[OpaquePtr][Clang] Add CLANG_ENABLE_OPAQUE_POINTERS cmake option

[nikic]

Summary

This option controls whether -opaque-pointers or -no-opaque-pointers is the default. Once opaque pointers are enabled by default, this will provide a simple way to temporarily opt-out of the change.

[nikic]

We might want to only actually land this one when the default gets switched -- I believe that cmake has this confusing behavior where changing an option default doesn't actually have an effect if the option has already been cached. IIRC when we switched to the NewPM this left a lot of people still using the LegacyPM by accident.

[dblaikie]

In D123122#3429318, @nikic wrote:

We might want to only actually land this one when the default gets switched -- I believe that cmake has this confusing behavior where changing an option default doesn't actually have an effect if the option has already been cached. IIRC when we switched to the NewPM this left a lot of people still using the LegacyPM by accident.

Yeah, bit unfortunate - I wonder if there's a way to add an option with 3 states, "default" (which we could change the semantics of), ON and OFF (so someone can explicitly opt in before the switch, and can explicitly opt out before the switch so they don't have to bundle a switch at the same time as the change, etc)?

[nikic]

In D123122#3430041, @dblaikie wrote:

In D123122#3429318, @nikic wrote:

We might want to only actually land this one when the default gets switched -- I believe that cmake has this confusing behavior where changing an option default doesn't actually have an effect if the option has already been cached. IIRC when we switched to the NewPM this left a lot of people still using the LegacyPM by accident.

Yeah, bit unfortunate - I wonder if there's a way to add an option with 3 states, "default" (which we could change the semantics of), ON and OFF (so someone can explicitly opt in before the switch, and can explicitly opt out before the switch so they don't have to bundle a switch at the same time as the change, etc)?

I think something like this would work:

# Manually handle default so we can change the meaning of a cached default. 
set(CLANG_ENABLE_OPAQUE_POINTERS "DEFAULT" CACHE STRING
    "Enable opaque pointers by default")
if(CLANG_ENABLE_OPAQUE_POINTERS STREQUAL "DEFAULT")
  set(CLANG_ENABLE_OPAQUE_POINTERS_INTERNAL OFF)
elseif(CLANG_ENABLE_OPAQUE_POINTERS)
  set(CLANG_ENABLE_OPAQUE_POINTERS_INTERNAL ON)
else()
  set(CLANG_ENABLE_OPAQUE_POINTERS_INTERNAL OFF)
endif()

This still has the annoying behavior that if you once specified -DCLANG_ENABLE_OPAQUE_POINTERS=false and then later drop it, you'll still keep implicitly using the old behavior, until you remove the build directory entirely. But I don't think one can do anything about that part.

[nikic]

Add intermediate option to make sure we can change a cached default value lateron.

I checked that modifying the default and running ninja does have an effect, unlike in the previous version.

clang/CMakeLists.txt

 set(PPC_LINUX_DEFAULT_IEEELONGDOUBLE OFF CACHE BOOL
     "Enable IEEE binary128 as default long double format on PowerPC Linux.")
 
 set(CLANG_SPAWN_CC1 OFF CACHE BOOL
     "Whether clang should use a new process for the CC1 invocation")
 
 option(CLANG_DEFAULT_PIE_ON_LINUX "Default to -fPIE and -pie on Linux" OFF)
 
+# Manually handle default so we can change the meaning of a cached default.
+set(CLANG_ENABLE_OPAQUE_POINTERS "DEFAULT" CACHE STRING
+    "Enable opaque pointers by default")
+if(CLANG_ENABLE_OPAQUE_POINTERS STREQUAL "DEFAULT")
+  set(CLANG_ENABLE_OPAQUE_POINTERS_INTERNAL OFF)
+elseif(CLANG_ENABLE_OPAQUE_POINTERS)
+  set(CLANG_ENABLE_OPAQUE_POINTERS_INTERNAL ON)
+else()
+  set(CLANG_ENABLE_OPAQUE_POINTERS_INTERNAL OFF)
+endif()
+
 # TODO: verify the values against LangStandards.def?
 set(CLANG_DEFAULT_STD_C "" CACHE STRING
   "Default standard to use for C/ObjC code (IDENT from LangStandards.def, empty for platform default)")
 set(CLANG_DEFAULT_STD_CXX "" CACHE STRING
   "Default standard to use for C++/ObjC++ code (IDENT from LangStandards.def, empty for platform default)")
 
 set(CLANG_DEFAULT_LINKER "" CACHE STRING
   "Default linker to use (linker name or absolute path, empty for platform default)")

clang/include/clang/Config/config.h.cmake

 /* Enable each functionality of modules */
 #cmakedefine01 CLANG_ENABLE_ARCMT
 #cmakedefine01 CLANG_ENABLE_OBJC_REWRITER
 #cmakedefine01 CLANG_ENABLE_STATIC_ANALYZER
 
 /* Spawn a new process clang.exe for the CC1 tool invocation, when necessary */
 #cmakedefine01 CLANG_SPAWN_CC1
 
+/* Whether to enable opaque pointers by default */
+#cmakedefine01 CLANG_ENABLE_OPAQUE_POINTERS_INTERNAL
+
 #endif

clang/include/clang/Driver/Options.td

defm enable_noundef_analysis : BoolOption<"",
   CodeGenOpts<"EnableNoundefAttrs">,
   DefaultTrue,
   PosFlag<SetTrue, [], "Enable">,
   NegFlag<SetFalse, [], "Disable">,
   BothFlags<[], " analyzing function argument and return types for mandatory definedness">>;
 defm opaque_pointers : BoolOption<"",
   "opaque-pointers",
   CodeGenOpts<"OpaquePointers">,
-  DefaultFalse,
+  Default<"CLANG_ENABLE_OPAQUE_POINTERS_INTERNAL">,
   PosFlag<SetTrue, [], "Enable">,
   NegFlag<SetFalse, [], "Disable">,
   BothFlags<[], " opaque pointers">>;
 def discard_value_names : Flag<["-"], "discard-value-names">,
   HelpText<"Discard value names in LLVM IR">,
   MarshallingInfoFlag<CodeGenOpts<"DiscardValueNames">>;
 def plugin_arg : JoinedAndSeparate<["-"], "plugin-arg-">,
     MetaVarName<"<name> <arg>">,