Index: lib/MC/WasmObjectWriter.cpp
===================================================================
--- lib/MC/WasmObjectWriter.cpp
+++ lib/MC/WasmObjectWriter.cpp
@@ -223,7 +223,7 @@
   }
 
   void startSection(SectionBookkeeping &Section, unsigned SectionId,
-                    const char *Name = nullptr);
+                    Optional<StringRef> Name = Optional<StringRef>());
   void endSection(SectionBookkeeping &Section);
 
 public:
@@ -286,6 +286,11 @@
       ArrayRef<std::pair<StringRef, uint32_t>> SymbolFlags,
       ArrayRef<std::pair<uint16_t, uint32_t>> InitFuncs,
       const std::map<StringRef, std::vector<WasmComdatEntry>>& Comdats);
+  void writeUserCustomSections(const
+                               std::vector<std::pair<StringRef,
+                                                     const
+                                                     SmallVectorImpl<char>*>>&
+                                   CustomSections);
 
   uint32_t getProvisionalValue(const WasmRelocationEntry &RelEntry);
   void applyRelocations(ArrayRef<WasmRelocationEntry> Relocations,
@@ -304,11 +309,14 @@
 // Write out a section header and a patchable section size field.
 void WasmObjectWriter::startSection(SectionBookkeeping &Section,
                                     unsigned SectionId,
-                                    const char *Name) {
-  assert((Name != nullptr) == (SectionId == wasm::WASM_SEC_CUSTOM) &&
+                                    Optional<StringRef> Name) {
+  assert((Name.hasValue()) == (SectionId == wasm::WASM_SEC_CUSTOM) &&
          "Only custom sections can have names");
 
-  DEBUG(dbgs() << "startSection " << SectionId << ": " << Name << "\n");
+  DEBUG(dbgs() << "startSection " << SectionId;
+        if (Name.hasValue())
+          dbgs() << ": " << Name.getValue();
+         dbgs() << "\n");
   encodeULEB128(SectionId, getStream());
 
   Section.SizeOffset = getStream().tell();
@@ -322,8 +330,7 @@
 
   // Custom sections in wasm also have a string identifier.
   if (SectionId == wasm::WASM_SEC_CUSTOM) {
-    assert(Name);
-    writeString(StringRef(Name));
+    writeString(Name.getValue());
   }
 }
 
@@ -837,7 +844,7 @@
     return;
 
   SectionBookkeeping Section;
-  startSection(Section, wasm::WASM_SEC_CUSTOM, "reloc.CODE");
+  startSection(Section, wasm::WASM_SEC_CUSTOM, StringRef("reloc.CODE"));
 
   encodeULEB128(wasm::WASM_SEC_CODE, getStream());
   encodeULEB128(CodeRelocations.size(), getStream());
@@ -855,7 +862,7 @@
     return;
 
   SectionBookkeeping Section;
-  startSection(Section, wasm::WASM_SEC_CUSTOM, "reloc.DATA");
+  startSection(Section, wasm::WASM_SEC_CUSTOM, StringRef("reloc.DATA"));
 
   encodeULEB128(wasm::WASM_SEC_DATA, getStream());
   encodeULEB128(DataRelocations.size(), getStream());
@@ -871,7 +878,7 @@
     ArrayRef<std::pair<uint16_t, uint32_t>> InitFuncs,
     const std::map<StringRef, std::vector<WasmComdatEntry>>& Comdats) {
   SectionBookkeeping Section;
-  startSection(Section, wasm::WASM_SEC_CUSTOM, "linking");
+  startSection(Section, wasm::WASM_SEC_CUSTOM, StringRef("linking"));
   SectionBookkeeping SubSection;
 
   if (SymbolFlags.size() != 0) {
@@ -929,6 +936,19 @@
   endSection(Section);
 }
 
+void WasmObjectWriter::writeUserCustomSections(
+    const std::vector<std::pair<StringRef,
+                                const SmallVectorImpl<char>*>>&
+                                    CustomSections
+) {
+  for (const auto &CustomSection : CustomSections) {
+    SectionBookkeeping Section;
+    startSection(Section, wasm::WASM_SEC_CUSTOM, CustomSection.first);
+    writeBytes(*CustomSection.second);
+    endSection(Section);
+  }
+}
+
 uint32_t WasmObjectWriter::getFunctionType(const MCSymbolWasm& Symbol) {
   assert(Symbol.isFunction());
   assert(TypeIndices.count(&Symbol));
@@ -969,6 +989,8 @@
   SmallVector<std::pair<uint16_t, uint32_t>, 2> InitFuncs;
   std::map<StringRef, std::vector<WasmComdatEntry>> Comdats;
   SmallVector<WasmDataSegment, 4> DataSegments;
+  std::vector<std::pair<StringRef, const SmallVectorImpl<char>*>>
+      CustomSections;
   uint32_t DataSize = 0;
 
