Index: ELF/LinkerScript.h
===================================================================
--- ELF/LinkerScript.h
+++ ELF/LinkerScript.h
@@ -205,7 +205,7 @@
     std::function<uint64_t()> LMAOffset;
   };
 
-  llvm::DenseMap<StringRef, OutputSection *> NameToOutputSection;
+  llvm::DenseMap<StringRef, std::vector<OutputSection *>> NameToOutputSection;
 
   void addSymbol(SymbolAssignment *Cmd);
   void assignSymbol(SymbolAssignment *Cmd, bool InSec);
@@ -242,8 +242,9 @@
   uint64_t Dot;
 
 public:
-  OutputSection *createOutputSection(StringRef Name, StringRef Location);
-  OutputSection *getOrCreateOutputSection(StringRef Name);
+  void declareOutputSection(StringRef Name);
+  OutputSection *defineOutputSection(StringRef Name, StringRef Location);
+  OutputSection *getOutputSection(StringRef Name);
 
   bool hasPhdrsCommands() { return !PhdrsCommands.empty(); }
   uint64_t getDot() { return Dot; }
Index: ELF/LinkerScript.cpp
===================================================================
--- ELF/LinkerScript.cpp
+++ ELF/LinkerScript.cpp
@@ -79,27 +79,45 @@
   return getValue() - getSecAddr();
 }
 
-OutputSection *LinkerScript::createOutputSection(StringRef Name,
+OutputSection *LinkerScript::defineOutputSection(StringRef Name,
                                                  StringRef Location) {
-  OutputSection *&SecRef = NameToOutputSection[Name];
-  OutputSection *Sec;
-  if (SecRef && SecRef->Location.empty()) {
-    // There was a forward reference.
-    Sec = SecRef;
-  } else {
-    Sec = make<OutputSection>(Name, SHT_PROGBITS, 0);
-    if (!SecRef)
-      SecRef = Sec;
+  std::vector<OutputSection *> &V = NameToOutputSection[Name];
+  // If we have no output sections with Name or all sections are already defined,
+  // we should create new section definition. This is used to handle
+  // multiple output section definitions when constraints are used, for example:
+  // bar : ONLY_IF_RO { *(...) }
+  // bar : ONLY_IF_RW { *(...) }
+  if (V.empty() || V.back()->isDefined()) {
+    OutputSection *Sec = make<OutputSection>(Name, SHT_PROGBITS, 0);
+    Sec->Location = Location;
+    V.push_back(Sec);
+    return Sec;
   }
+
+  // When we have forward declaration we should define it and return.
+  OutputSection *Sec = V.back();
   Sec->Location = Location;
   return Sec;
 }
 
-OutputSection *LinkerScript::getOrCreateOutputSection(StringRef Name) {
-  OutputSection *&CmdRef = NameToOutputSection[Name];
-  if (!CmdRef)
-    CmdRef = make<OutputSection>(Name, SHT_PROGBITS, 0);
-  return CmdRef;
+// Returns first live section by Name. Normally only one section should be live.
+// Otherwise commands like SIZEOF(section) will use first alive output section
+// for calculation. Ideally we would like to report an error then, but GNU
+// linkers allow that and we just follow for simplicity.
+OutputSection *LinkerScript::getOutputSection(StringRef Name) {
+  std::vector<OutputSection *> &V = NameToOutputSection[Name];
+  for (OutputSection *Sec : V)
+    if (Sec->Live)
+      return Sec;
+  return V.front();
+}
+
+// Used for creating forward references to output section.
+void LinkerScript::declareOutputSection(StringRef Name) {
+  std::vector<OutputSection *> &V = NameToOutputSection[Name];
+  if (!V.empty())
+    return;
+  V.push_back(make<OutputSection>(Name, SHT_PROGBITS, 0));
 }
 
 void LinkerScript::setDot(Expr E, const Twine &Loc, bool InSec) {
@@ -429,7 +447,7 @@
 
 static OutputSection *createSection(InputSectionBase *IS,
                                     StringRef OutsecName) {
-  OutputSection *Sec = Script->createOutputSection(OutsecName, "<internal>");
+  OutputSection *Sec = Script->defineOutputSection(OutsecName, "<internal>");
   Sec->addSection(cast<InputSection>(IS));
   return Sec;
 }
Index: ELF/OutputSections.h
===================================================================
--- ELF/OutputSections.h
+++ ELF/OutputSections.h
@@ -52,6 +52,10 @@
   uint64_t getLMA() const { return Addr + LMAOffset; }
   template <typename ELFT> void writeHeaderTo(typename ELFT::Shdr *SHdr);
 
+  // Linker script might create forward declarations for output sections.
+  // This method returns true if section known to have definition.
+  bool isDefined() { return !Location.empty(); }
+
   unsigned SectionIndex;
   unsigned SortRank;
 
