llvm dev - May 2021 - DataFlowSanitizer label overwriting

Hi,

When using DataFlowSanitizer, I've noticed that assigning to a
labelled location overwrites or removes that label.

e.g. in this sample, the assert fails:

int a = 5;     
dfsan_label a_label = dfsan_create_label("a", 0);
dfsan_set_label(a_label, &a, sizeof(a));
a = 6;
dfsan_label check = dfsan_read_label(&a, sizeof(a));
assert(dfsan_has_label(check, a_label));

Remove the a = 6 statement and the assert passes.

This makes sense if the intended use of DataFlowSanitizer is taint
analysis alone, but isn't helpful if I want to track how a particular
memory location is used throughout a program and don't care what is
stored in it or when.

Is there a dfsan flag that retains the original label after a store?
I've tried the obvious ones with no luck.

Many thanks,
Magnus