Index: include/clang/Basic/DiagnosticSemaKinds.td
===================================================================
--- include/clang/Basic/DiagnosticSemaKinds.td
+++ include/clang/Basic/DiagnosticSemaKinds.td
@@ -259,6 +259,9 @@
   "a pointer as the first parameter|a %2 type as the second parameter}1">;
 def err_anyx86_interrupt_called : Error<
   "interrupt service routine cannot be called directly">;
+def warn_arm_interrupt_calling_convention : Warning<
+   "call to function without interrupt attribute could clobber interruptee's VFP registers">,
+   InGroup<Extra>;
 def warn_mips_interrupt_attribute : Warning<
    "MIPS 'interrupt' attribute only applies to functions that have "
    "%select{no parameters|a 'void' return type}0">,
Index: lib/Sema/SemaExpr.cpp
===================================================================
--- lib/Sema/SemaExpr.cpp
+++ lib/Sema/SemaExpr.cpp
@@ -5395,6 +5395,15 @@
     return ExprError();
   }
 
+  // Interrupt handlers don't save off the VFP regs automatically on ARM,
+  // so there's some risk when calling out to non-interrupt handler functions
+  // that the callee might not preserve them. This is easy to diagnose here,
+  // but can be very challenging to debug.
+  if (auto *Caller = getCurFunctionDecl())
+    if (Caller->hasAttr<ARMInterruptAttr>())
+      if (!FDecl->hasAttr<ARMInterruptAttr>())
+        Diag(Fn->getExprLoc(), diag::warn_arm_interrupt_calling_convention);
+
   // Promote the function operand.
   // We special-case function promotion here because we only allow promoting
   // builtin functions to function pointers in the callee of a call.
Index: test/Sema/arm-interrupt-attr.c
===================================================================
--- test/Sema/arm-interrupt-attr.c
+++ test/Sema/arm-interrupt-attr.c
@@ -17,3 +17,14 @@
 __attribute__((interrupt)) void foo8() {}
 __attribute__((interrupt())) void foo9() {}
 __attribute__((interrupt(""))) void foo10() {}
+
+void callee1();
+__attribute__((interrupt("IRQ"))) void callee2();
+void caller1() {
+  callee1();
+  callee2();
+}
+__attribute__((interrupt("IRQ"))) void caller2() {
+  callee1(); // expected-warning {{call to function without interrupt attribute could clobber interruptee's VFP registers}}
+  callee2();
+}