Index: ELF/ScriptParser.cpp
===================================================================
--- ELF/ScriptParser.cpp
+++ ELF/ScriptParser.cpp
@@ -664,7 +664,7 @@
 
 OutputSection *ScriptParser::readOutputSectionDescription(StringRef OutSec) {
   OutputSection *Cmd =
-      Script->createOutputSection(OutSec, getCurrentLocation());
+      Script->defineOutputSection(OutSec, getCurrentLocation());
 
   if (peek() != ":")
     readSectionAddressType(Cmd);
@@ -971,8 +971,9 @@
   }
   if (Tok == "ADDR") {
     StringRef Name = readParenLiteral();
-    OutputSection *Sec = Script->getOrCreateOutputSection(Name);
+    Script->declareOutputSection(Name);
     return [=]() -> ExprValue {
+      OutputSection *Sec = Script->getOutputSection(Name);
       checkIfExists(Sec, Location);
       return {Sec, false, 0, Location};
     };
@@ -995,10 +996,11 @@
   }
   if (Tok == "ALIGNOF") {
     StringRef Name = readParenLiteral();
-    OutputSection *Cmd = Script->getOrCreateOutputSection(Name);
+    Script->declareOutputSection(Name);
     return [=] {
-      checkIfExists(Cmd, Location);
-      return Cmd->Alignment;
+      OutputSection *Sec = Script->getOutputSection(Name);
+      checkIfExists(Sec, Location);
+      return Sec->Alignment;
     };
   }
   if (Tok == "ASSERT")
@@ -1045,10 +1047,11 @@
   }
   if (Tok == "LOADADDR") {
     StringRef Name = readParenLiteral();
-    OutputSection *Cmd = Script->getOrCreateOutputSection(Name);
+    Script->declareOutputSection(Name);
     return [=] {
-      checkIfExists(Cmd, Location);
-      return Cmd->getLMA();
+      OutputSection *Sec = Script->getOutputSection(Name);
+      checkIfExists(Sec, Location);
+      return Sec->getLMA();
     };
   }
   if (Tok == "ORIGIN") {
@@ -1067,11 +1070,14 @@
   }
   if (Tok == "SIZEOF") {
     StringRef Name = readParenLiteral();
-    OutputSection *Cmd = Script->getOrCreateOutputSection(Name);
+    Script->declareOutputSection(Name);
     // Linker script does not create an output section if its content is empty.
     // We want to allow SIZEOF(.foo) where .foo is a section which happened to
     // be empty.
-    return [=] { return Cmd->Size; };
+    return [=] {
+      OutputSection *Sec = Script->getOutputSection(Name);
+      return Sec->Size;
+    };
   }
   if (Tok == "SIZEOF_HEADERS")
     return [=] { return elf::getHeaderSize(); };
Index: test/ELF/linkerscript/sections-constraint6.s
===================================================================
--- test/ELF/linkerscript/sections-constraint6.s
+++ test/ELF/linkerscript/sections-constraint6.s
@@ -0,0 +1,37 @@
+# REQUIRES: x86
+# RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-linux %s -o %t.o
+# RUN: echo "SECTIONS { . = 0x10000;                         \
+# RUN:         symSize1 = SIZEOF(bar); symAddr1 = ADDR(bar); \
+# RUN:         bar : ONLY_IF_RO { *(unknown) }               \
+# RUN:         bar : ONLY_IF_RW { *(foo_aw) }                \
+# RUN:         symSize2 = SIZEOF(bar); symAddr2 = ADDR(bar); \
+# RUN:       }" > %t.script
+# RUN: ld.lld -o %t -T %t.script %t.o
+# RUN: llvm-readobj -s -t %t | FileCheck %s
+
+## Check values of symbols are (1) not zeroes and (2) corresponds to
+## size and address of bar that contains foo_aw input section.
+# CHECK: Sections [
+# CHECK:      Name: bar
+# CHECK-NEXT: Type: SHT_PROGBITS
+# CHECK-NEXT: Flags [
+# CHECK-NEXT:   SHF_ALLOC
+# CHECK-NEXT:   SHF_WRITE
+# CHECK-NEXT: ]
+# CHECK-NEXT: Address: 0x10001
+# CHECK-NEXT: Offset:
+# CHECK-NEXT: Size: 8
+# CHECK:      Name: symSize1
+# CHECK-NEXT:  Value: 0x8
+# CHECK:      Name: symAddr1
+# CHECK-NEXT:  Value: 0x10001
+# CHECK:      Name: symSize2
+# CHECK-NEXT:  Value: 0x8
+# CHECK:      Name: symAddr2
+# CHECK-NEXT:  Value: 0x10001
+
+.section foo_a,"a"
+.byte 0
+
+.section foo_aw,"aw"
+.quad 0