diff --git a/COFF/InputFiles.cpp b/COFF/InputFiles.cpp
index 74d4a26..8687102 100644
--- a/COFF/InputFiles.cpp
+++ b/COFF/InputFiles.cpp
@@ -126,6 +126,13 @@
   initializeSymbols();
 }
 
+const coff_section* ObjFile::getSection(uint32_t I) {
+  const coff_section *Sec;
+  if (auto EC = COFFObj->getSection(I, Sec))
+    fatal("getSection failed: #" + Twine(I) + ": " + EC.message());
+  return Sec;
+}
+
 // We set SectionChunk pointers in the SparseChunks vector to this value
 // temporarily to mark comdat sections as having an unknown resolution. As we
 // walk the object file's symbol table, once we visit either a leader symbol or
@@ -139,10 +146,7 @@
   Chunks.reserve(NumSections);
   SparseChunks.resize(NumSections + 1);
   for (uint32_t I = 1; I < NumSections + 1; ++I) {
-    const coff_section *Sec;
-    if (auto EC = COFFObj->getSection(I, Sec))
-      fatal("getSection failed: #" + Twine(I) + ": " + EC.message());
-
+    const coff_section *Sec = getSection(I);
     if (Sec->Characteristics & IMAGE_SCN_LNK_COMDAT)
       SparseChunks[I] = PendingComdat;
     else
@@ -153,9 +157,7 @@
 SectionChunk *ObjFile::readSection(uint32_t SectionNumber,
                                    const coff_aux_section_definition *Def,
                                    StringRef LeaderName) {
-  const coff_section *Sec;
-  if (auto EC = COFFObj->getSection(SectionNumber, Sec))
-    fatal("getSection failed: #" + Twine(SectionNumber) + ": " + EC.message());
+  const coff_section *Sec = getSection(SectionNumber);
 
   StringRef Name;
   if (auto EC = COFFObj->getSectionName(Sec, Name))
@@ -231,8 +233,7 @@
     StringRef Name, ParentName;
     COFFObj->getSymbolName(Sym, Name);
 
-    const coff_section *ParentSec;
-    COFFObj->getSection(ParentIndex, ParentSec);
+    const coff_section *ParentSec = getSection(ParentIndex);
     COFFObj->getSectionName(ParentSec, ParentName);
     error(toString(this) + ": associative comdat " + Name + " (sec " +
           Twine(SectionNumber) + ") has invalid reference to section " +
@@ -429,10 +430,21 @@
     fatal(toString(this) + ": " + GetName() +
           " should not refer to non-existent section " + Twine(SectionNumber));
 
-  // Handle comdat leader symbols.
+  // Comdat handling.
+  // A comdat symbol consists of two symbol table entries.
+  // The first symbol entry has the name of the section (e.g. .text), fixed
+  // values for the other fields, and one auxilliary record.
+  // The second symbol entry has the name of the comdat symbol, called the
+  // "comdat leader".
+  // When this function is called for the first symbol entry of a comdat,
+  // it sets ComdatDefs and returns None, and when it's called for the second
+  // symbol entry it reads ComdatDefs and then sets it back to nullptr.
+
+  // Handle comdat leader.
   if (const coff_aux_section_definition *Def = ComdatDefs[SectionNumber]) {
     ComdatDefs[SectionNumber] = nullptr;
-    Symbol *Leader;
+    DefinedRegular *Leader;
+
     if (Sym.isExternal()) {
       std::tie(Leader, Prevailing) =
           Symtab->addComdat(this, GetName(), Sym.getGeneric());
@@ -442,10 +454,111 @@
       Prevailing = true;
     }
 
