diff --git a/llvm/include/llvm/MC/MCStreamer.h b/llvm/include/llvm/MC/MCStreamer.h
--- a/llvm/include/llvm/MC/MCStreamer.h
+++ b/llvm/include/llvm/MC/MCStreamer.h
@@ -19,6 +19,7 @@
 #include "llvm/ADT/Optional.h"
 #include "llvm/ADT/SmallVector.h"
 #include "llvm/ADT/StringRef.h"
+#include "llvm/ADT/StringSet.h"
 #include "llvm/MC/MCDirectives.h"
 #include "llvm/MC/MCLinkerOptimizationHint.h"
 #include "llvm/MC/MCPseudoProbe.h"
@@ -237,6 +238,19 @@
   /// discussion for future inclusion.
   bool AllowAutoPadding = false;
 
+  /// Tracks the symbols we have emitted for conditional assignments.
+  StringSet<> emittedSymbols;
+
+  struct PendingAssignment {
+    MCSymbol *Symbol;
+    const MCExpr *Value;
+    MCSymbolAttr Attr;
+  };
+
+  /// A list of conditional assignments we may need to emit if the target
+  /// symbol is later emitted.
+  StringMap<SmallVector<PendingAssignment, 1>> pendingAssignments;
+
 protected:
   MCStreamer(MCContext &Ctx);
 
@@ -256,6 +270,10 @@
   /// Returns true if the the .cv_loc directive is in the right section.
   bool checkCVLocSection(unsigned FuncId, unsigned FileNo, SMLoc Loc);
 
+  /// Emits pending conditional assignments that depend on \p Symbol
+  /// being emitted.
+  void emitPendingAssignments(MCSymbol *Symbol);
+
 public:
   MCStreamer(const MCStreamer &) = delete;
   MCStreamer &operator=(const MCStreamer &) = delete;
@@ -516,6 +534,11 @@
   /// \param Value - The value for the symbol.
   virtual void emitAssignment(MCSymbol *Symbol, const MCExpr *Value);
 
+  /// Emit an assignment of \p Value to \p Symbol, but only if \p Value is also
+  /// emitted. If \p Attr is specified, also emit the symbol attribute.
+  virtual void emitConditionalAssignment(MCSymbol *Symbol, const MCExpr *Value,
+                                         MCSymbolAttr Attr = MCSA_Invalid);
+
   /// Emit an weak reference from \p Alias to \p Symbol.
   ///
   /// This corresponds to an assembler statement such as:
diff --git a/llvm/lib/MC/MCParser/AsmParser.cpp b/llvm/lib/MC/MCParser/AsmParser.cpp
--- a/llvm/lib/MC/MCParser/AsmParser.cpp
+++ b/llvm/lib/MC/MCParser/AsmParser.cpp
@@ -357,7 +357,7 @@
   StringRef parseStringToComma();
 
   bool parseAssignment(StringRef Name, bool allow_redef,
-                       bool NoDeadStrip = false);
+                       bool NoDeadStrip = false, bool Cond = false);
 
   unsigned getBinOpPrecedence(AsmToken::TokenKind K,
                               MCBinaryExpr::Opcode &Kind);
@@ -534,6 +534,7 @@
     DK_ADDRSIG_SYM,
     DK_PSEUDO_PROBE,
     DK_LTO_DISCARD,
+    DK_SET_CONDITIONAL,
     DK_END
   };
 
