[cmake] Disable GCC 9's -Wredundant-move

Summary:
This new warning (enabled by -Wextra) fires when a std::move is
redundant, as the default compiler behavior would be to select a move
operation anyway (e.g., when returning a local variable). Unlike
-Wpessimizing-move, it has no performance impact -- it just adds noise.

Currently llvm has about 1500 of these warnings. Unfortunately, the
suggested fix -- removing std::move -- does not work because of some
older compilers we still support. Specifically clang<=3.8 will not use a
move operation if an implicit conversion is needed (Core issue 1579). In
code like "A f(ConvertibleToA a) { return a; }" it will prefer a copy,
or fail to compile if a copy is not possible.

This patch disables that warning to get a meaningful signal out of a GCC
9 build.

Reviewers: rnk, aaron.ballman, xbolva00

Subscribers: mgorny, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D70963

llvm/cmake/modules/HandleLLVMOptions.cmake

@@ -610,6 +610,16 @@ if (LLVM_ENABLE_WARNINGS AND (LLVM_COMPILER_IS_GCC_COMPATIBLE OR CLANG_CL))
   check_cxx_compiler_flag("-Wclass-memaccess" CXX_SUPPORTS_CLASS_MEMACCESS_FLAG)
   append_if(CXX_SUPPORTS_CLASS_MEMACCESS_FLAG "-Wno-class-memaccess" CMAKE_CXX_FLAGS)
 
+  # Disable -Wredundant-move on GCC>=9. GCC wants to remove std::move in code
+  # like "A foo(ConvertibleToA a) { return std::move(a); }", but this code does
+  # not compile (or uses the copy constructor instead) on clang<=3.8. Clang also
+  # has a -Wredundant-move, but it only fires when the types match exactly, so
+  # we can keep it here.
+  if (CMAKE_CXX_COMPILER_ID STREQUAL "GNU")
+    check_cxx_compiler_flag("-Wredundant-move" CXX_SUPPORTS_REDUNDANT_MOVE_FLAG)
+    append_if(CXX_SUPPORTS_REDUNDANT_MOVE_FLAG "-Wno-redundant-move" CMAKE_CXX_FLAGS)
+  endif()
+
   # The LLVM libraries have no stable C++ API, so -Wnoexcept-type is not useful.
   check_cxx_compiler_flag("-Wnoexcept-type" CXX_SUPPORTS_NOEXCEPT_TYPE_FLAG)
   append_if(CXX_SUPPORTS_NOEXCEPT_TYPE_FLAG "-Wno-noexcept-type" CMAKE_CXX_FLAGS)