+    if (Def->Selection < (int)IMAGE_COMDAT_SELECT_NODUPLICATES ||
+        // Intentionally ends at IMAGE_COMDAT_SELECT_LARGEST: link.exe
+        // doesn't understand IMAGE_COMDAT_SELECT_NEWEST either.
+        Def->Selection > (int)IMAGE_COMDAT_SELECT_LARGEST) {
+      fatal("unknown comdat type " + std::to_string((int)Def->Selection) +
+            " for " + GetName() + " in " + toString(this));
+    }
+    COMDATType Selection = (COMDATType)Def->Selection;
+
+    if (!Prevailing && Leader->isCOMDAT()) {
+      // There's already an existing comdat for this symbol: `Leader`.
+      // Use the comdats's selection field to determine if the new
+      // symbol in `Sym` should be discarded, produce a duplicate symbol
+      // error, etc.
+
+      SectionChunk *LeaderChunk = nullptr;
+      COMDATType LeaderSelection = IMAGE_COMDAT_SELECT_ANY;
+
+      if (Leader->Data) {
+        LeaderChunk = Leader->getChunk();
+        LeaderSelection = LeaderChunk->Selection;
+      } else {
+        // FIXME: comdats from LTO files don't know their selection; treat them
+        // as "any".
+        Selection = LeaderSelection;
+      }
+
+      // Requiring selections to match exactly is a bit more strict than
+      // link.exe which allows merging "any" and "largest" if "any" is the first
+      // symbol the linker sees, and it allows merging "largest" with everything
+      // (!) if "largest" is the first symbol the linker sees. Making this
+      // symmetric independent of which selection is seen first seems better
+      // though, and if we can get away with not allowing merging "any" and
+      // "largest" that keeps things more regular too.
+      // (ModuleLinker::getComdatResult() also does comdat type merging in a
+      // different way and it's also a bit more permissive.)
+      if (Selection != LeaderSelection) {
+        std::string Msg = ("conflicting comdat type for " + toString(*Leader) +
+                           ": " + Twine((int)LeaderSelection) + " in " +
+                           toString(Leader->getFile()) + " and " +
+                           Twine((int)Selection) + " in " + toString(this))
+                              .str();
+        if (Config->ForceMultiple)
+          warn(Msg);
+        else
+          error(Msg);
+      }
+
+      switch (Selection) {
+      case IMAGE_COMDAT_SELECT_NODUPLICATES:
+        Symtab->reportDuplicate(Leader, this);
+        break;
+
+      case IMAGE_COMDAT_SELECT_ANY:
+        // Nothing to do.
+        break;
+
+      case IMAGE_COMDAT_SELECT_SAME_SIZE:
+        if (LeaderChunk->getSize() != getSection(SectionNumber)->SizeOfRawData)
+          Symtab->reportDuplicate(Leader, this);
+        break;
+
+      case IMAGE_COMDAT_SELECT_EXACT_MATCH: {
+        SectionChunk NewChunk(this, getSection(SectionNumber));
+        // link.exe only compares section contents here and doesn't complain
+        // if the two comdat sections have e.g. different alignment.
+        // Match that.
+        if (LeaderChunk->getContents() != NewChunk.getContents())
+          Symtab->reportDuplicate(Leader, this);
+        break;
+      }
+
+      case IMAGE_COMDAT_SELECT_ASSOCIATIVE:
+        // createDefined() is never called for IMAGE_COMDAT_SELECT_ASSOCIATIVE.
+        // (This means lld-link doesn't produce duplicate symbol errors for
+        // associative comdats while link.exe does, but associate comdats
+        // are never extern in practice.)
+        llvm_unreachable("createDefined not called for associative comdats");
+
+      case IMAGE_COMDAT_SELECT_LARGEST:
+        if (LeaderChunk->getSize() < getSection(SectionNumber)->SizeOfRawData) {
+          // Replace the existing comdat symbol with the new one.
+          // FIXME: This is incorrect: With /opt:noref, the previous sections
+          // make it into the final executable as well. Correct handling would
+          // be to undo reading of the whole old section that's being replaced,
+          // or doing one pass that determines what the final largest comdat
+          // is for all IMAGE_COMDAT_SELECT_LARGEST comdats and then reading
+          // only the largest one.
+          replaceSymbol<DefinedRegular>(
+              Leader, this, GetName(), /*IsCOMDAT*/ true,
+              /*IsExternal*/ true, Sym.getGeneric(), nullptr);
+          Prevailing = true;
+        }
+        break;
+
+      case IMAGE_COMDAT_SELECT_NEWEST:
+        llvm_unreachable("should have been rejected earlier");
+      }
+    }
+
     if (Prevailing) {
       SectionChunk *C = readSection(SectionNumber, Def, GetName());
       SparseChunks[SectionNumber] = C;
       C->Sym = cast<DefinedRegular>(Leader);
+      C->Selection = Selection;
       cast<DefinedRegular>(Leader)->Data = &C->Repl;
     } else {
       SparseChunks[SectionNumber] = nullptr;
@@ -534,6 +647,8 @@
       MB.getBuffer(), Saver.save(ParentName + MB.getBufferIdentifier()))));
   std::vector<std::pair<Symbol *, bool>> Comdat(Obj->getComdatTable().size());
   for (size_t I = 0; I != Obj->getComdatTable().size(); ++I)
+    // FIXME: lto::InputFile doesn't keep enough data to do correct comdat
+    // selection handling.
     Comdat[I] = Symtab->addComdat(this, Saver.save(Obj->getComdatTable()[I]));
   for (const lto::InputFile::Symbol &ObjSym : Obj->symbols()) {
     StringRef SymName = Saver.save(ObjSym.getName());
diff --git a/COFF/InputFiles.h b/COFF/InputFiles.h
index 1b1e99a..6bf0af3 100644
--- a/COFF/InputFiles.h
+++ b/COFF/InputFiles.h
@@ -156,6 +156,8 @@
   llvm::Optional<uint32_t> PCHSignature;
 
