[PATCH] D42874: [hwasan] Add a paragraph on stack instrumentation.

[Evgenii Stepanov via Phabricator via cfe-commits]

This revision was automatically updated to reflect the committed changes.
Closed by commit rL324163: [hwasan] Add a paragraph on stack instrumentation. (authored by eugenis, committed by ).
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Changed prior to commit:
  https://reviews.llvm.org/D42874?vs=132708&id=132711#toc

Repository:
  rL LLVM

https://reviews.llvm.org/D42874

Files:
  cfe/trunk/docs/HardwareAssistedAddressSanitizerDesign.rst


Index: cfe/trunk/docs/HardwareAssistedAddressSanitizerDesign.rst
===================================================================
--- cfe/trunk/docs/HardwareAssistedAddressSanitizerDesign.rst
+++ cfe/trunk/docs/HardwareAssistedAddressSanitizerDesign.rst
@@ -77,11 +77,18 @@
 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
 -------


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