commit 43ff2c1f637f2c8d444f88e982f04a5b354ec0c2
author Mohit Saini <mohitms@google.com> Sun Mar 13 21:28:39 2022 +0000
committer Mohit Saini <mohitms@google.com> Mon Mar 14 11:50:26 2022 +0000
tree a0ffc347e02f784a0e047776c2e8a73b59940af4
parent 6d204989009bbb1da10ad0d5339aafd0bcd208d5

Fix 'weak-vtables' warning in TrackEvent

warning: 'TrackEvent' has no out-of-line virtual method definitions;
its vtable will be emitted in every translation unit [-Wweak-vtables]

This warning can be reproduced by compiling example.cc using
clang compiler that supports `-Wweak-vtables` warning. It was
suppressed earlier by supplying
`configs -= [ "//gn/standalone:extra_warnings" ]` option in
`examples/sdk/BUILD.gn`.

The root cause of this warning is demonstrated in CL aosp/2019905

Note that this warning is NOT serious because linker will anyway merge
those vtable into a single vtable. i.e. duplicated vtable won't increase
the size of final executable.

The root cause is, `TrackEvent` don't have any non-inline virtual
function. So compiler cannot decide where to place vtable.

Itanium CXX ABI, section 5.2.3:

The virtual table for a class is emitted in the same object containing
the definition of its key function, i.e. the first non-pure virtual
function that is not inline at the point of class definition. If there
is no key function, it is emitted everywhere used. The emitted virtual
table includes the full virtual table group for the class, any new
construction virtual tables required for subobjects, and the VTT for
the class. They are emitted in a COMDAT group, with the virtual table
mangled name as the identifying symbol. Note that if the key function
is not declared inline in the class definition, but its definition later
is always declared inline, it will be emitted in every object containing
the definition.
NOTE: In the abstract, a pure virtual destructor could be used as the
key function, as it must be defined even though it is pure. However,
the ABI committee did not realize this fact until after the
specification of key function was complete; therefore a pure virtual
destructor cannot be the key function.

Ref: https://itanium-cxx-abi.github.io/cxx-abi/abi.html#vague-vtable

Change-Id: I29f3308bdb91b135e3cc91c3a0a3d60dc3de4d86