 private:
+  const coff_section* getSection(uint32_t I);
+
   void initializeChunks();
   void initializeSymbols();
 
diff --git a/COFF/SymbolTable.cpp b/COFF/SymbolTable.cpp
index 76e122d..1828929 100644
--- a/COFF/SymbolTable.cpp
+++ b/COFF/SymbolTable.cpp
@@ -399,7 +399,7 @@
   return S;
 }
 
-std::pair<Symbol *, bool>
+std::pair<DefinedRegular *, bool>
 SymbolTable::addComdat(InputFile *F, StringRef N,
                        const coff_symbol_generic *Sym) {
   Symbol *S;
@@ -408,11 +408,12 @@
   if (WasInserted || !isa<DefinedRegular>(S)) {
     replaceSymbol<DefinedRegular>(S, F, N, /*IsCOMDAT*/ true,
                                   /*IsExternal*/ true, Sym, nullptr);
-    return {S, true};
+    return {cast<DefinedRegular>(S), true};
   }
-  if (!cast<DefinedRegular>(S)->isCOMDAT())
+  auto *ExistingSymbol = cast<DefinedRegular>(S);
+  if (!ExistingSymbol->isCOMDAT())
     reportDuplicate(S, F);
-  return {S, false};
+  return {ExistingSymbol, false};
 }
 
