When a variable goes into scope several times within a single function
or when two variables from different scopes share a stack slot it may
be incorrect to poison such scoped locals at the beginning of the
In the former case it may lead to false negatives (see
https://github.com/google/sanitizers/issues/590), in the latter - to
incorrect reports (because only one origin remains on the stack).
If Clang emits lifetime intrinsics with known size for such scoped
variables we insert code poisoning them after each call to
llvm.lifetime.start(). This is done on a best-effort basis.
If for a certain intrinsic we fail to find a corresponding alloca, we
fall back to poisoning allocas for the whole function, as it's now
impossible to tell which alloca was missed.
The new instrumentation may slow down hot loops containing local
variables with lifetime intrinsics, so we allow disabling it with