Summary: Document potential implementation of CFI in hardware.

Reviewers: eugenis, pcc

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D25455

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Differential Revision: http://reviews.llvm.org/D21054

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Clang-side cross-DSO CFI.

* Adds a command line flag -f[no-]sanitize-cfi-cross-dso.
* Links a runtime library when enabled.
* Emits __cfi_slowpath calls is bitset test fails.
* Emits extra hash-based bitsets for external CFI checks.
* Sets a module flag to enable __cfi_check generation during LTO.

This mode does not yet support diagnostics.

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This flag causes the compiler to emit bit set entries for functions as well
as runtime bitset checks at indirect call sites. Depends on the new function
bitset mechanism.

Differential Revision: http://reviews.llvm.org/D11857

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This patch introduces the -fsanitize=cfi-vptr flag, which enables a control
flow integrity scheme that checks that virtual calls take place using a vptr of
the correct dynamic type. More details in the new docs/ControlFlowIntegrity.rst
file.

It also introduces the -fsanitize=cfi flag, which is currently a synonym for
-fsanitize=cfi-vptr, but will eventually cover all CFI checks implemented
in Clang.

Differential Revision: http://reviews.llvm.org/D7424

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