Index: include/clang/Basic/DiagnosticSemaKinds.td
===================================================================
--- include/clang/Basic/DiagnosticSemaKinds.td
+++ include/clang/Basic/DiagnosticSemaKinds.td
@@ -444,6 +444,10 @@
   "change the argument to be the free space in the destination buffer minus " 
   "the terminating null byte">;
 
+def warn_assume_side_effects : Warning<
+  "the argument to __assume has side effects that will be discarded">,
+  InGroup<DiagGroup<"assume">>;
+
 /// main()
 // static main() is not an error in C, just in C++.
 def warn_static_main : Warning<"'main' should not be declared static">,
Index: include/clang/Sema/Sema.h
===================================================================
--- include/clang/Sema/Sema.h
+++ include/clang/Sema/Sema.h
@@ -8244,6 +8244,7 @@
 
 private:
   bool SemaBuiltinPrefetch(CallExpr *TheCall);
+  bool SemaBuiltinAssume(CallExpr *TheCall);
   bool SemaBuiltinLongjmp(CallExpr *TheCall);
   ExprResult SemaBuiltinAtomicOverloaded(ExprResult TheCallResult);
   ExprResult SemaAtomicOpsOverloaded(ExprResult TheCallResult,
Index: lib/Sema/SemaChecking.cpp
===================================================================
--- lib/Sema/SemaChecking.cpp
+++ lib/Sema/SemaChecking.cpp
@@ -175,6 +175,10 @@
     if (SemaBuiltinPrefetch(TheCall))
       return ExprError();
     break;
+  case Builtin::BI__assume:
+    if (SemaBuiltinAssume(TheCall))
+      return ExprError();
+    break;
   case Builtin::BI__builtin_object_size:
     if (SemaBuiltinConstantArgRange(TheCall, 1, 0, 3))
       return ExprError();
@@ -1942,6 +1946,20 @@
   return false;
 }
 
+/// SemaBuiltinAssume - Handle __assume (MS Extension).
+// __assume does not evaluate its arguments, and should warn if its argument
+// has side effects.
+bool Sema::SemaBuiltinAssume(CallExpr *TheCall) {
+  Expr *Arg = TheCall->getArg(0);
+  if (Arg->isTypeDependent() || Arg->isValueDependent()) return false;
+
+  if (Arg->HasSideEffects(Context))
+    return Diag(Arg->getLocStart(), diag::warn_assume_side_effects)
+      << Arg->getSourceRange();
+
+  return false;
+}
+
 /// SemaBuiltinConstantArg - Handle a check if argument ArgNum of CallExpr
 /// TheCall is a constant expression.
 bool Sema::SemaBuiltinConstantArg(CallExpr *TheCall, int ArgNum,
Index: test/Sema/builtin-assume.c
===================================================================
--- /dev/null
+++ test/Sema/builtin-assume.c
@@ -0,0 +1,8 @@
+// RUN: %clang_cc1 -triple i386-mingw32 -fms-extensions -fsyntax-only -verify %s
+
+int foo(int *a, int i) {
+  __assume(i != 4);
+  __assume(++i > 2); //expected-warning {{the argument to __assume has side effects that will be discarded}}
+  return a[i];
+}
+