AArch64: Add a tagged-globals backend feature.

This feature instructs the backend to allow locally defined global variable
addresses to contain a pointer tag in bits 56-63 that will be ignored by
the hardware (i.e. TBI), but may be used by an instrumentation pass such
as HWASAN. It works by adding a MOVK instruction to the regular ADRP/ADD
sequence that sets bits 48-63 to the corresponding bits of the global, with
the linker bounds check disabled on the ADRP instruction to prevent the tag
from causing a link failure.

This implementation of the feature omits the MOVK when loading from or storing
to a global, which is sufficient for TBI. If the same approach is extended
to MTE, assuming that 0 is not configured as a catch-all tag, we will most
likely also need the MOVK in this case in order to avoid a tag mismatch.

Differential Revision: https://reviews.llvm.org/D65364

llvm-svn: 367475

1 files changed, 7 insertions, 0 deletions

diff --git a/llvm/lib/Target/AArch64/AArch64Subtarget.cpp b/llvm/lib/Target/AArch64/AArch64Subtarget.cpp
index 6a45b4e57d7..fb82b264706 100644
--- a/llvm/lib/Target/AArch64/AArch64Subtarget.cpp
+++ b/llvm/lib/Target/AArch64/AArch64Subtarget.cpp
@@ -229,6 +229,13 @@ AArch64Subtarget::ClassifyGlobalReference(const GlobalValue *GV,
       GV->hasExternalWeakLinkage())
     return AArch64II::MO_GOT;
 
+  // References to tagged globals are marked with MO_NC | MO_TAGGED to indicate
+  // that their nominal addresses are tagged and outside of the code model. In
+  // AArch64ExpandPseudo::expandMI we emit an additional instruction to set the
+  // tag if necessary based on MO_TAGGED.
+  if (AllowTaggedGlobals && !isa<FunctionType>(GV->getValueType()))
+    return AArch64II::MO_NC | AArch64II::MO_TAGGED;
+
   return AArch64II::MO_NO_FLAG;
 }