@@ -564,8 +565,8 @@
                                const fltSemantics &); // ".single", ...
   bool parseDirectiveFill(); // ".fill"
   bool parseDirectiveZero(); // ".zero"
-  // ".set", ".equ", ".equiv"
-  bool parseDirectiveSet(StringRef IDVal, bool allow_redef);
+  // ".set", ".equ", ".equiv", ".set_conditional"
+  bool parseDirectiveSet(StringRef IDVal, bool allow_redef, bool Cond = false);
   bool parseDirectiveOrg(); // ".org"
   // ".align{,32}", ".p2align{,w,l}"
   bool parseDirectiveAlign(bool IsPow2, unsigned ValueSize);
@@ -2030,6 +2031,8 @@
       return parseDirectiveSet(IDVal, true);
     case DK_EQUIV:
       return parseDirectiveSet(IDVal, false);
+    case DK_SET_CONDITIONAL:
+      return parseDirectiveSet(IDVal, true, true);
     case DK_ASCII:
       return parseDirectiveAscii(IDVal, false);
     case DK_ASCIZ:
@@ -2926,9 +2929,10 @@
 }
 
 bool AsmParser::parseAssignment(StringRef Name, bool allow_redef,
-                                bool NoDeadStrip) {
+                                bool NoDeadStrip, bool Cond) {
   MCSymbol *Sym;
   const MCExpr *Value;
+  SMLoc ExprLoc = getTok().getLoc();
   if (MCParserUtils::parseAssignmentExpression(Name, allow_redef, *this, Sym,
                                                Value))
     return true;
@@ -2944,9 +2948,17 @@
     return false;
 
   // Do the assignment.
-  Out.emitAssignment(Sym, Value);
-  if (NoDeadStrip)
-    Out.emitSymbolAttribute(Sym, MCSA_NoDeadStrip);
+  if (Cond) {
+    if (Value->getKind() != MCExpr::SymbolRef)
+      return Error(ExprLoc, "expected identifier");
+
+    Out.emitConditionalAssignment(
+        Sym, Value, NoDeadStrip ? MCSA_NoDeadStrip : MCSA_Invalid);
+  } else {
+    Out.emitAssignment(Sym, Value);
+    if (NoDeadStrip)
+      Out.emitSymbolAttribute(Sym, MCSA_NoDeadStrip);
+  }
 
   return false;
 }
@@ -2998,10 +3010,12 @@
 ///   ::= .equ identifier ',' expression
 ///   ::= .equiv identifier ',' expression
 ///   ::= .set identifier ',' expression
-bool AsmParser::parseDirectiveSet(StringRef IDVal, bool allow_redef) {
+///   ::= .set_conditional identifier ',' expression
+bool AsmParser::parseDirectiveSet(StringRef IDVal, bool allow_redef,
+                                  bool Cond) {
   StringRef Name;
   if (check(parseIdentifier(Name), "expected identifier") || parseComma() ||
-      parseAssignment(Name, allow_redef, true))
+      parseAssignment(Name, allow_redef, true, Cond))
     return true;
   return false;
 }
@@ -5581,6 +5595,7 @@
   DirectiveKindMap[".addrsig_sym"] = DK_ADDRSIG_SYM;
   DirectiveKindMap[".pseudoprobe"] = DK_PSEUDO_PROBE;
   DirectiveKindMap[".lto_discard"] = DK_LTO_DISCARD;
+  DirectiveKindMap[".set_conditional"] = DK_SET_CONDITIONAL;
 }
 
 MCAsmMacro *AsmParser::parseMacroLikeBody(SMLoc DirectiveLoc) {
diff --git a/llvm/lib/MC/MCStreamer.cpp b/llvm/lib/MC/MCStreamer.cpp
--- a/llvm/lib/MC/MCStreamer.cpp
+++ b/llvm/lib/MC/MCStreamer.cpp
@@ -429,6 +429,38 @@
   MCTargetStreamer *TS = getTargetStreamer();
   if (TS)
     TS->emitLabel(Symbol);
+
+  emitPendingAssignments(Symbol);
+}
+
+void MCStreamer::emitPendingAssignments(MCSymbol *Symbol) {
+  auto Assignments = pendingAssignments.find(Symbol->getName());
+  if (Assignments != pendingAssignments.end()) {
+    for (const auto &A : Assignments->second) {
+      emitAssignment(A.Symbol, A.Value);
+      if (A.Attr != MCSA_Invalid)
+        emitSymbolAttribute(A.Symbol, A.Attr);
+    }
+
+    pendingAssignments.erase(Assignments);
+  }
+
+  emittedSymbols.insert(Symbol->getName());
+}
+
+void MCStreamer::emitConditionalAssignment(MCSymbol *Symbol,
+                                           const MCExpr *Value,
+                                           MCSymbolAttr Attr) {
+  StringRef Target = cast<MCSymbolRefExpr>(*Value).getSymbol().getName();
+
+  // If the symbol already exists, emit the assignment. Otherwise, emit it
+  // later only if the symbol is also emitted.
+  if (emittedSymbols.contains(Target)) {
+    emitAssignment(Symbol, Value);
+    if (Attr != MCSA_Invalid)
+      emitSymbolAttribute(Symbol, Attr);
+  } else
+    pendingAssignments[Target].push_back({Symbol, Value, Attr});
 }
 
