[gn] Tag unneeded variable.

[bryant]

Summary

When only building targets without assembly parsers (e.g.,
llvm_targets_to_build = ["NVPTX"]), all_targets is empty and causes GN to
warn about an assigned-but-unused variable.

[thakis]

Thanks!

llvm/utils/gn/secondary/llvm/include/llvm/Config/BUILD.gn

389

nit: I'd keep the loop out of the if and do

  if (all_targets == []) {
    not_needed(invoker, [ "value" ])
  }
``

after the loop instead.

[bryant]

Moved for-loop out of branch.

[thakis]

lgtm, thanks. Do you need someone to land this?

[bryant]

I checked this in at https://reviews.llvm.org/rL365897 but made a copy-paste mistake that referenced https://reviews.llvm.org/D31727 in the commit message instead of this differential. ):

llvm/utils/gn/secondary/llvm/include/llvm/Config/BUILD.gn

write_cmake_config(target_name) {
     # Build something like
     # `LLVM_ENUM_ASM_PARSERS=LLVM_ASM_PARSER(ARM)\nLLVM_ASM_PARSER(X86)\n`. Note
     # that \n is a literal '\' followed by a literal 'n', not a newline
     # character.  (write_cmake_config.py replaces that with a real newline).
     value = ""
     foreach(target, all_targets) {
       value = "$value${invoker.value}($target)\n"
     }
+    if (all_targets == []) {
+      not_needed(invoker, [ "value" ])
+    }
     values = [ "${invoker.key}=$value" ]
   }
 }
 
 write_target_def_file("AsmParsers.def") {
   key = "LLVM_ENUM_ASM_PARSERS"
   value = "LLVM_ASM_PARSER"
   all_targets = targets_with_asm_parsers
 }