Index: lib/Target/AArch64/AArch64ISelLowering.cpp
===================================================================
--- lib/Target/AArch64/AArch64ISelLowering.cpp
+++ lib/Target/AArch64/AArch64ISelLowering.cpp
@@ -2016,6 +2016,19 @@
     return false;
   }
 
+  // Externally-defined functions with weak linkage should not be
+  // tail-called on AArch64 when the OS does not support dynamic
+  // pre-emption of symbols, as the AAELF spec requires normal calls
+  // to undefined weak functions to be replaced with a NOP or jump to the
+  // next instruction. The behaviour of branch instructions in this
+  // situation (as used for tail calls) is implementation-defined, so we
+  // cannot rely on the linker replacing the tail call with a return.
+  if (GlobalAddressSDNode *G = dyn_cast<GlobalAddressSDNode>(Callee)) {
+    const GlobalValue *GV = G->getGlobal();
+    if (GV->hasExternalWeakLinkage())
+      return false;
+  }
+
   // Now we search for cases where we can use a tail call without changing the
   // ABI. Sibcall is used in some places (particularly gcc) to refer to this
   // concept.
Index: lib/Target/ARM/ARMISelLowering.cpp
===================================================================
--- lib/Target/ARM/ARMISelLowering.cpp
+++ lib/Target/ARM/ARMISelLowering.cpp
@@ -1970,6 +1970,19 @@
   if (Subtarget->isThumb1Only())
     return false;
 
+  // Externally-defined functions with weak linkage should not be
+  // tail-called on ARM when the OS does not support dynamic
+  // pre-emption of symbols, as the AAELF spec requires normal calls
+  // to undefined weak functions to be replaced with a NOP or jump to the
+  // next instruction. The behaviour of branch instructions in this
+  // situation (as used for tail calls) is implementation-defined, so we
+  // cannot rely on the linker replacing the tail call with a return.
+  if (GlobalAddressSDNode *G = dyn_cast<GlobalAddressSDNode>(Callee)) {
+    const GlobalValue *GV = G->getGlobal();
+    if (GV->hasExternalWeakLinkage())
+      return false;
+  }
+
   // If the calling conventions do not match, then we'd better make sure the
   // results are returned in the same way as what the caller expects.
   if (!CCMatch) {
Index: test/CodeGen/AArch64/tail-call.ll
===================================================================
--- test/CodeGen/AArch64/tail-call.ll
+++ test/CodeGen/AArch64/tail-call.ll
@@ -3,6 +3,7 @@
 declare fastcc void @callee_stack0()
 declare fastcc void @callee_stack8([8 x i32], i64)
 declare fastcc void @callee_stack16([8 x i32], i64, i64)
+declare extern_weak fastcc void @callee_weak()
 
 define fastcc void @caller_to0_from0() nounwind {
 ; CHECK-LABEL: caller_to0_from0:
@@ -92,3 +93,13 @@
 ; CHECK-NEXT: add sp, sp, #16
 ; CHECK-NEXT: b callee_stack16
 }
+
+
+; Weakly-referenced extern functions cannot be tail-called, as AAELF does
+; not define the behaviour of branch instructions to undefined weak symbols.
+define fastcc void @caller_weak() {
+; CHECK-LABEL: caller_weak:
+; CHECK: bl callee_weak
+  tail call void @callee_weak()
+  ret void
+}
Index: test/CodeGen/ARM/tail-call.ll
===================================================================
--- test/CodeGen/ARM/tail-call.ll
+++ test/CodeGen/ARM/tail-call.ll
@@ -3,6 +3,7 @@
 ; RUN:   | FileCheck %s -check-prefix CHECK-NO-TAIL
 
 declare i32 @callee(i32 %i)
+declare extern_weak fastcc void @callee_weak()
 
 define i32 @caller(i32 %i) {
 entry:
@@ -19,3 +20,12 @@
 ; CHECK-NO-TAIL: pop {lr}
 ; CHECK-NO-TAIL: bx lr
 
+
+; Weakly-referenced extern functions cannot be tail-called, as AAELF does
+; not define the behaviour of branch instructions to undefined weak symbols.
+define fastcc void @caller_weak() {
+; CHECK-LABEL: caller_weak:
+; CHECK: bl callee_weak
+  tail call void @callee_weak()
+  ret void
+}