Add -fstrict-vtable-pointers to UserManual

[Prazek]

Summary

It would be good to merge it to 4.0 branch.

[hans]

Would it be possible to expand on, or give some kind of pointer to, what "strict rules for overwriting polymorphic C++ objects" means?

[Prazek]

suggested fix

[Prazek]

Removed [no-] so hyperlink would work

[hans]

lgtm with comment

docs/UsersManual.rst

1103 ↗

(On Diff #84492)

e.g should be i.e. (or you could even spell it out: "that is")

I think it should also be "*the* vptr" and "*the* object* or "during an object's lifetime".

[Prazek]

I also have sent it on the branch

cfe/trunk/docs/UsersManual.rst

.. option:: -fsanitize-undefined-trap-on-error
    Deprecated alias for ``-fsanitize-trap=undefined``.
 
 .. option:: -fsanitize-cfi-cross-dso
 
    Enable cross-DSO control flow integrity checks. This flag modifies
    the behavior of sanitizers in the ``cfi`` group to allow checking
    of cross-DSO virtual and indirect calls.
 
+
+.. option:: -fstrict-vtable-pointers
+   Enable optimizations based on the strict rules for overwriting polymorphic
+   C++ objects, i.e. the vptr is invariant during an object's lifetime.
+   This enables better devirtualization. Turned off by default, because it is
+   still experimental.
+
 .. option:: -ffast-math
 
    Enable fast-math mode. This defines the ``__FAST_MATH__`` preprocessor
    macro, and lets the compiler make aggressive, potentially-lossy assumptions
    about floating-point math.  These include:
 
    * Floating-point math obeys regular algebraic rules for real numbers (e.g.
      ``+`` and ``*`` are associative, ``x/y == x * (1/y)``, and