Index: ELF/Config.h
===================================================================
--- ELF/Config.h
+++ ELF/Config.h
@@ -151,6 +151,7 @@
   bool Verbose;
   bool WarnCommon;
   bool WarnMissingEntry;
+  bool WarnSymbolOrdering;
   bool ZCombreloc;
   bool ZExecstack;
   bool ZNocopyreloc;
Index: ELF/Driver.cpp
===================================================================
--- ELF/Driver.cpp
+++ ELF/Driver.cpp
@@ -670,6 +670,8 @@
   Config->Verbose = Args.hasArg(OPT_verbose);
   errorHandler().Verbose = Config->Verbose;
   Config->WarnCommon = Args.hasArg(OPT_warn_common);
+  Config->WarnSymbolOrdering =
+      Args.hasFlag(OPT_warn_symbol_ordering, OPT_no_warn_symbol_ordering, true);
   Config->ZCombreloc = !hasZOption(Args, "nocombreloc");
   Config->ZExecstack = hasZOption(Args, "execstack");
   Config->ZNocopyreloc = hasZOption(Args, "nocopyreloc");
Index: ELF/Options.td
===================================================================
--- ELF/Options.td
+++ ELF/Options.td
@@ -208,6 +208,9 @@
 def no_threads: F<"no-threads">,
   HelpText<"Do not run the linker multi-threaded">;
 
+def no_warn_symbol_ordering: F<"no-warn-symbol-ordering">,
+  HelpText<"Do not warn about problems with the symbol ordering file">;
+
 def no_whole_archive: F<"no-whole-archive">,
   HelpText<"Restores the default behavior of loading archive members">;
 
@@ -323,6 +326,9 @@
 def warn_common: F<"warn-common">,
   HelpText<"Warn about duplicate common symbols">;
 
+def warn_symbol_ordering: F<"warn-symbol-ordering">,
+  HelpText<"Warn about problems with the symbol ordering file">;
+
 def warn_unresolved_symbols: F<"warn-unresolved-symbols">,
   HelpText<"Report unresolved symbols as warnings">;
 
Index: ELF/Writer.cpp
===================================================================
--- ELF/Writer.cpp
+++ ELF/Writer.cpp
@@ -1029,21 +1029,54 @@
   // Build a map from symbols to their priorities. Symbols that didn't
   // appear in the symbol ordering file have the lowest priority 0.
   // All explicitly mentioned symbols have negative (higher) priorities.
-  DenseMap<StringRef, int> SymbolOrder;
+  // The values are of the form {priority, present in inputs}.
+  DenseMap<StringRef, std::pair<int, bool>> SymbolOrder;
   int Priority = -Config->SymbolOrderingFile.size();
-  for (StringRef S : Config->SymbolOrderingFile)
-    SymbolOrder.insert({S, Priority++});
+  for (StringRef S : Config->SymbolOrderingFile) {
+    auto Inserted = SymbolOrder.insert({S, {Priority++, false}});
+    if (Config->WarnSymbolOrdering && !Inserted.second)
+      warn("symbol ordering file: symbol '" + S + "' specified multiple times");
+  }
 
   // Build a map from sections to their priorities.
   for (InputFile *File : ObjectFiles) {
     for (Symbol *Sym : File->getSymbols()) {
       auto *D = dyn_cast<Defined>(Sym);
+      auto OrderEntry = SymbolOrder.find(Sym->getName());
+      if (Config->WarnSymbolOrdering && OrderEntry != SymbolOrder.end()) {
+        if (Sym->isUndefined())
+          warn("symbol ordering file: unable to order undefined symbol '" +
+               Sym->getName() + "' from file '" + File->getName() + "'");
+        else if (Sym->isShared())
+          warn("symbol ordering file: unable to order shared symbol '" +
+               Sym->getName() + "' from file '" + File->getName() + "'");
+        else if (D && !D->Section)
+          warn("symbol ordering file: unable to order absolute symbol '" +
+               Sym->getName() + "' from file '" + File->getName() + "'");
+        else if (D && !D->Section->Live)
+          warn("symbol ordering file: unable to order discarded symbol '" +
+               Sym->getName() + "' from file '" + File->getName() + "'");
+        OrderEntry->second.second = true;
+      }
       if (!D || !D->Section)
         continue;
+
       int &Priority = SectionOrder[D->Section];
-      Priority = std::min(Priority, SymbolOrder.lookup(D->getName()));
+      int OrderPriority = 0;
+      if (OrderEntry != SymbolOrder.end())
+        OrderPriority = OrderEntry->second.first;
+      Priority = std::min(Priority, OrderPriority);
     }
   }
+
+  if (Config->WarnSymbolOrdering)
+    for (auto V : SymbolOrder) {
+      bool Used = V.second.second;
+      if (!Used)
+        warn("symbol ordering file: no symbol '" + V.first +
+             "' found in input");
+    }
+
   return SectionOrder;
 }
 
