLLVMToLLVMIRTranslation
   MLIRSCFToControlFlow
   ${dialect_libs}
 
   LINK_COMPONENTS
+  Analysis
   Option
   Support
+  Target
   FrontendOpenACC
   FrontendOpenMP
 )