Index: lld/trunk/ELF/CMakeLists.txt
===================================================================
--- lld/trunk/ELF/CMakeLists.txt
+++ lld/trunk/ELF/CMakeLists.txt
@@ -31,6 +31,7 @@
   Core
   IPO
   Linker
+  LTO
   Object
   Option
   Passes
Index: lld/trunk/ELF/LTO.h
===================================================================
--- lld/trunk/ELF/LTO.h
+++ lld/trunk/ELF/LTO.h
@@ -47,6 +47,7 @@
   llvm::IRMover Mover;
   std::vector<SmallString<0>> OwningData;
   llvm::StringSet<> InternalizedSyms;
+  llvm::StringSet<> AsmUndefinedRefs;
   std::string TheTriple;
 };
 }
Index: lld/trunk/ELF/LTO.cpp
===================================================================
--- lld/trunk/ELF/LTO.cpp
+++ lld/trunk/ELF/LTO.cpp
@@ -25,6 +25,7 @@
 #include "llvm/IR/LegacyPassManager.h"
 #include "llvm/IR/PassManager.h"
 #include "llvm/IR/Verifier.h"
+#include "llvm/LTO/UpdateCompilerUsed.h"
 #include "llvm/Linker/IRMover.h"
 #include "llvm/Passes/PassBuilder.h"
 #include "llvm/Support/StringSaver.h"
@@ -153,6 +154,21 @@
   replaceBody<Undefined>(S, S->body()->getName(), STV_DEFAULT, S->body()->Type);
 }
 
+static void handleUndefinedAsmRefs(const BasicSymbolRef &Sym, GlobalValue *GV,
+                                   StringSet<> &AsmUndefinedRefs) {
+  // GV associated => not an assembly symbol, bail out.
+  if (GV)
+    return;
+
+  // This is an undefined reference to a symbol in asm. We put that in
+  // compiler.used, so that we can preserve it from being dropped from
+  // the output, without necessarily preventing its internalization.
+  SmallString<64> Name;
+  raw_svector_ostream OS(Name);
+  Sym.printName(OS);
+  AsmUndefinedRefs.insert(Name.str());
+}
+
 void BitcodeCompiler::add(BitcodeFile &F) {
   std::unique_ptr<IRObjectFile> Obj = std::move(F.Obj);
   std::vector<GlobalValue *> Keep;
@@ -181,8 +197,10 @@
     if (BitcodeFile::shouldSkip(Flags))
       continue;
     Symbol *S = Syms[BodyIndex++];
-    if (Flags & BasicSymbolRef::SF_Undefined)
+    if (Flags & BasicSymbolRef::SF_Undefined) {
+      handleUndefinedAsmRefs(Sym, GV, AsmUndefinedRefs);
       continue;
+    }
     auto *B = dyn_cast<DefinedBitcode>(S->body());
     if (!B || B->File != &F)
       continue;
@@ -271,9 +289,6 @@
     internalize(*GV);
   }
 
-  if (Config->SaveTemps)
-    saveBCFile(*Combined, ".lto.bc");
-
   std::string Msg;
   const Target *T = TargetRegistry::lookupTarget(TheTriple, Msg);
   if (!T)
@@ -291,6 +306,14 @@
   };
 
   std::unique_ptr<TargetMachine> TM = CreateTargetMachine();
+
+  // Update llvm.compiler.used so that optimizations won't strip
+  // off AsmUndefinedReferences.
+  UpdateCompilerUsed(*Combined, *TM, AsmUndefinedRefs);
+
+  if (Config->SaveTemps)
+    saveBCFile(*Combined, ".lto.bc");
+
   runLTOPasses(*Combined, *TM);
   if (HasError)
     return {};
Index: lld/trunk/test/ELF/lto/asmundef.ll
===================================================================
--- lld/trunk/test/ELF/lto/asmundef.ll
+++ lld/trunk/test/ELF/lto/asmundef.ll
@@ -0,0 +1,25 @@
+; REQUIRES: x86
+; RUN: llvm-as %s -o %t.o
+; RUN: ld.lld -m elf_x86_64 %t.o -o %t -save-temps
+; RUN: llvm-dis %t.lto.bc -o - | FileCheck %s
+
+target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-unknown-linux-gnu"
+
+module asm ".weak patatino"
+module asm ".equ patatino, foo"
+
+declare void @patatino()
+
+define void @foo() {
+  ret void
+}
+
+define void @_start() {
+  call void @patatino()
+  ret void
+}
+
+; CHECK: @llvm.compiler.used = appending global [1 x i8*] [i8* bitcast (void ()* @foo to i8*)], section "llvm.metadata"
+; CHECK: define internal void @foo
+