[hwasan] Add a paragraph on stack instrumentation.

[kcc]

LGTM++

cfe/trunk/docs/HardwareAssistedAddressSanitizerDesign.rst

 
 Tagging the heap memory/pointers is done by `malloc`.
 This can be based on any malloc that forces all objects to be N-aligned.
 `free` tags the memory with a different tag.
 
 Stack
 -----
 
-Special compiler instrumentation is required to align the local variables
-by N, tag the memory and the pointers.
+Stack frames are instrumented by aligning all non-promotable allocas
+by `N` and tagging stack memory in function prologue and epilogue.
+
+Tags for different allocas in one function are **not** generated
+independently; doing that in a function with `M` allocas would require
+maintaining `M` live stack pointers, significantly increasing register
+pressure. Instead we generate a single base tag value in the prologue,
+and build the tag for alloca number `M` as `ReTag(BaseTag, M)`, where
+ReTag can be as simple as exclusive-or with constant `M`.
+
 Stack instrumentation is expected to be a major source of overhead,
 but could be optional.
-TODO: details.
 
 Globals
 -------
 
 TODO: details.
 
 Error reporting
 ---------------