Index: lld/trunk/COFF/Driver.cpp
===================================================================
--- lld/trunk/COFF/Driver.cpp
+++ lld/trunk/COFF/Driver.cpp
@@ -896,50 +896,51 @@
   if (auto *Arg = Args.getLastArg(OPT_implib))
     Config->Implib = Arg->getValue();
 
-  // Handle /opt
+  // Handle /opt.
+  bool DoGC = !Args.hasArg(OPT_debug);
+  unsigned ICFLevel = 1; // 0: off, 1: limited, 2: on
   for (auto *Arg : Args.filtered(OPT_opt)) {
     std::string Str = StringRef(Arg->getValue()).lower();
     SmallVector<StringRef, 1> Vec;
     StringRef(Str).split(Vec, ',');
     for (StringRef S : Vec) {
-      if (S == "noref") {
-        Config->DoGC = false;
-        Config->DoICF = false;
-        continue;
-      }
-      if (S == "icf" || S.startswith("icf=")) {
-        Config->DoICF = true;
-        continue;
-      }
-      if (S == "noicf") {
-        Config->DoICF = false;
-        continue;
-      }
-      if (S.startswith("lldlto=")) {
+      if (S == "ref") {
+        DoGC = true;
+      } else if (S == "noref") {
+        DoGC = false;
+      } else if (S == "icf" || S.startswith("icf=")) {
+        ICFLevel = 2;
+      } else if (S == "noicf") {
+        ICFLevel = 0;
+      } else if (S.startswith("lldlto=")) {
         StringRef OptLevel = S.substr(7);
         if (OptLevel.getAsInteger(10, Config->LTOOptLevel) ||
             Config->LTOOptLevel > 3)
           error("/opt:lldlto: invalid optimization level: " + OptLevel);
-        continue;
-      }
-      if (S.startswith("lldltojobs=")) {
+      } else if (S.startswith("lldltojobs=")) {
         StringRef Jobs = S.substr(11);
         if (Jobs.getAsInteger(10, Config->LTOJobs) || Config->LTOJobs == 0)
           error("/opt:lldltojobs: invalid job count: " + Jobs);
-        continue;
-      }
-      if (S.startswith("lldltopartitions=")) {
+      } else if (S.startswith("lldltopartitions=")) {
         StringRef N = S.substr(17);
         if (N.getAsInteger(10, Config->LTOPartitions) ||
             Config->LTOPartitions == 0)
           error("/opt:lldltopartitions: invalid partition count: " + N);
-        continue;
-      }
-      if (S != "ref" && S != "lbr" && S != "nolbr")
+      } else if (S != "lbr" && S != "nolbr")
         error("/opt: unknown option: " + S);
     }
   }
 
+  // Limited ICF is enabled if GC is enabled and ICF was never mentioned
+  // explicitly.
+  // FIXME: LLD only implements "limited" ICF, i.e. it only merges identical
+  // code. If the user passes /OPT:ICF explicitly, LLD should merge identical
+  // comdat readonly data.
+  if (ICFLevel == 1 && !DoGC)
+    ICFLevel = 0;
+  Config->DoGC = DoGC;
+  Config->DoICF = ICFLevel > 0;
+
   // Handle /lldsavetemps
   if (Args.hasArg(OPT_lldsavetemps))
     Config->SaveTemps = true;
Index: lld/trunk/test/COFF/icf-associative.test
===================================================================
--- lld/trunk/test/COFF/icf-associative.test
+++ lld/trunk/test/COFF/icf-associative.test
@@ -1,5 +1,5 @@
 # RUN: yaml2obj < %s > %t.obj
-# RUN: lld-link /entry:foo /out:%t.exe /subsystem:console /include:bar \
+# RUN: lld-link /opt:icf /entry:foo /out:%t.exe /subsystem:console /include:bar \
 # RUN:   /debug /verbose %t.obj > %t.log 2>&1
 # RUN: FileCheck %s < %t.log
 
