Index: include/llvm/Transforms/IPO/DeadArgumentElimination.h
===================================================================
--- include/llvm/Transforms/IPO/DeadArgumentElimination.h
+++ include/llvm/Transforms/IPO/DeadArgumentElimination.h
@@ -32,6 +32,7 @@
 namespace llvm {
 
 class Module;
+class TargetLibraryInfo;
 class Use;
 class Value;
 
@@ -128,7 +129,7 @@
                      unsigned RetValNum = -1U);
   Liveness SurveyUses(const Value *V, UseVector &MaybeLiveUses);
 
-  void SurveyFunction(const Function &F);
+  void SurveyFunction(const Function &F, const TargetLibraryInfo &TLI);
   void MarkValue(const RetOrArg &RA, Liveness L,
                  const UseVector &MaybeLiveUses);
   void MarkLive(const RetOrArg &RA);
Index: lib/Transforms/IPO/ArgumentPromotion.cpp
===================================================================
--- lib/Transforms/IPO/ArgumentPromotion.cpp
+++ lib/Transforms/IPO/ArgumentPromotion.cpp
@@ -816,7 +816,8 @@
 promoteArguments(Function *F, function_ref<AAResults &(Function &F)> AARGetter,
                  unsigned MaxElements,
                  Optional<function_ref<void(CallSite OldCS, CallSite NewCS)>>
-                     ReplaceCallSite) {
+                     ReplaceCallSite,
+                 const TargetLibraryInfo &TLI) {
   // Don't perform argument promotion for naked functions; otherwise we can end
   // up removing parameters that are seemingly 'not used' as they are referred
   // to in the assembly.
@@ -827,6 +828,12 @@
   if (!F->hasLocalLinkage())
     return nullptr;
 
+  // Don't promote arguments for library functions. We can make assumptions
+  // about library function signatures in later passes.
+  LibFunc LF;
+  if (TLI.getLibFunc(*F, LF))
+    return nullptr;
+
   // 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
@@ -978,8 +985,10 @@
         assert(&F == &OldF && "Called with an unexpected function!");
         return FAM.getResult<AAManager>(F);
       };
+      const TargetLibraryInfo *TLI = &FAM.getResult<TargetLibraryAnalysis>(OldF);
 
-      Function *NewF = promoteArguments(&OldF, AARGetter, MaxElements, None);
+      Function *NewF = promoteArguments(&OldF, AARGetter, MaxElements, None,
+                                        *TLI);
       if (!NewF)
         continue;
       LocalChange = true;
@@ -1056,6 +1065,8 @@
   // Get the callgraph information that we need to update to reflect our
   // changes.
   CallGraph &CG = getAnalysis<CallGraphWrapperPass>().getCallGraph();
+  TargetLibraryInfo const& TLI =
+    getAnalysis<TargetLibraryInfoWrapperPass>().getTLI();
 
   LegacyAARGetter AARGetter(*this);
 
@@ -1079,7 +1090,7 @@
       };
 
       if (Function *NewF = promoteArguments(OldF, AARGetter, MaxElements,
-                                            {ReplaceCallSite})) {
+                                            {ReplaceCallSite}, TLI)) {
         LocalChange = true;
 
