Index: cfe/trunk/include/clang/Basic/Attr.td
===================================================================
--- cfe/trunk/include/clang/Basic/Attr.td
+++ cfe/trunk/include/clang/Basic/Attr.td
@@ -450,6 +450,15 @@
   let Documentation = [XRayDocs];
 }
 
+def XRayLogArgs : InheritableAttr {
+  let Spellings = [GNU<"xray_log_args">, CXX11<"clang", "xray_log_args">];
+  let Subjects = SubjectList<
+      [CXXMethod, ObjCMethod, Function], WarnDiag, "ExpectedFunctionOrMethod"
+  >;
+  let Args = [UnsignedArgument<"ArgumentCount">];
+  let Documentation = [XRayDocs];
+}
+
 def TLSModel : InheritableAttr {
   let Spellings = [GCC<"tls_model">];
   let Subjects = SubjectList<[TLSVar], ErrorDiag, "ExpectedTLSVar">;
Index: cfe/trunk/include/clang/Basic/AttrDocs.td
===================================================================
--- cfe/trunk/include/clang/Basic/AttrDocs.td
+++ cfe/trunk/include/clang/Basic/AttrDocs.td
@@ -2862,13 +2862,15 @@
 
 def XRayDocs : Documentation {
   let Category = DocCatFunction;
-  let Heading = "xray_always_instrument (clang::xray_always_instrument), xray_never_instrument (clang::xray_never_instrument)";
+  let Heading = "xray_always_instrument (clang::xray_always_instrument), xray_never_instrument (clang::xray_never_instrument), xray_log_args (clang::xray_log_args)";
   let Content = [{
 ``__attribute__((xray_always_instrument))`` or ``[[clang::xray_always_instrument]]`` is used to mark member functions (in C++), methods (in Objective C), and free functions (in C, C++, and Objective C) to be instrumented with XRay. This will cause the function to always have space at the beginning and exit points to allow for runtime patching.
 
 Conversely, ``__attribute__((xray_never_instrument))`` or ``[[clang::xray_never_instrument]]`` will inhibit the insertion of these instrumentation points.
 
 If a function has neither of these attributes, they become subject to the XRay heuristics used to determine whether a function should be instrumented or otherwise.
+
+``__attribute__((xray_log_args(N)))`` or ``[[clang::xray_log_args(N)]]`` is used to preserve N function arguments for the logging function.  Currently, only N==1 is supported.
   }];
 }
 
Index: cfe/trunk/lib/CodeGen/CodeGenFunction.cpp
===================================================================
--- cfe/trunk/lib/CodeGen/CodeGenFunction.cpp
+++ cfe/trunk/lib/CodeGen/CodeGenFunction.cpp
@@ -779,6 +779,10 @@
         Fn->addFnAttr("function-instrument", "xray-always");
       if (XRayAttr->neverXRayInstrument())
         Fn->addFnAttr("function-instrument", "xray-never");
+      if (const auto *LogArgs = D->getAttr<XRayLogArgsAttr>()) {
+        Fn->addFnAttr("xray-log-args",
+                      llvm::utostr(LogArgs->getArgumentCount()));
+      }
     } else {
       Fn->addFnAttr(
           "xray-instruction-threshold",
Index: cfe/trunk/lib/Sema/SemaDeclAttr.cpp
===================================================================
--- cfe/trunk/lib/Sema/SemaDeclAttr.cpp
+++ cfe/trunk/lib/Sema/SemaDeclAttr.cpp
@@ -4424,6 +4424,19 @@
                                     Attr.getAttributeSpellingListIndex()));
 }
 