Index: lld/trunk/test/COFF/icf-simple.test
===================================================================
--- lld/trunk/test/COFF/icf-simple.test
+++ lld/trunk/test/COFF/icf-simple.test
@@ -1,5 +1,5 @@
 # RUN: yaml2obj < %s > %t.obj
-# RUN: lld-link /entry:foo /out:%t.exe /subsystem:console /include:bar \
+# RUN: lld-link /opt:icf /entry:foo /out:%t.exe /subsystem:console /include:bar \
 # RUN:   /verbose %t.obj > %t.log 2>&1
 # RUN: FileCheck -check-prefix=ICF %s < %t.log
 
@@ -13,6 +13,31 @@
 # RUN:   /verbose /opt:noref,noicf %t.obj > %t.log 2>&1
 # RUN: FileCheck -check-prefix=NOICF %s < %t.log
 
+# ICF is on by default (no /opt: flags).
+# RUN: lld-link /entry:foo /out:%t.exe /subsystem:console \
+# RUN:   /include:bar /verbose %t.obj > %t.log 2>&1
+# RUN: FileCheck -check-prefix=ICF %s < %t.log
+
+# /debug disables ICF.
+# RUN: lld-link /debug /entry:foo /out:%t.exe /subsystem:console \
+# RUN:   /include:bar /verbose %t.obj > %t.log 2>&1
+# RUN: FileCheck -check-prefix=NOICF %s < %t.log
+
+# /opt:noref disables ICF.
+# RUN: lld-link /opt:noref /entry:foo /out:%t.exe /subsystem:console \
+# RUN:   /include:bar /verbose %t.obj > %t.log 2>&1
+# RUN: FileCheck -check-prefix=NOICF %s < %t.log
+
+# /debug /opt:ref enables ICF.
+# RUN: lld-link /debug /opt:ref /entry:foo /out:%t.exe /subsystem:console \
+# RUN:   /include:bar /verbose %t.obj > %t.log 2>&1
+# RUN: FileCheck -check-prefix=ICF %s < %t.log
+
+# /debug /opt:noicf,ref disables ICF.
+# RUN: lld-link /debug /opt:noicf,ref /entry:foo /out:%t.exe /subsystem:console \
+# RUN:   /include:bar /verbose %t.obj > %t.log 2>&1
+# RUN: FileCheck -check-prefix=NOICF %s < %t.log
+
 # NOICF-NOT: Removed foo
 # NOICF-NOT: Removed bar
 
Index: lld/trunk/test/COFF/pdb-global-gc.yaml
===================================================================
--- lld/trunk/test/COFF/pdb-global-gc.yaml
+++ lld/trunk/test/COFF/pdb-global-gc.yaml
@@ -1,7 +1,7 @@
 # RUN: yaml2obj %s -o %t.obj
 # RUN: llvm-mc %S/Inputs/pdb-global-gc.s -triple x86_64-windows-msvc -filetype=obj -o %t2.obj
 # RUN: lld-link %t.obj %t2.obj -debug -entry:main \
-# RUN:          -nodefaultlib -debug -out:%t.exe -pdb:%t.pdb -verbose
+# RUN:          -nodefaultlib -opt:ref -out:%t.exe -pdb:%t.pdb -verbose
 # RUN: llvm-pdbutil dump -symbols -globals %t.pdb | FileCheck %s
 
 # This tests the case where an __imp_ chunk is discarded by linker GC. The debug
Index: lld/trunk/test/COFF/pdb-import-gc.yaml
===================================================================
--- lld/trunk/test/COFF/pdb-import-gc.yaml
+++ lld/trunk/test/COFF/pdb-import-gc.yaml
@@ -1,6 +1,6 @@
 # RUN: yaml2obj %s -o %t.obj
 # RUN: lld-link %t.obj %S/Inputs/pdb-import-gc.lib -debug -entry:main \
-# RUN:          -nodefaultlib -debug -out:%t.exe -pdb:%t.pdb
+# RUN:          -nodefaultlib -opt:ref -out:%t.exe -pdb:%t.pdb
 # RUN: llvm-pdbutil dump -globals -symbols %t.pdb | FileCheck %s
 
 # This tests the case where an __imp_ chunk is discarded by linker GC. The debug