 void MCStreamer::emitCFISections(bool EH, bool Debug) {}
@@ -1046,6 +1078,8 @@
   MCTargetStreamer *TS = getTargetStreamer();
   if (TS)
     TS->emitAssignment(Symbol, Value);
+
+  emitPendingAssignments(Symbol);
 }
 
 void MCTargetStreamer::prettyPrintAsm(MCInstPrinter &InstPrinter,
diff --git a/llvm/lib/Transforms/IPO/ThinLTOBitcodeWriter.cpp b/llvm/lib/Transforms/IPO/ThinLTOBitcodeWriter.cpp
--- a/llvm/lib/Transforms/IPO/ThinLTOBitcodeWriter.cpp
+++ b/llvm/lib/Transforms/IPO/ThinLTOBitcodeWriter.cpp
@@ -87,7 +87,7 @@
     if (isa<Function>(&ExportGV) && allowPromotionAlias(OldName)) {
       // Create a local alias with the original name to avoid breaking
       // references from inline assembly.
-      std::string Alias = ".set " + OldName + "," + NewName + "\n";
+      std::string Alias = ".set_conditional " + OldName + "," + NewName + "\n";
       ExportM.appendModuleInlineAsm(Alias);
     }
   }
diff --git a/llvm/test/MC/ELF/set-conditional.s b/llvm/test/MC/ELF/set-conditional.s
new file mode 100644
--- /dev/null
+++ b/llvm/test/MC/ELF/set-conditional.s
@@ -0,0 +1,31 @@
+# RUN: llvm-mc -triple x86_64-pc-linux-gnu < %s | FileCheck %s
+# RUN: not llvm-mc -filetype=obj -triple x86_64-pc-linux-gnu --defsym ERR=1 %s 2>&1 |\
+# RUN:         FileCheck %s --check-prefix=ERR
+
+# CHECK: .set b, a
+# CHECK: .set c, b
+# CHECK: .set d, a
+.set_conditional b, a
+.set_conditional d, a
+.set_conditional c, b
+
+# CHECK-NOT: .set e, n
+.set_conditional e, n
+
+# CHECK: a:
+# CHECK-NEXT: .byte 1
+a:
+  .byte 1
+
+# CHECK: .set f, a
+.set_conditional f, a
+# CHECK: .set g, b
+.set_conditional g, b
+# CHECK-NOT: .set h, m
+.set_conditional h, m
+
+.ifdef ERR
+.text
+# ERR: {{.*}}.s:[[#@LINE+1]]:21: error: expected identifier
+.set_conditional i, ERR
+.endif
diff --git a/llvm/test/MC/MachO/set-conditional.s b/llvm/test/MC/MachO/set-conditional.s
new file mode 100644
--- /dev/null
+++ b/llvm/test/MC/MachO/set-conditional.s
@@ -0,0 +1,37 @@
+# RUN: llvm-mc -triple i386-apple-darwin9 < %s | FileCheck %s
+# RUN: not llvm-mc -filetype=obj -triple i386-apple-darwin9 --defsym ERR=1 %s 2>&1 |\
+# RUN:         FileCheck %s --check-prefix=ERR
+