+static void handleXRayLogArgsAttr(Sema &S, Decl *D,
+                                  const AttributeList &Attr) {
+  uint64_t ArgCount;
+  if (!checkFunctionOrMethodParameterIndex(S, D, Attr, 1, Attr.getArgAsExpr(0),
+                                           ArgCount))
+    return;
+
+  // ArgCount isn't a parameter index [0;n), it's a count [1;n] - hence + 1.
+  D->addAttr(::new (S.Context)
+             XRayLogArgsAttr(Attr.getRange(), S.Context, ++ArgCount,
+             Attr.getAttributeSpellingListIndex()));
+}
+
 //===----------------------------------------------------------------------===//
 // Checker-specific attribute handlers.
 //===----------------------------------------------------------------------===//
@@ -6285,6 +6298,9 @@
   case AttributeList::AT_XRayInstrument:
     handleSimpleAttribute<XRayInstrumentAttr>(S, D, Attr);
     break;
+  case AttributeList::AT_XRayLogArgs:
+    handleXRayLogArgsAttr(S, D, Attr);
+    break;
   }
 }
 
Index: cfe/trunk/test/CodeGen/xray-log-args.cpp
===================================================================
--- cfe/trunk/test/CodeGen/xray-log-args.cpp
+++ cfe/trunk/test/CodeGen/xray-log-args.cpp
@@ -0,0 +1,13 @@
+// RUN: %clang_cc1 %s -fxray-instrument -std=c++11 -x c++ -emit-llvm -o - -triple x86_64-unknown-linux-gnu | FileCheck %s
+
+// Make sure that the LLVM attribute for XRay-annotated functions do show up.
+[[clang::xray_always_instrument,clang::xray_log_args(1)]] void foo(int a) {
+// CHECK: define void @_Z3fooi(i32 %a) #0
+};
+
+[[clang::xray_log_args(1)]] void bar(int a) {
+// CHECK: define void @_Z3bari(i32 %a) #1
+};
+
+// CHECK: #0 = {{.*}}"function-instrument"="xray-always"{{.*}}"xray-log-args"="1"
+// CHECK-NOT: #1 = {{.*}}"xray-log-args"="1"
Index: cfe/trunk/test/Sema/xray-log-args-oob.c
===================================================================
--- cfe/trunk/test/Sema/xray-log-args-oob.c
+++ cfe/trunk/test/Sema/xray-log-args-oob.c
@@ -0,0 +1,9 @@
+// RUN: %clang_cc1 %s -verify -fsyntax-only -std=c11
+void foo(int) __attribute__((xray_log_args(1)));
+struct __attribute__((xray_log_args(1))) a { int x; }; // expected-warning {{'xray_log_args' attribute only applies to functions and methods}}
+
+void fop() __attribute__((xray_log_args(1))); // expected-error {{'xray_log_args' attribute parameter 1 is out of bounds}}
+
+void foq() __attribute__((xray_log_args(-1))); // expected-error {{'xray_log_args' attribute parameter 1 is out of bounds}}
+
+void fos() __attribute__((xray_log_args(0))); // expected-error {{'xray_log_args' attribute parameter 1 is out of bounds}}
Index: cfe/trunk/test/Sema/xray-log-args-oob.cpp
===================================================================
--- cfe/trunk/test/Sema/xray-log-args-oob.cpp
+++ cfe/trunk/test/Sema/xray-log-args-oob.cpp
@@ -0,0 +1,9 @@
+// RUN: %clang_cc1 %s -verify -fsyntax-only -std=c++11 -x c++
+void foo [[clang::xray_log_args(1)]] (int);
+struct [[clang::xray_log_args(1)]] a { int x; }; // expected-warning {{'xray_log_args' attribute only applies to functions and methods}}
+
+void fop [[clang::xray_log_args(1)]] (); // expected-error {{'xray_log_args' attribute parameter 1 is out of bounds}}
+
+void foq [[clang::xray_log_args(-1)]] (); // expected-error {{'xray_log_args' attribute parameter 1 is out of bounds}}
+
+void fos [[clang::xray_log_args(0)]] (); // expected-error {{'xray_log_args' attribute parameter 1 is out of bounds}}