[cmake] Update experimental target error message

[hintonda]

Summary

Update this error message indicate this test only ensures experimental
targets were passed via LLVM_EXPERIMENTAL_TARGETS_TO_BUILD.

Originally, this test validated all targets, but in r184923, it was moved
after the LLVMBUILDTOOL test, which also validates all targets, making
that part of the test redundant.

[asb]

I have the same concern with this as I described in the discussion around r320413.

With this change, I can directly specify an experimental backend using -DLLVM_TARGETS_TO_BUILD and it will "just work". I think there is value in requiring experimental backends to be specified separately with -DLLVM_EXPERIMENTAL_TARGETS_TO_BUILD. It means that anyone auditing the build configuration can see at a glance when an experimental backend is being built, which I think is important given the lack of stability guarantees for these backends. Did you intend that change in behaviour?

[hintonda]

In D41273#956447, @asb wrote:

I have the same concern with this as I described in the discussion around r320413.

With this change, I can directly specify an experimental backend using -DLLVM_TARGETS_TO_BUILD and it will "just work". I think there is value in requiring experimental backends to be specified separately with -DLLVM_EXPERIMENTAL_TARGETS_TO_BUILD. It means that anyone auditing the build configuration can see at a glance when an experimental backend is being built, which I think is important given the lack of stability guarantees for these backends. Did you intend that change in behaviour?

If you find it useful, I'm happy to leave it in, but we should at least change the error message to reflect what's really going on, i.e., the target was found but not explicitly passed as experimental.

[hintonda]

• Restore target test, but update error message to reflect that the target is experimental and must be passed via LLVM_EXPERIMENTAL_TARGETS_TO_BUILD.

[asb]

This seems fine, but would be best with a comment that indicates that llvm-build will have already errored out if a totally unrecognised target was given.

[hintonda]

• Add comment per Alex's suggestion.

CMakeLists.txt

 set(LLVM_ENUM_ASM_PARSERS "")
 set(LLVM_ENUM_DISASSEMBLERS "")
 foreach(t ${LLVM_TARGETS_TO_BUILD})
   set( td ${LLVM_MAIN_SRC_DIR}/lib/Target/${t} )
 
   list(FIND LLVM_ALL_TARGETS ${t} idx)
   list(FIND LLVM_EXPERIMENTAL_TARGETS_TO_BUILD ${t} idy)
   if( idx LESS 0 AND idy LESS 0 )
-    message(FATAL_ERROR "The target `${t}' does not exist.
-    It should be one of\n${LLVM_ALL_TARGETS}")
+    message(FATAL_ERROR "The target `${t}' is experimental and must be passed "
+      "via LLVM_EXPERIMENTAL_TARGETS_TO_BUILD.")
   else()
     set(LLVM_ENUM_TARGETS "${LLVM_ENUM_TARGETS}LLVM_TARGET(${t})\n")
   endif()
 
   file(GLOB asmp_file "${td}/*AsmPrinter.cpp")
   if( asmp_file )
     set(LLVM_ENUM_ASM_PRINTERS
       "${LLVM_ENUM_ASM_PRINTERS}LLVM_ASM_PRINTER(${t})\n")