diff --git a/lib/Transforms/IPO/ArgumentPromotion.cpp b/lib/Transforms/IPO/ArgumentPromotion.cpp
index 7c40ecd98d1..d2f407b9e55 100644
--- a/lib/Transforms/IPO/ArgumentPromotion.cpp
+++ b/lib/Transforms/IPO/ArgumentPromotion.cpp
@@ -156,6 +156,13 @@ CallGraphNode *ArgPromotion::PromoteArguments(CallGraphNode *CGN) {
       isSelfRecursive = true;
   }
   
+  // Don't promote arguments for variadic functions. Adding, removing, or
+  // changing non-pack parameters can change the classification of pack
+  // parameters. Frontends encode that classification at the call site in the
+  // IR, while in the callee the classification is determined dynamically based
+  // on the number of registers consumed so far.
+  if (F->isVarArg()) return nullptr;
+
   // Check to see which arguments are promotable.  If an argument is promotable,
   // add it to ArgsToPromote.
   SmallPtrSet<Argument*, 8> ArgsToPromote;
diff --git a/test/Transforms/ArgumentPromotion/variadic.ll b/test/Transforms/ArgumentPromotion/variadic.ll
new file mode 100644
index 00000000000..0ae52b3bbbd
--- /dev/null
+++ b/test/Transforms/ArgumentPromotion/variadic.ll
@@ -0,0 +1,28 @@
+; RUN: opt < %s -argpromotion -S | FileCheck %s
+
+; Unused arguments from variadic functions cannot be eliminated as that changes
+; their classiciation according to the SysV amd64 ABI. Clang and other frontends
+; bake in the classification when they use things like byval, as in this test.
+
+target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-unknown-linux-gnu"
+
+%struct.tt0 = type { i64, i64 }
+%struct.__va_list_tag = type { i32, i32, i8*, i8* }
+
+@t45 = internal global %struct.tt0 { i64 1335139741, i64 438042995 }, align 8
+
+; Function Attrs: nounwind uwtable
+define i32 @main(i32 %argc, i8** nocapture readnone %argv) #0 {
+entry:
+  tail call void (i8*, i8*, i8*, i8*, i8*, ...)* @callee_t0f(i8* undef, i8* undef, i8* undef, i8* undef, i8* undef, %struct.tt0* byval align 8 @t45)
+  ret i32 0
+}
+
+; Function Attrs: nounwind uwtable
+define internal void @callee_t0f(i8* nocapture readnone %tp13, i8* nocapture readnone %tp14, i8* nocapture readnone %tp15, i8* nocapture readnone %tp16, i8* nocapture readnone %tp17, ...) {
+entry:
+  ret void
+}
+
+; CHECK-LABEL: define internal void @callee_t0f(i8* nocapture readnone %tp13, i8* nocapture readnone %tp14, i8* nocapture readnone %tp15, i8* nocapture readnone %tp16, i8* nocapture readnone %tp17, ...)