[clang] [CodeGen] Disable exact dynamic_cast for duplicable vtables (PR #205929)

[Oliver Hunt via cfe-commits]

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@@ -1662,12 +1662,13 @@ def : Flag<["-"], "shared-libasan">, Alias<shared_libsan>;
 def : Flag<["-"], "static-libasan">, Alias<static_libsan>;
 def fasm : Flag<["-"], "fasm">, Group<f_Group>;
 
-defm assume_unique_vtables : BoolFOption<"assume-unique-vtables",
-  CodeGenOpts<"AssumeUniqueVTables">, DefaultTrue,
-  PosFlag<SetTrue>,
-  NegFlag<SetFalse, [], [ClangOption, CC1Option],
-          "Disable optimizations based on vtable pointer identity">,
-  BothFlags<[], [ClangOption, CLOption]>>;
+def fassume_unique_vtables : Flag<["-"], "fassume-unique-vtables">,
+  Group<f_Group>, Visibility<[ClangOption, CC1Option, CLOption]>,
+  HelpText<"Assume each polymorphic class has a single vtable with a unique "
+           "address, enabling optimizations based on vtable pointer identity">;
+def fno_assume_unique_vtables : Flag<["-"], "fno-assume-unique-vtables">,
+  Group<f_Group>, Visibility<[ClangOption, CC1Option, CLOption]>,
+  HelpText<"Disable optimizations based on vtable pointer identity">;
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ojhunt wrote:

It's almost worth making this match the actual options: never, always, vtables-with-key-function -- do you have an opinion? (e.g it would be the fassume-unique-vtables={no, yes, key function flag})

https://github.com/llvm/llvm-project/pull/205929