[XRay] Support AArch64 in compiler-rt

This patch adds XRay support in compiler-rt for AArch64 targets.
This patch is one of a series:

LLVM: https://reviews.llvm.org/D26412
Clang: https://reviews.llvm.org/D26415

Author: rSerge

Reviewers: rengolin, dberris

Subscribers: aemerson, mgorny, llvm-commits, iid_iunknown

Differential Revision: https://reviews.llvm.org/D26413

llvm-svn: 287517

1 files changed, 7 insertions, 0 deletions

diff --git a/compiler-rt/include/xray/xray_interface.h b/compiler-rt/include/xray/xray_interface.h
index 680fcfdd523..9e712b1fa2a 100644
--- a/compiler-rt/include/xray/xray_interface.h
+++ b/compiler-rt/include/xray/xray_interface.h
@@ -32,6 +32,13 @@ enum XRayEntryType { ENTRY = 0, EXIT = 1, TAIL = 2 };
 //                 (function entry, function exit, etc.). See the enum
 //                 XRayEntryType for more details.
 //
+// The user handler must handle correctly spurious calls after this handler is
+// removed or replaced with another handler, because it would be too costly for
+// XRay runtime to avoid spurious calls.
+// To prevent circular calling, the handler function itself and all its
+// direct&indirect callees must not be instrumented with XRay, which can be
+// achieved by marking them all with: __attribute__((xray_never_instrument))
+//
 // Returns 1 on success, 0 on error.
 extern int __xray_set_handler(void (*entry)(int32_t, XRayEntryType));