         // Update the call graph for the newly promoted function.
Index: lib/Transforms/IPO/DeadArgumentElimination.cpp
===================================================================
--- lib/Transforms/IPO/DeadArgumentElimination.cpp
+++ lib/Transforms/IPO/DeadArgumentElimination.cpp
@@ -20,6 +20,7 @@
 #include "llvm/Transforms/IPO/DeadArgumentElimination.h"
 #include "llvm/ADT/SmallVector.h"
 #include "llvm/ADT/Statistic.h"
+#include "llvm/Analysis/TargetLibraryInfo.h"
 #include "llvm/IR/Argument.h"
 #include "llvm/IR/Attributes.h"
 #include "llvm/IR/BasicBlock.h"
@@ -79,6 +80,7 @@
         return false;
       DeadArgumentEliminationPass DAEP(ShouldHackArguments());
       ModuleAnalysisManager DummyMAM;
+      DummyMAM.registerPass([&] { return TargetLibraryAnalysis(); });
       PreservedAnalyses PA = DAEP.run(M, DummyMAM);
       return !PA.areAllPreserved();
     }
@@ -480,7 +482,8 @@
 //
 // We consider arguments of non-internal functions to be intrinsically alive as
 // well as arguments to functions which have their "address taken".
-void DeadArgumentEliminationPass::SurveyFunction(const Function &F) {
+void DeadArgumentEliminationPass::SurveyFunction(const Function &F,
+                                                 const TargetLibraryInfo &TLI) {
   // Functions with inalloca parameters are expecting args in a particular
   // register and memory layout.
   if (F.getAttributes().hasAttrSomewhere(Attribute::InAlloca)) {
@@ -533,7 +536,10 @@
                       << " has musttail calls\n");
   }
 
-  if (!F.hasLocalLinkage() && (!ShouldHackArguments || F.isIntrinsic())) {
+  bool CanTouchArguments = ShouldHackArguments && !F.isIntrinsic();
+  LibFunc LF;
+  bool IsLibFunc = TLI.getLibFunc(F, LF);
+  if ((!F.hasLocalLinkage() && !CanTouchArguments) || IsLibFunc) {
     MarkLive(F);
     return;
   }
@@ -1079,8 +1085,9 @@
 }
 
 PreservedAnalyses DeadArgumentEliminationPass::run(Module &M,
-                                                   ModuleAnalysisManager &) {
+                                                   ModuleAnalysisManager &AM) {
   bool Changed = false;
+  const TargetLibraryInfo *TLI = &AM.getResult<TargetLibraryAnalysis>(M);
 
   // First pass: Do a simple check to see if any functions can have their "..."
   // removed.  We can do this if they never call va_start.  This loop cannot be
@@ -1099,7 +1106,7 @@
   //
   LLVM_DEBUG(dbgs() << "DeadArgumentEliminationPass - Determining liveness\n");
   for (auto &F : M)
-    SurveyFunction(F);
+    SurveyFunction(F, *TLI);
 
   // Now, remove all dead arguments and return values from each function in
   // turn.
Index: test/Other/ipo-libfunc-signature.ll
===================================================================
--- /dev/null
+++ test/Other/ipo-libfunc-signature.ll
@@ -0,0 +1,29 @@
+; RUN: opt %s -S -internalize -internalize-public-api-list=main -std-link-opts | FileCheck %s
+
+@Msg = hidden global [32 x i8] zeroinitializer, align 16
+@main.Parms = internal global [64 x i8] zeroinitializer, align 16
+
+; Function Attrs: nounwind
+define hidden void @main() local_unnamed_addr {
+entry:
+  %call150 = call i8* @strcat(
+    i8* getelementptr inbounds ([64 x i8], [64 x i8]* @main.Parms, i32 0, i32 0),
+    i8* getelementptr inbounds ([32 x i8], [32 x i8]* @Msg, i32 0, i32 0))
+  ret void
+}
+
+declare hidden i8* @strcpy(i8* noalias returned, i8* noalias) local_unnamed_addr
+declare hidden i32 @strlen(i8* %s) local_unnamed_addr
+
+
+; CHECK: define{{.*}}i8* @strcat(i8*{{.*}}%dest, i8*{{.*}}%src)
+; Function Attrs: noinline nounwind optsize
+define hidden i8* @strcat(i8* noalias returned %dest, i8* noalias %src) local_unnamed_addr #0 {
+entry:
+  %call = tail call i32 @strlen(i8* %dest)
+  %add.ptr = getelementptr inbounds i8, i8* %dest, i32 %call
+  %call1 = tail call i8* @strcpy(i8* %add.ptr, i8* %src)
+  ret i8* %dest
+}
+
+attributes #0 = { noinline nounwind optsize }