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

[mstorsjo]

Summary

Similar to the existing code for disabling GCC's -Wredudant-move,
also check for the -Wpessimizing-move option and disable it if
possible.

This silences another bunch of noisy warnings when building LLVM
with GCC 9.

As noted for -Wredundant-move, the cases can't be fixed while
retaining support for older compilers.

[xbolva00]

llvm/cmake/modules/HandleLLVMOptions.cmake

700

Maybe we can drop this? LLVM supports clang 3.6 or newer.

[labath]

looks good to me.

llvm/cmake/modules/HandleLLVMOptions.cmake

683–687

This comment could use some rephrasing/generalization.

700

Possibly. But that definitely sounds like a separate patch.

llvm/cmake/modules/HandleLLVMOptions.cmake

if (LLVM_ENABLE_WARNINGS AND (LLVM_COMPILER_IS_GCC_COMPATIBLE OR CLANG_CL))
   append_if(USE_NO_UNINITIALIZED "-Wno-uninitialized" CMAKE_CXX_FLAGS)
   append_if(USE_NO_MAYBE_UNINITIALIZED "-Wno-maybe-uninitialized" CMAKE_CXX_FLAGS)
 
   # Disable -Wclass-memaccess, a C++-only warning from GCC 8 that fires on
   # LLVM's ADT classes.
   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)
+    check_cxx_compiler_flag("-Wpessimizing-move" CXX_SUPPORTS_PESSIMIZING_MOVE_FLAG)
+    append_if(CXX_SUPPORTS_PESSIMIZING_MOVE_FLAG "-Wno-pessimizing-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)
 
   # Check if -Wnon-virtual-dtor warns even though the class is marked final.
   # If it does, don't add it. So it won't be added on clang 3.4 and older.
   # This also catches cases when -Wnon-virtual-dtor isn't supported by
   # the compiler at all.  This flag is not activated for gcc since it will
   # incorrectly identify a protected non-virtual base when there is a friend
   # declaration. Don't activate this in general on Windows as this warning has
   # too many false positives on COM-style classes, which are destroyed with
   # Release() (PR32286).
   if (NOT CMAKE_COMPILER_IS_GNUCXX AND NOT WIN32)
     set(OLD_CMAKE_REQUIRED_FLAGS ${CMAKE_REQUIRED_FLAGS})