 Symbol *SymbolTable::addCommon(InputFile *F, StringRef N, uint64_t Size,
diff --git a/COFF/SymbolTable.h b/COFF/SymbolTable.h
index 3424c2b..7f74863 100644
--- a/COFF/SymbolTable.h
+++ b/COFF/SymbolTable.h
@@ -27,6 +27,7 @@
 class CommonChunk;
 class Defined;
 class DefinedAbsolute;
+class DefinedRegular;
 class DefinedRelative;
 class Lazy;
 class SectionChunk;
@@ -88,7 +89,7 @@
   Symbol *addRegular(InputFile *F, StringRef N,
                      const llvm::object::coff_symbol_generic *S = nullptr,
                      SectionChunk *C = nullptr);
-  std::pair<Symbol *, bool>
+  std::pair<DefinedRegular *, bool>
   addComdat(InputFile *F, StringRef N,
             const llvm::object::coff_symbol_generic *S = nullptr);
   Symbol *addCommon(InputFile *F, StringRef N, uint64_t Size,
diff --git a/test/COFF/comdat-selection-associative-largest.s b/test/COFF/comdat-selection-associative-largest.s
new file mode 100644
index 0000000..301aaec
--- /dev/null
+++ b/test/COFF/comdat-selection-associative-largest.s
@@ -0,0 +1,45 @@
+# REQUIRES: x86
+
+# Tests handling of several comdats with "largest" selection type that each
+# has an associative comdat.
+
+# Create obj files.
+# RUN: sed -e s/TYPE/.byte/  -e s/SIZE/1/ %s | llvm-mc -triple x86_64-pc-win32 -filetype=obj -o %t.1.obj
+# RUN: sed -e s/TYPE/.short/ -e s/SIZE/2/ %s | llvm-mc -triple x86_64-pc-win32 -filetype=obj -o %t.2.obj
+# RUN: sed -e s/TYPE/.long/  -e s/SIZE/4/ %s | llvm-mc -triple x86_64-pc-win32 -filetype=obj -o %t.4.obj
+
+        .section .text$ac, "", associative, symbol
+assocsym:
+        .long SIZE
+
+        .section .text$nm, "", largest, symbol
+        .globl symbol
+symbol:
+        TYPE SIZE
+
+# Pass the obj files in different orders and check that only the associative
+# comdat of the largest obj file makes it into the output, independent of
+# the order of the obj files on the command line.
+
+# FIXME: Make these pass when /opt:noref is passed.
+
+# RUN: lld-link /include:symbol /dll /noentry /nodefaultlib %t.1.obj %t.2.obj %t.4.obj /out:%t.exe
+# RUN: llvm-objdump -s %t.exe | FileCheck --check-prefix=ALL124 %s
+# ALL124: Contents of section .text:
+# ALL124:   180001000 04000000 04000000 ....
+
+# RUN: lld-link /include:symbol /dll /noentry /nodefaultlib %t.4.obj %t.2.obj %t.1.obj /out:%t.exe
+# RUN: llvm-objdump -s %t.exe | FileCheck --check-prefix=ALL421 %s
+# ALL421: Contents of section .text:
+# ALL421:   180001000 04000000 04000000 ....
+
+# RUN: lld-link /include:symbol /dll /noentry /nodefaultlib %t.2.obj %t.4.obj %t.1.obj /out:%t.exe
+# RUN: llvm-objdump -s %t.exe | FileCheck --check-prefix=ALL241 %s
+# ALL241: Contents of section .text:
+# ALL241:   180001000 04000000 04000000 ....
+
+# RUN: lld-link /include:symbol /dll /noentry /nodefaultlib %t.2.obj %t.1.obj /out:%t.exe
+# RUN: llvm-objdump -s %t.exe | FileCheck --check-prefix=JUST21 %s
+# JUST21: Contents of section .text:
+# JUST21:   180001000 02000000 0200 ....
+
diff --git a/test/COFF/comdat-selection.s b/test/COFF/comdat-selection.s
new file mode 100644
index 0000000..b9d0301
--- /dev/null
+++ b/test/COFF/comdat-selection.s
@@ -0,0 +1,87 @@
+# REQUIRES: x86
+
+# Tests handling of the comdat selection type.
+# (Except associative which is tested in associative-comdat.s and
+# comdat-selection-associate-largest.s instead.)
+
+# Create obj files with each selection type.
+# RUN: sed -e s/SEL/discard/ %s | llvm-mc -triple x86_64-pc-win32 -filetype=obj -o %t.discard.obj
+# RUN: sed -e s/SEL/one_only/ %s | llvm-mc -triple x86_64-pc-win32 -filetype=obj -o %t.one_only.obj
+# RUN: sed -e s/SEL/same_size/ %s | llvm-mc -triple x86_64-pc-win32 -filetype=obj -o %t.same_size.obj
+# RUN: sed -e s/SEL/same_size/ -e s/.long/.short/ %s | llvm-mc -triple x86_64-pc-win32 -filetype=obj -o %t.same_size.short.obj
+# RUN: sed -e s/SEL/same_contents/ %s | llvm-mc -triple x86_64-pc-win32 -filetype=obj -o %t.same_contents.obj
+# RUN: sed -e s/SEL/same_contents/ -e s/.long/.short/ %s | llvm-mc -triple x86_64-pc-win32 -filetype=obj -o %t.same_contents.short.obj
+# RUN: sed -e s/SEL/same_contents/ -e s/1/2/ %s | llvm-mc -triple x86_64-pc-win32 -filetype=obj -o %t.same_contents.2.obj
+# RUN: sed -e s/SEL/largest/ %s | llvm-mc -triple x86_64-pc-win32 -filetype=obj -o %t.largest.obj
+# RUN: sed -e s/SEL/largest/ -e s/.long/.short/ -e s/1/2/ %s | llvm-mc -triple x86_64-pc-win32 -filetype=obj -o %t.largest.short.2.obj
+# RUN: sed -e s/SEL/newest/ %s | llvm-mc -triple x86_64-pc-win32 -filetype=obj -o %t.newest.obj
