Index: include/clang/Basic/DiagnosticDriverKinds.td
===================================================================
--- include/clang/Basic/DiagnosticDriverKinds.td
+++ include/clang/Basic/DiagnosticDriverKinds.td
@@ -361,4 +361,12 @@
 def note_drv_verify_prefix_spelling : Note<
   "-verify prefixes must start with a letter and contain only alphanumeric"
   " characters, hyphens, and underscores">;
+
+def warn_drv_experimental_isel_incomplete : Warning<
+  "-fexperimental-isel support for the '%0' architecture is incomplete">,
+  InGroup<ExperimentalISel>;
+
+def warn_drv_experimental_isel_incomplete_opt : Warning<
+  "-fexperimental-isel support is incomplete for this architecture at the current optimization level">,
+  InGroup<ExperimentalISel>;
 }
Index: include/clang/Basic/DiagnosticGroups.td
===================================================================
--- include/clang/Basic/DiagnosticGroups.td
+++ include/clang/Basic/DiagnosticGroups.td
@@ -985,3 +985,6 @@
 // A warning group for warnings about code that clang accepts when
 // compiling OpenCL C/C++ but which is not compatible with the SPIR spec.
 def SpirCompat : DiagGroup<"spir-compat">;
+
+// Warning for the experimental-isel options.
+def ExperimentalISel : DiagGroup<"experimental-isel">;
Index: include/clang/Driver/Options.td
===================================================================
--- include/clang/Driver/Options.td
+++ include/clang/Driver/Options.td
@@ -1033,6 +1033,8 @@
 def finline_hint_functions: Flag<["-"], "finline-hint-functions">, Group<f_clang_Group>, Flags<[CC1Option]>,
   HelpText<"Inline functions which are (explicitly or implicitly) marked inline">;
 def finline : Flag<["-"], "finline">, Group<clang_ignored_f_Group>;
+def fexperimental_isel : Flag<["-"], "fexperimental-isel">, Group<f_clang_Group>,
+  HelpText<"Enables the experimental global instruction selector">;
 def fexperimental_new_pass_manager : Flag<["-"], "fexperimental-new-pass-manager">,
   Group<f_clang_Group>, Flags<[CC1Option]>,
   HelpText<"Enables an experimental new pass manager in LLVM.">;
@@ -1244,6 +1246,8 @@
 def fno_gnu_keywords : Flag<["-"], "fno-gnu-keywords">, Group<f_Group>, Flags<[CC1Option]>;
 def fno_inline_functions : Flag<["-"], "fno-inline-functions">, Group<f_clang_Group>, Flags<[CC1Option]>;
 def fno_inline : Flag<["-"], "fno-inline">, Group<f_clang_Group>, Flags<[CC1Option]>;
+def fno_experimental_isel : Flag<["-"], "fno-experimental-isel">, Group<f_clang_Group>,
+  HelpText<"Disables the experimental global instruction selector">;
 def fno_experimental_new_pass_manager : Flag<["-"], "fno-experimental-new-pass-manager">,
   Group<f_clang_Group>, Flags<[CC1Option]>,
   HelpText<"Disables an experimental new pass manager in LLVM.">;
Index: lib/Driver/ToolChains/Clang.cpp
===================================================================
--- lib/Driver/ToolChains/Clang.cpp
+++ lib/Driver/ToolChains/Clang.cpp
@@ -4694,6 +4694,37 @@
     CmdArgs.push_back("-fwhole-program-vtables");
   }
 
+  if (Arg *A = Args.getLastArg(options::OPT_fexperimental_isel,
+                               options::OPT_fno_experimental_isel)) {
+    CmdArgs.push_back("-mllvm");
+    if (A->getOption().matches(options::OPT_fexperimental_isel)) {
+      CmdArgs.push_back("-global-isel=1");
+
+      // GISel is on by default on AArch64 -O0, so don't bother adding
+      // the fallback remarks for it. Other combinations will add a warning of
+      // some kind.
+      bool IsArchSupported = Triple.getArch() == llvm::Triple::aarch64;
+      bool IsOptLevelSupported = false;
+
+      Arg *A = Args.getLastArg(options::OPT_O_Group);
+      if (Triple.getArch() == llvm::Triple::aarch64) {
+        if (!A || A->getOption().matches(options::OPT_O0))
+          IsOptLevelSupported = true;
+      }
+      if (!IsArchSupported || !IsOptLevelSupported) {
+        CmdArgs.push_back("-mllvm");
+        CmdArgs.push_back("-global-isel-abort=2");
+
+        if (!IsArchSupported)
+          D.Diag(diag::warn_drv_experimental_isel_incomplete) << Triple.getArchName();
+        else
+          D.Diag(diag::warn_drv_experimental_isel_incomplete_opt);
+      }
+    } else {
+      CmdArgs.push_back("-global-isel=0");
+    }
+  }
+
   // Finally add the compile command to the compilation.
   if (Args.hasArg(options::OPT__SLASH_fallback) &&
       Output.getType() == types::TY_Object &&
Index: test/Driver/global-isel.c
===================================================================
--- test/Driver/global-isel.c
+++ test/Driver/global-isel.c
@@ -0,0 +1,24 @@
+// REQUIRES: x86-registered-target,aarch64-registered-target
+
+// RUN: %clang -fexperimental-isel -S -### %s 2>&1 | FileCheck --check-prefix=ENABLED %s
+// RUN: %clang -fno-experimental-isel -S -### %s 2>&1 | FileCheck --check-prefix=DISABLED %s
+
+// RUN: %clang -target aarch64 -fexperimental-isel -S %s -### 2>&1 | FileCheck --check-prefix=ARM64-DEFAULT %s
+// RUN: %clang -target aarch64 -fexperimental-isel -S -O0 %s -### 2>&1 | FileCheck --check-prefix=ARM64-O0 %s
+// RUN: %clang -target aarch64 -fexperimental-isel -S -O2 %s -### 2>&1 | FileCheck --check-prefix=ARM64-O2 %s
+// RUN: %clang -target aarch64 -fexperimental-isel -Wno-experimental-isel -S -O2 %s -### 2>&1 | FileCheck --check-prefix=ARM64-O2-NOWARN %s
+
+// RUN: %clang -target x86_64 -fexperimental-isel -S %s -### 2>&1 | FileCheck --check-prefix=X86_64 %s
+
+// ENABLED: "-mllvm" "-global-isel=1"
+// DISABLED: "-mllvm" "-global-isel=0"
+
+// ARM64-DEFAULT-NOT: warning: -fexperimental-sel
+// ARM64-DEFAULT-NOT: "-global-isel-abort=2"
+// ARM64-O0-NOT: warning: -fexperimental-sel
+// ARM64-O2: warning: -fexperimental-isel support is incomplete for this architecture at the current optimization level
+// ARM64-O2: "-mllvm" "-global-isel-abort=2"
+// ARM64-O2-NOWARN-NOT: warning: -fexperimental-isel
+
+// X86_64: -fexperimental-isel support for the 'x86_64' architecture is incomplete
+// X86_64: "-mllvm" "-global-isel-abort=2"