Index: test/ELF/Inputs/symbol-ordering-file-warnings1.s
===================================================================
--- test/ELF/Inputs/symbol-ordering-file-warnings1.s
+++ test/ELF/Inputs/symbol-ordering-file-warnings1.s
@@ -0,0 +1,19 @@
+# This is a "bad" (absolute) instance of the symbol
+multi = 1234
+
+.text
+.global shared
+.type shared, @function
+shared:
+  movq  %rax, multi
+  ret
+
+.section .text.comdat,"axG",@progbits,comdat,comdat
+.weak comdat
+comdat:
+  ret
+
+.section .text.glob_or_wk,"ax",@progbits
+.global glob_or_wk
+glob_or_wk:
+  ret
Index: test/ELF/Inputs/symbol-ordering-file-warnings2.s
===================================================================
--- test/ELF/Inputs/symbol-ordering-file-warnings2.s
+++ test/ELF/Inputs/symbol-ordering-file-warnings2.s
@@ -0,0 +1,6 @@
+.text
+.global missing
+missing:
+  callq undefined
+  # This is a "bad" (undefined) instance of the symbol
+  callq multi
Index: test/ELF/symbol-ordering-file-warnings.s
===================================================================
--- test/ELF/symbol-ordering-file-warnings.s
+++ test/ELF/symbol-ordering-file-warnings.s
@@ -0,0 +1,141 @@
+# REQUIRES: x86
+# RUN: llvm-mc -filetype=obj -triple=x86_64-pc-linux %s -o %t.o
+# RUN: llvm-mc -filetype=obj -triple=x86_64-pc-linux %S/Inputs/symbol-ordering-file-warnings1.s -o %t1.o
+# RUN: llvm-mc -filetype=obj -triple=x86_64-pc-linux %S/Inputs/symbol-ordering-file-warnings2.s -o %t2.o
+# RUN: ld.lld -shared %t1.o -o %t.so
+
+# Check that a warning is emitted for entries in the file that are not present in any used input.
+# RUN: echo "missing" > %t-order-missing.txt
+# RUN: ld.lld %t.o -o %t --symbol-ordering-file %t-order-missing.txt --unresolved-symbols=ignore-all 2>&1 | \
+# RUN:   FileCheck %s --check-prefixes=WARN,MISSING
+
+# Check that the warning can be disabled.
+# RUN: ld.lld %t.o -o %t --symbol-ordering-file %t-order-missing.txt --unresolved-symbols=ignore-all --no-warn-symbol-ordering 2>&1 | \
+# RUN:   FileCheck %s --check-prefixes=WARN --allow-empty
+
+# Check that the warning can be re-enabled
+# RUN: ld.lld %t.o -o %t --symbol-ordering-file %t-order-missing.txt --unresolved-symbols=ignore-all --no-warn-symbol-ordering --warn-symbol-ordering 2>&1 | \
+# RUN:   FileCheck %s --check-prefixes=WARN,MISSING
+
+# Check that a warning is emitted for undefined symbols.
+# RUN: echo "undefined" > %t-order-undef.txt
+# RUN: ld.lld %t.o %t2.o -o %t --symbol-ordering-file %t-order-undef.txt --unresolved-symbols=ignore-all 2>&1 | \
+# RUN:   FileCheck %s --check-prefixes=WARN,UNDEFINED
+
+# Check that a warning is emitted for imported shared symbols.
+# RUN: echo "shared" > %t-order-shared.txt
+# RUN: ld.lld %t.o %t.so -o %t --symbol-ordering-file %t-order-shared.txt --unresolved-symbols=ignore-all 2>&1 | \
+# RUN:   FileCheck %s --check-prefixes=WARN,SHARED
+
+# Check that a warning is emitted for absolute symbols.
+# RUN: echo "absolute" > %t-order-absolute.txt
+# RUN: ld.lld %t.o -o %t --symbol-ordering-file %t-order-absolute.txt --unresolved-symbols=ignore-all 2>&1 | \
+# RUN:   FileCheck %s --check-prefixes=WARN,ABSOLUTE
+
+# Check that a warning is emitted for symbols discarded due to --gc-sections.
+# RUN: echo "gc" > %t-order-gc.txt
+# RUN: ld.lld %t.o -o %t --symbol-ordering-file %t-order-gc.txt --gc-sections --unresolved-symbols=ignore-all 2>&1 | \
+# RUN:   FileCheck %s --check-prefixes=WARN,GC
+
+# Check that a warning is emitted for symbols discarded due to a linker script /DISCARD/ section.
+# RUN: echo "discard" > %t-order-discard.txt
+# RUN: echo "SECTIONS { /DISCARD/ : { *(.text.discard) } }" > %t.script
+# RUN: ld.lld %t.o -o %t --symbol-ordering-file %t-order-discard.txt -T %t.script --unresolved-symbols=ignore-all 2>&1 | \
+# RUN:   FileCheck %s --check-prefixes=WARN,DISCARD
+
+# Check that LLD does not warn for discarded COMDAT symbols, if at least instance was kept.
+# RUN: echo "comdat" > %t-order-comdat.txt
+# RUN: ld.lld %t.o %t1.o -o %t --symbol-ordering-file %t-order-comdat.txt --unresolved-symbols=ignore-all 2>&1 | \
