Index: include/clang/Basic/DiagnosticSemaKinds.td
===================================================================
--- include/clang/Basic/DiagnosticSemaKinds.td
+++ include/clang/Basic/DiagnosticSemaKinds.td
@@ -6418,6 +6418,8 @@
   def err_asm_non_addr_value_in_memory_constraint : Error <
     "reference to a %select{bit-field|vector element|global register variable}0"
     " in asm %select{input|output}1 with a memory constraint '%2'">;
+  def err_asm_input_duplicate_match : Error<
+    "more than one input constraint matches the same output '%0'">;
 
   def warn_asm_label_on_auto_decl : Warning<
     "ignored asm label '%0' on automatic variable">;
@@ -6432,6 +6434,8 @@
 
   def note_asm_missing_constraint_modifier : Note<
     "use constraint modifier \"%0\"">;
+  def note_asm_input_duplicate_first : Note<
+    "constraint '%0' is already present here">;
 }
 
 let CategoryName = "Semantic Issue" in {
Index: lib/Sema/SemaStmtAsm.cpp
===================================================================
--- lib/Sema/SemaStmtAsm.cpp
+++ lib/Sema/SemaStmtAsm.cpp
@@ -420,6 +420,8 @@
                             diag::err_asm_unexpected_constraint_alternatives)
                        << NumAlternatives << AltCount);
   }
+  SmallVector<size_t, 4> InputMatchedToOutput(OutputConstraintInfos.size(),
+                                              ~0U);
   for (unsigned i = 0, e = InputConstraintInfos.size(); i != e; ++i) {
     TargetInfo::ConstraintInfo &Info = InputConstraintInfos[i];
     StringRef ConstraintStr = Info.getConstraintStr();
@@ -441,6 +443,18 @@
     Expr *OutputExpr = Exprs[TiedTo];
     Expr *InputExpr = Exprs[InputOpNo];
 
+    // Make sure no more than one input constraint matches each output.
+    if (InputMatchedToOutput[TiedTo] != ~0U) {
+      Diag(NS->getInputExpr(i)->getLocStart(),
+           diag::err_asm_input_duplicate_match)
+          << TiedTo;
+      Diag(NS->getInputExpr(InputMatchedToOutput[TiedTo])->getLocStart(),
+           diag::note_asm_input_duplicate_first)
+          << TiedTo;
+      return StmtError();
+    }
+    InputMatchedToOutput[TiedTo] = i;
+
     if (OutputExpr->isTypeDependent() || InputExpr->isTypeDependent())
       continue;
 
Index: test/Sema/asm.c
===================================================================
--- test/Sema/asm.c
+++ test/Sema/asm.c
@@ -236,3 +236,15 @@
           : "m" (test16_baz)); // expected-error {{reference to a global register variable in asm output with a memory constraint 'm'}}
 }
 
+int test17(int t0)
+{
+  int r0, r1;
+  __asm ("addl %2, %2\n\t"
+         "movl $123, %0"
+         : "=a" (r0),
+           "=&r" (r1)
+         : "1" (t0),   // expected-note {{constraint '1' is already present here}}
+           "1" (t0));  // expected-error {{more than one input constraint matches the same output '1'}}
+  return r0 + r1;
+}
+