   // For now, always emit the memory import, since loads and stores are not
@@ -1037,6 +1059,24 @@
 
   for (MCSection &Sec : Asm) {
     auto &Section = static_cast<MCSectionWasm &>(Sec);
+
+    if (cast<MCSectionWasm>(Sec).getSectionName().startswith(".custom_section.")) {
+      if (Section.getFragmentList().empty())
+        continue;
+      if (Section.getFragmentList().size() != 1)
+        report_fatal_error("only one .custom_section section fragment supported");
+      const MCFragment &Frag = *Section.begin();
+      if (Frag.hasInstructions() || Frag.getKind() != MCFragment::FT_Data)
+        report_fatal_error("only data supported in .custom_section section");
+      const auto &DataFrag = cast<MCDataFragment>(Frag);
+      if (!DataFrag.getFixups().empty())
+        report_fatal_error("fixups not supported in .custom_section section");
+      const SmallVectorImpl<char> &Contents = DataFrag.getContents();
+      StringRef UserName = Section.getSectionName().substr(16);
+      CustomSections.push_back(std::make_pair(UserName, &Contents));
+      continue;
+    }
+
     if (!Section.isWasmData())
       continue;
 
@@ -1294,6 +1334,7 @@
   writeDataRelocSection();
   writeLinkingMetaDataSection(DataSegments, DataSize, SymbolFlags,
                               InitFuncs, Comdats);
+  writeUserCustomSections(CustomSections);
 
   // TODO: Translate the .comment section to the output.
   // TODO: Translate debug sections to the output.
Index: lib/Target/WebAssembly/WebAssemblyAsmPrinter.cpp
===================================================================
--- lib/Target/WebAssembly/WebAssemblyAsmPrinter.cpp
+++ lib/Target/WebAssembly/WebAssemblyAsmPrinter.cpp
@@ -32,6 +32,7 @@
 #include "llvm/IR/GlobalVariable.h"
 #include "llvm/MC/MCContext.h"
 #include "llvm/MC/MCStreamer.h"
+#include "llvm/MC/MCSectionWasm.h"
 #include "llvm/MC/MCSymbol.h"
 #include "llvm/MC/MCSymbolWasm.h"
 #include "llvm/MC/MCSymbolELF.h"
@@ -99,6 +100,26 @@
       }
     }
   }
+
+  if (const NamedMDNode* Named = M.getNamedMetadata("wasm.custom_sections")) {
+    for (const Metadata *MD : Named->operands()) {
+      const MDTuple *Tuple = dyn_cast<MDTuple>(MD);
+      if (!Tuple || Tuple->getNumOperands() != 2)
+        continue;
+      const MDString *Name = dyn_cast<MDString>(Tuple->getOperand(0));
+      const MDString *Contents = dyn_cast<MDString>(Tuple->getOperand(1));
+      if (!Name || !Contents)
+        continue;
+
+      OutStreamer->PushSection();
+      std::string SectionName = (".custom_section." + Name->getString()).str();
+      MCSectionWasm *mySection =
+        OutContext.getWasmSection(SectionName, SectionKind::getMetadata());
+      OutStreamer->SwitchSection(mySection);
+      OutStreamer->EmitBytes(Contents->getString());
+      OutStreamer->PopSection();
+    }
+  }
 }
 
 void WebAssemblyAsmPrinter::EmitConstantPool() {
Index: test/CodeGen/WebAssembly/custom-sections.ll
===================================================================
--- test/CodeGen/WebAssembly/custom-sections.ll
+++ test/CodeGen/WebAssembly/custom-sections.ll
@@ -0,0 +1,20 @@
+; RUN: llc < %s -asm-verbose=false | FileCheck %s
+
+; Test the mechanism for defining user custom sections.
+
+target datalayout = "e-m:e-p:32:32-i64:64-n32:64-S128"
+target triple = "wasm32-unknown-unknown-wasm"
+
+!0 = !{ !"red", !"foo" }
+!1 = !{ !"green", !"bar" }
+!2 = !{ !"green", !"qux" }
+!wasm.custom_sections = !{ !0, !1, !2 }
+
+; CHECK:      .section	.custom_section.red,"",@
+; CHECK-NEXT: .ascii	"foo"
+
+; CHECK:      .section	.custom_section.green,"",@
+; CHECK-NEXT: .ascii	"bar"
+
+; CHECK:      .section	.custom_section.green,"",@
+; CHECK-NEXT: .ascii	"qux"