+
+        .section .text$nm, "", SEL, symbol
+        .globl symbol
+symbol:
+        .long 1
+
+# First, pass each selection type twice. All should link fine except for
+# one_only which should report a duplicate symbol error and newest which
+# link.exe (and hence lld-link) doesn't understand.
+
+# RUN: cp %t.discard.obj %t.obj && lld-link /dll /noentry /nodefaultlib %t.discard.obj %t.obj
+# RUN: cp %t.one_only.obj %t.obj && not lld-link /dll /noentry /nodefaultlib %t.one_only.obj %t.obj 2>&1 | FileCheck --check-prefix=ONEONE %s
+# ONEONE: lld-link: error: duplicate symbol: symbol
+# RUN: cp %t.same_size.obj %t.obj && lld-link /dll /noentry /nodefaultlib %t.same_size.obj %t.obj
+# RUN: cp %t.same_contents.obj %t.obj && lld-link /dll /noentry /nodefaultlib %t.same_contents.obj %t.obj
+# RUN: cp %t.largest.obj %t.obj && lld-link /dll /noentry /nodefaultlib %t.largest.obj %t.obj
+# RUN: cp %t.newest.obj %t.obj && not lld-link /dll /noentry /nodefaultlib %t.newest.obj %t.obj 2>&1 | FileCheck --check-prefix=NEWNEW %s
+# NEWNEW: lld-link: error: unknown comdat type 7 for symbol
+
+# /force doesn't affect errors about unknown comdat types.
+# RUN: cp %t.newest.obj %t.obj && not lld-link /force /dll /noentry /nodefaultlib %t.newest.obj %t.obj 2>&1 | FileCheck --check-prefix=NEWNEWFORCE %s
+# NEWNEWFORCE: lld-link: error: unknown comdat type 7 for symbol
+
+# Check that same_size, same_contents, largest do what they're supposed to.
+
+# Check that the "same_size" selection produces an error if passed two symbols
+# with different size.
+# RUN: not lld-link /dll /noentry /nodefaultlib %t.same_size.obj %t.same_size.short.obj 2>&1 | FileCheck --check-prefix=SAMESIZEDUPE %s
+# SAMESIZEDUPE: lld-link: error: duplicate symbol: symbol
+
+# Check that the "same_contents" selection produces an error if passed two
+# symbols with different contents.
+# RUN: not lld-link /dll /noentry /nodefaultlib %t.same_contents.obj %t.same_contents.2.obj 2>&1 | FileCheck --check-prefix=SAMECONTENTSDUPE1 %s
+# SAMECONTENTSDUPE1: lld-link: error: duplicate symbol: symbol
+# RUN: not lld-link /dll /noentry /nodefaultlib %t.same_contents.obj %t.same_contents.2.obj 2>&1 | FileCheck --check-prefix=SAMECONTENTSDUPE2 %s
+# SAMECONTENTSDUPE2: lld-link: error: duplicate symbol: symbol
+
+# Check that the "largest" selection picks the larger comdat (independent of
+# the order the .obj files are passed on the commandline).
+# RUN: lld-link /opt:noref /include:symbol /dll /noentry /nodefaultlib %t.largest.obj %t.largest.short.2.obj /out:%t.exe
+# RUN: llvm-objdump -s %t.exe | FileCheck --check-prefix=LARGEST1 %s
+# LARGEST1: Contents of section .text:
+# LARGEST1:   180001000 01000000 ....
+
+# FIXME: Make this pass when /opt:noref is passed.
+# RUN: lld-link /include:symbol /dll /noentry /nodefaultlib %t.largest.short.2.obj %t.largest.obj /out:%t.exe
+# RUN: llvm-objdump -s %t.exe | FileCheck --check-prefix=LARGEST2 %s
+# LARGEST2: Contents of section .text:
+# LARGEST2:   180001000 01000000 ....
+
+
+# Test linking the same symbol with different comdat selection types.
+# link.exe generally rejects this, except for "largest" which is allowed to
+# combine with everything (https://bugs.llvm.org/show_bug.cgi?id=40094#c7).
+# lld-link rejects all comdat selection type mismatches. Spot-test just a few
+# combinations.
+
+# RUN: not lld-link /dll /noentry /nodefaultlib %t.discard.obj %t.one_only.obj 2>&1 | FileCheck --check-prefix=DISCARDONE %s
+# DISCARDONE: lld-link: error: conflicting comdat type for symbol: 2 in
+# RUN: lld-link /force /dll /noentry /nodefaultlib %t.discard.obj %t.one_only.obj 2>&1 | FileCheck --check-prefix=DISCARDONEFORCE %s
+# DISCARDONEFORCE: lld-link: warning: conflicting comdat type for symbol: 2 in
+
+# RUN: not lld-link /dll /noentry /nodefaultlib %t.same_contents.obj %t.same_size.obj 2>&1 | FileCheck --check-prefix=CONTENTSSIZE %s
+# CONTENTSSIZE: lld-link: error: conflicting comdat type for symbol: 4 in
+
+# These cases are accepted by link.exe but not by lld-link.
+# RUN: not lld-link /dll /noentry /nodefaultlib %t.discard.obj %t.largest.obj 2>&1 | FileCheck --check-prefix=DISCARDLARGEST %s
+# DISCARDLARGEST: lld-link: error: conflicting comdat type for symbol: 2 in
+# RUN: not lld-link /dll /noentry /nodefaultlib %t.largest.obj %t.one_only.obj 2>&1 | FileCheck --check-prefix=LARGESTONE %s
+# LARGESTONE: lld-link: error: conflicting comdat type for symbol: 6 in