[hwasan] Add a paragraph on stack instrumentation.

Reviewers: kcc

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D42874

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@324163 91177308-0d34-0410-b5e6-96231b3b80d8

diff --git a/docs/HardwareAssistedAddressSanitizerDesign.rst b/docs/HardwareAssistedAddressSanitizerDesign.rst
index 5904cceaead201f353dbd7627d1057a78f068e93..423ec6f889b007d0fa5316fb8b9760f60ffa2df3 100644 (file)
--- a/docs/HardwareAssistedAddressSanitizerDesign.rst
+++ b/docs/HardwareAssistedAddressSanitizerDesign.rst
@@ -77,11 +77,18 @@ This can be based on any malloc that forces all objects to be N-aligned.
 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
 -------