+# CHECK: .set b, a
+# CHECK: .set c, b
+# CHECK: .no_dead_strip c
+# CHECK: .no_dead_strip b
+# CHECK: .set d, a
+# CHECK: .no_dead_strip d
+.set_conditional b, a
+.set_conditional d, a
+.set_conditional c, b
+# CHECK-NOT: .set e, n
+# CHECK-NOT: .no_dead_strip e
+.set_conditional e, n
+
+# CHECK: a:
+# CHECK-NEXT: .byte 1
+a:
+  .byte 1
+
+# CHECK: .set f, a
+# CHECK: .no_dead_strip f
+.set_conditional f, a
+# CHECK: .set g, b
+# CHECK: .no_dead_strip g
+.set_conditional g, b
+# CHECK-NOT: .set h, m
+# CHECK-NOT: .no_dead_strip h
+.set_conditional h, m
+
+.ifdef ERR
+.text
+# ERR: {{.*}}.s:[[#@LINE+1]]:21: error: expected identifier
+.set_conditional i, ERR
+.endif
diff --git a/llvm/test/ThinLTO/X86/devirt2.ll b/llvm/test/ThinLTO/X86/devirt2.ll
--- a/llvm/test/ThinLTO/X86/devirt2.ll
+++ b/llvm/test/ThinLTO/X86/devirt2.ll
@@ -131,12 +131,10 @@
 ; RUN:   -r=%t1.o,_ZN1D1mEi, \
 ; RUN:   -r=%t1.o,test2, \
 ; RUN:   -r=%t2.o,_ZN1A1nEi,p \
-; RUN:   -r=%t2.o,_ZN1A1nEi, \
 ; RUN:   -r=%t2.o,_ZN1B1fEi,p \
 ; RUN:   -r=%t2.o,_ZN1C1fEi,p \
 ; RUN:   -r=%t2.o,_ZN1D1mEi,p \
 ; RUN:   -r=%t2.o,_ZN1E1mEi,p \
-; RUN:   -r=%t2.o,_ZN1E1mEi, \
 ; RUN:   -r=%t2.o,_ZTV1B, \
 ; RUN:   -r=%t2.o,_ZTV1C, \
 ; RUN:   -r=%t2.o,_ZTV1D, \
@@ -169,12 +167,10 @@
 ; RUN:   -r=%t1.o,_ZN1D1mEi, \
 ; RUN:   -r=%t1.o,test2, \
 ; RUN:   -r=%t2.o,_ZN1A1nEi,p \
-; RUN:   -r=%t2.o,_ZN1A1nEi, \
 ; RUN:   -r=%t2.o,_ZN1B1fEi,p \
 ; RUN:   -r=%t2.o,_ZN1C1fEi,p \
 ; RUN:   -r=%t2.o,_ZN1D1mEi,p \
 ; RUN:   -r=%t2.o,_ZN1E1mEi,p \
-; RUN:   -r=%t2.o,_ZN1E1mEi, \
 ; RUN:   -r=%t2.o,_ZTV1B, \
 ; RUN:   -r=%t2.o,_ZTV1C, \
 ; RUN:   -r=%t2.o,_ZTV1D, \
diff --git a/llvm/test/Transforms/ThinLTOBitcodeWriter/cfi-icall-static-inline-asm.ll b/llvm/test/Transforms/ThinLTOBitcodeWriter/cfi-icall-static-inline-asm.ll
--- a/llvm/test/Transforms/ThinLTOBitcodeWriter/cfi-icall-static-inline-asm.ll
+++ b/llvm/test/Transforms/ThinLTOBitcodeWriter/cfi-icall-static-inline-asm.ll
@@ -3,7 +3,7 @@
 
 target triple = "x86_64-unknown-linux-gnu"
 
-; CHECK: module asm ".set a,a.[[HASH:[0-9a-f]+]]"
+; CHECK: module asm ".set_conditional a,a.[[HASH:[0-9a-f]+]]"
 
 define void @b() {
   %f = alloca void ()*, align 8