[PATCH] D70963: [cmake] Disable GCC 9's -Wredundant-move

[Pavel Labath via Phabricator via llvm-commits]

labath created this revision.
labath added reviewers: rnk, aaron.ballman, xbolva00.
Herald added a subscriber: mgorny.
Herald added a project: LLVM.

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.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D70963

Files:
  llvm/cmake/modules/HandleLLVMOptions.cmake


Index: llvm/cmake/modules/HandleLLVMOptions.cmake
===================================================================
--- llvm/cmake/modules/HandleLLVMOptions.cmake
+++ llvm/cmake/modules/HandleLLVMOptions.cmake
@@ -610,6 +610,12 @@
   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, a GCC 9 warning. 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.
+  check_cxx_compiler_flag("-Wredundant-move" CXX_SUPPORTS_REDUNDANT_MOVE_FLAG)
+  append_if(CXX_SUPPORTS_REDUNDANT_MOVE_FLAG "-Wno-redundant-move" CMAKE_CXX_FLAGS)
+
   # 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)


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