+# RUN:   FileCheck %s --check-prefixes=WARN --allow-empty
+
+# Check that if a COMDAT was unused and discarded via --gc-sections, warn for each instance.
+# RUN: ld.lld %t.o %t1.o -o %t --symbol-ordering-file %t-order-comdat.txt --gc-sections --unresolved-symbols=ignore-all 2>&1 | \
+# RUN:   FileCheck %s --check-prefixes=WARN,COMDAT
+
+# Check that if a weak symbol is not kept, because of an equivalent global symbol, no warning is emitted.
+# RUN: echo "glob_or_wk" > %t-order-weak.txt
+# RUN: ld.lld %t.o %t1.o -o %t --symbol-ordering-file %t-order-weak.txt --unresolved-symbols=ignore-all 2>&1 | \
+# RUN:   FileCheck %s --check-prefixes=WARN --allow-empty
+
+# Check that symbols only in unused archive members result in a warning.
+# RUN: rm -f %t.a
+# RUN: llvm-ar rc %t.a %t2.o
+# RUN: ld.lld %t.o %t.a -o %t --symbol-ordering-file %t-order-missing.txt --unresolved-symbols=ignore-all 2>&1 | \
+# RUN:   FileCheck %s --check-prefixes=WARN,MISSING --allow-empty
+
+# Check that a warning for each same-named symbol with an issue.
+# RUN: echo "multi" > %t-order-same-name.txt
+# RUN: ld.lld %t.o %t1.o %t2.o -o %t --symbol-ordering-file %t-order-same-name.txt --unresolved-symbols=ignore-all 2>&1 | \
+# RUN:   FileCheck %s --check-prefixes=WARN,MULTI
+
+# Check that a warning is emitted if the same symbol is mentioned multiple times in the ordering file.
+# RUN: echo "_start" > %t-order-multiple-same.txt
+# RUN: echo "_start" >> %t-order-multiple-same.txt
+# RUN: ld.lld %t.o -o %t --symbol-ordering-file %t-order-multiple-same.txt --unresolved-symbols=ignore-all 2>&1 | \
+# RUN:   FileCheck %s --check-prefixes=WARN,SAMESYM
+
+# Check that all warnings can be emitted from the same input.
+# RUN: echo "missing_sym" > %t-order-multi.txt
+# RUN: echo "undefined" >> %t-order-multi.txt
+# RUN: echo "_start" >> %t-order-multi.txt
+# RUN: echo "shared" >> %t-order-multi.txt
+# RUN: echo "absolute" >> %t-order-multi.txt
+# RUN: echo "gc" >> %t-order-multi.txt
+# RUN: echo "discard" >> %t-order-multi.txt
+# RUN: echo "_start" >> %t-order-multi.txt
+# RUN: ld.lld %t.o %t2.o %t.so -o %t --symbol-ordering-file %t-order-multi.txt --gc-sections -T %t.script --unresolved-symbols=ignore-all 2>&1 | \
+# RUN:   FileCheck %s --check-prefixes=WARN,SAMESYM,ABSOLUTE,SHARED,UNDEFINED,GC,DISCARD,MISSING2
+
+# WARN-NOT:    warning:
+# SAMESYM:     warning: symbol ordering file: symbol '_start' specified multiple times
+# WARN-NOT:    warning:
+# ABSOLUTE:    warning: symbol ordering file: unable to order absolute symbol 'absolute' from file
+# WARN-NOT:    warning:
+# DISCARD:     warning: symbol ordering file: unable to order discarded symbol 'discard' from file
+# WARN-NOT:    warning:
+# GC:          warning: symbol ordering file: unable to order discarded symbol 'gc' from file
+# WARN-NOT:    warning:
+# SHARED:      warning: symbol ordering file: unable to order shared symbol 'shared' from file
+# WARN-NOT:    warning:
+# UNDEFINED:   warning: symbol ordering file: unable to order undefined symbol 'undefined' from file
+# WARN-NOT:    warning:
+# MISSING:     warning: symbol ordering file: no symbol 'missing' found in input
+# MISSING2:    warning: symbol ordering file: no symbol 'missing_sym' found in input
+# COMDAT:      warning: symbol ordering file: unable to order discarded symbol 'comdat' from file
+# COMDAT-NEXT: warning: symbol ordering file: unable to order discarded symbol 'comdat' from file
+# MULTI:       warning: symbol ordering file: unable to order absolute symbol 'multi' from file
+# MULTI-NEXT:  warning: symbol ordering file: unable to order undefined symbol 'multi' from file
+# WARN-NOT:    warning:
+
+absolute = 0x1234
+.global absolute
+
+.section .text.gc,"ax",@progbits
+.global gc
+gc:
+  nop
+
+.section .text.discard,"ax",@progbits
+.global discard
+discard:
+  nop
+
+.section .text.comdat,"axG",@progbits,comdat,comdat
+.weak comdat
+comdat:
+  nop
+
+.section .text.glob_or_wk,"ax",@progbits
+.weak glob_or_wk
+glob_or_wk:
+  nop
+
+.text
+.global _start
+_start:
+  movq  %rax, absolute
+  callq shared
+
+# This is a "good" instance of the symbol
+multi:
+  nop