Index: lib/Target/AArch64/AArch64InstructionSelector.h
===================================================================
--- lib/Target/AArch64/AArch64InstructionSelector.h
+++ lib/Target/AArch64/AArch64InstructionSelector.h
@@ -11,8 +11,6 @@
 /// AArch64.
 //===----------------------------------------------------------------------===//
 
-#ifdef LLVM_BUILD_GLOBAL_ISEL
-
 #ifndef LLVM_LIB_TARGET_AARCH64_AARCH64INSTRUCTIONSELECTOR_H
 #define LLVM_LIB_TARGET_AARCH64_AARCH64INSTRUCTIONSELECTOR_H
 
@@ -68,4 +66,3 @@
 } // end namespace llvm
 
 #endif // LLVM_LIB_TARGET_AARCH64_AARCH64INSTRUCTIONSELECTOR_H
-#endif // LLVM_BUILD_GLOBAL_ISEL
Index: lib/Target/AArch64/CMakeLists.txt
===================================================================
--- lib/Target/AArch64/CMakeLists.txt
+++ lib/Target/AArch64/CMakeLists.txt
@@ -15,8 +15,9 @@
 tablegen(LLVM AArch64GenSystemOperands.inc -gen-searchable-tables)
 if(LLVM_BUILD_GLOBAL_ISEL)
   tablegen(LLVM AArch64GenRegisterBank.inc FORMAT OPTIONS -gen-register-bank)
-  tablegen(LLVM AArch64GenGlobalISel.inc FORMAT OPTIONS -gen-global-isel)
 endif()
+# -gen-global-isel handles LLVM_BUILD_GLOBAL_ISEL internally.
+tablegen(LLVM AArch64GenGlobalISel.inc FORMAT OPTIONS -gen-global-isel)
 
 add_public_tablegen_target(AArch64CommonTableGen)
 
Index: lib/Target/X86/CMakeLists.txt
===================================================================
--- lib/Target/X86/CMakeLists.txt
+++ lib/Target/X86/CMakeLists.txt
@@ -13,8 +13,9 @@
 tablegen(LLVM X86GenEVEX2VEXTables.inc -gen-x86-EVEX2VEX-tables)
 if(LLVM_BUILD_GLOBAL_ISEL)
   tablegen(LLVM X86GenRegisterBank.inc -gen-register-bank)
-  tablegen(LLVM X86GenGlobalISel.inc -gen-global-isel)
 endif()
+# -gen-global-isel handles LLVM_BUILD_GLOBAL_ISEL internally.
+tablegen(LLVM X86GenGlobalISel.inc -gen-global-isel)
 
 add_public_tablegen_target(X86CommonTableGen)
 
Index: lib/Target/X86/X86InstructionSelector.h
===================================================================
--- lib/Target/X86/X86InstructionSelector.h
+++ lib/Target/X86/X86InstructionSelector.h
@@ -10,7 +10,6 @@
 /// This file declares the targeting of the InstructionSelector class for X86.
 //===----------------------------------------------------------------------===//
 
-#ifdef LLVM_BUILD_GLOBAL_ISEL
 #ifndef LLVM_LIB_TARGET_X86_X86INSTRUCTIONSELECTOR_H
 #define LLVM_LIB_TARGET_X86_X86INSTRUCTIONSELECTOR_H
 
@@ -60,4 +59,3 @@
 } // end namespace llvm
 
 #endif // LLVM_LIB_TARGET_X86_X86INSTRUCTIONSELECTOR_H
-#endif // LLVM_BUILD_GLOBAL_ISEL
Index: utils/TableGen/GlobalISelEmitter.cpp
===================================================================
--- utils/TableGen/GlobalISelEmitter.cpp
+++ utils/TableGen/GlobalISelEmitter.cpp
@@ -59,6 +59,9 @@
     cl::desc("Explain why a pattern was skipped for inclusion "
              "in the GlobalISel selector"),
     cl::init(false), cl::cat(GlobalISelEmitterCat));
+static cl::opt<bool> BuildGlobalISelEnabled(
+    "build-global-isel", cl::desc("Is GlobalISel enabled for the compiler"),
+    cl::init(LLVM_BUILD_GLOBAL_ISEL), cl::cat(GlobalISelEmitterCat));
 
 //===- Helper functions ---------------------------------------------------===//
 
@@ -1146,6 +1149,22 @@
 
   emitSourceFileHeader(("Global Instruction Selector for the " +
                        Target.getName() + " target").str(), OS);
+
+  if (!BuildGlobalISelEnabled) {
+    OS << "// GlobalISel is not enabled. See the -build-global-isel option\n"
+          "// and/or the cmake variable LLVM_BUILD_GLOBAL_ISEL which controls\n"
+          "// the default value of -build-global-isel\n";
+
+    OS << "#ifdef GET_GLOBALISEL_IMPL\n"
+       << "bool " << Target.getName()
+       << "InstructionSelector::selectImpl(MachineInstr &I) const {\n"
+       << "  return false;\n"
+       << "}\n"
+       << "#endif // GET_GLOBALISEL_IMPL\n";
+
+    return;
+  }
+
   std::vector<RuleMatcher> Rules;
   // Look through the SelectionDAG patterns we found, possibly emitting some.
   for (const PatternToMatch &Pat : CGP.ptms()) {