[all-commits] [llvm/llvm-project] 9d525b: Optimize emission of `dynamic_cast` to final classes.

[Richard Smith via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: 9d525bf94b255df89587db955b5fa2d3c03c2c3e
      https://github.com/llvm/llvm-project/commit/9d525bf94b255df89587db955b5fa2d3c03c2c3e
  Author: Richard Smith <richard at metafoo.co.uk>
  Date:   2023-07-21 (Fri, 21 Jul 2023)

  Changed paths:
    M clang/include/clang/Basic/CodeGenOptions.def
    M clang/include/clang/Driver/Options.td
    M clang/lib/AST/ExprCXX.cpp
    M clang/lib/CodeGen/CGCXXABI.h
    M clang/lib/CodeGen/CGExprCXX.cpp
    M clang/lib/CodeGen/CodeGenModule.cpp
    M clang/lib/CodeGen/CodeGenModule.h
    M clang/lib/CodeGen/ItaniumCXXABI.cpp
    M clang/lib/CodeGen/MicrosoftCXXABI.cpp
    M clang/lib/Driver/ToolChains/Clang.cpp
    M clang/test/CodeGenCXX/dynamic-cast-always-null.cpp
    A clang/test/CodeGenCXX/dynamic-cast-exact-disabled.cpp
    A clang/test/CodeGenCXX/dynamic-cast-exact.cpp
    A clang/test/Driver/fassume-unique-vtables.cpp

  Log Message:
  -----------
  Optimize emission of `dynamic_cast` to final classes.

- When the destination is a final class type that does not derive from
  the source type, the cast always fails and is now emitted as a null
  pointer or call to __cxa_bad_cast.

- When the destination is a final class type that does derive from the
  source type, emit a direct comparison against the corresponding base
  class vptr value(s). There may be more than one such value in the case
  of multiple inheritance; check them all.

For now, this is supported only for the Itanium ABI. I expect the same thing is
possible for the MS ABI too, but I don't know what guarantees are made about
vfptr uniqueness.

Reviewed By: rjmccall

Differential Revision: https://reviews.llvm.org/D154658