diff --git a/llvm/utils/gn/build/BUILD.gn b/llvm/utils/gn/build/BUILD.gn
--- a/llvm/utils/gn/build/BUILD.gn
+++ b/llvm/utils/gn/build/BUILD.gn
@@ -42,17 +42,36 @@
   cflags = target_flags
   ldflags = target_flags + target_ldflags
 
-  if (host_os == "mac" && clang_base_path != "") {
+  if ((current_os == "ios" || current_os == "mac") && clang_base_path != "") {
+    if (current_os == "ios" && current_cpu == "arm64") {
+      sdk_path = ios_sdk_path
+    } else if (current_os == "ios" && current_cpu == "x64") {
+      sdk_path = iossim_sdk_path
+    } else if (current_os == "mac") {
+      sdk_path = mac_sdk_path
+    }
     cflags += [
       "-isysroot",
-      mac_sdk_path,
+      sdk_path,
     ]
     ldflags += [
       "-isysroot",
-      mac_sdk_path,
+      sdk_path,
     ]
   }
 
+  # Mostly for compiler-rt, see compiler-rt/cmake/config-ix.cmake
+  if (current_os == "ios") {
+    asmflags += [ "-miphoneos-version-min=8.0" ]
+    cflags += [ "-miphoneos-version-min=8.0" ]
+    ldflags += [ "-miphoneos-version-min=8.0" ]
+  }
+  if (current_os == "mac") {
+    asmflags += [ "-mmacosx-version-min=10.10" ]
+    cflags += [ "-mmacosx-version-min=10.10" ]
+    ldflags += [ "-mmacosx-version-min=10.10" ]
+  }
+
   if (host_os != "win") {
     if (is_debug) {
       cflags += [ "-g" ]
@@ -296,7 +315,7 @@
 # To make an archive that can be distributed, you need to remove this config and
 # set complete_static_lib.
 config("thin_archive") {
-  if (current_os != "win" && current_os != "mac") {
+  if (current_os != "ios" && current_os != "mac" && current_os != "win") {
     arflags = [ "-T" ]
   }
 }
diff --git a/llvm/utils/gn/build/mac_sdk.gni b/llvm/utils/gn/build/mac_sdk.gni
--- a/llvm/utils/gn/build/mac_sdk.gni
+++ b/llvm/utils/gn/build/mac_sdk.gni
@@ -9,7 +9,12 @@
 # but that makes `gn gen` take twice as long and almost everyone has Xcode
 # installed.  So require that people who don't have it installed set a gn arg.
 if (mac_use_commandline_tools_sdk) {
+  ios_sdk_path = "/Library/Developer/CommandLineTools/SDKs/iPhoneOS.sdk"
+  iossim_sdk_path =
+      "/Library/Developer/CommandLineTools/SDKs/iPhoneSimulator.sdk"
   mac_sdk_path = "/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk"
 } else {
+  ios_sdk_path = "/Applications/Xcode.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/SDKs/iPhoneOS.sdk"
+  iossim_sdk_path = "/Applications/Xcode.app/Contents/Developer/Platforms/iPhoneSimulator.platform/Developer/SDKs/iPhoneSimulator.sdk"
   mac_sdk_path = "/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk"
 }
diff --git a/llvm/utils/gn/build/toolchain/BUILD.gn b/llvm/utils/gn/build/toolchain/BUILD.gn
--- a/llvm/utils/gn/build/toolchain/BUILD.gn
+++ b/llvm/utils/gn/build/toolchain/BUILD.gn
@@ -56,7 +56,7 @@
     }
 
     tool("alink") {
-      if (current_os == "mac") {
+      if (current_os == "ios" || current_os == "mac") {
         command = "libtool -D -static -no_warning_for_no_symbols {{arflags}} -o {{output}} {{inputs}}"
       } else {
         # Remove the output file first so that ar doesn't try to modify the
@@ -70,7 +70,7 @@
       default_output_dir = "{{root_out_dir}}/lib"
     }
 
-    if (current_os == "mac") {
+    if (current_os == "ios" || current_os == "mac") {
       # gn < 1693 (e214b5d35898) doesn't support |frameworks|, requiring
       # frameworks to be listed in |libs|, but gn >= 1808 (3028c6a426a4) forbids
       # frameworks from appearing in |libs|. This assertion provides a helpful
@@ -89,7 +89,7 @@
 
     tool("solink") {
       outfile = "{{output_dir}}/{{target_output_name}}{{output_extension}}"
-      if (current_os == "mac") {
+      if (current_os == "ios" || current_os == "mac") {
         command = "$ld -shared {{ldflags}} -o $outfile {{inputs}} {{libs}} {{frameworks}}"
         default_output_extension = ".dylib"
       } else {
@@ -105,7 +105,7 @@
 
     tool("solink_module") {
       outfile = "{{output_dir}}/{{target_output_name}}{{output_extension}}"
-      if (current_os == "mac") {
+      if (current_os == "ios" || current_os == "mac") {
         command = "$ld -shared {{ldflags}} -Wl,-flat_namespace -Wl,-undefined,suppress -o $outfile {{inputs}} {{libs}} {{frameworks}}"
         default_output_extension = ".dylib"
       } else {
@@ -120,7 +120,7 @@
 
     tool("link") {
       outfile = "{{output_dir}}/{{target_output_name}}{{output_extension}}"
-      if (current_os == "mac") {
+      if (current_os == "ios" || current_os == "mac") {
         command =
             "$ld {{ldflags}} -o $outfile {{inputs}} {{libs}} {{frameworks}}"
       } else {
@@ -141,7 +141,7 @@
       description = "COPY {{source}} {{output}}"
     }
 
-    if (current_os == "mac") {
+    if (current_os == "ios" || current_os == "mac") {
       tool("copy_bundle_data") {
         # http://serverfault.com/q/209888/43689
         _copydir = "mkdir -p {{output}} && cd {{source}} && " +
@@ -164,7 +164,7 @@
 }
 
 unix_toolchain("unix") {
-  if (current_os != "mac") {
+  if (current_os != "ios" && current_os != "mac") {
     ar = "ar"
   }
 
@@ -189,7 +189,7 @@
       "//:clang($host_toolchain)",
       "//:lld($host_toolchain)",
     ]
-    if (current_os != "mac") {
+    if (current_os != "ios" && current_os != "mac") {
       ar = "bin/llvm-ar"
       deps += [ "//:llvm-ar($host_toolchain)" ]
     }
@@ -219,6 +219,22 @@
   }
 }
 
+if (host_os == "mac") {
+  stage2_unix_toolchain("stage2_ios_aarch64") {
+    toolchain_args = {
+      current_os = "ios"
+      current_cpu = "arm64"
+    }
+  }
+
+  stage2_unix_toolchain("stage2_iossim_x64") {
+    toolchain_args = {
+      current_os = "ios"
+      current_cpu = "x64"
+    }
+  }
+}
+
 toolchain("win") {
   cl = "cl"
   link = "link"
diff --git a/llvm/utils/gn/build/toolchain/target_flags.gni b/llvm/utils/gn/build/toolchain/target_flags.gni
--- a/llvm/utils/gn/build/toolchain/target_flags.gni
+++ b/llvm/utils/gn/build/toolchain/target_flags.gni
@@ -1,6 +1,11 @@
 import("//llvm/triples.gni")
 import("//llvm/utils/gn/build/toolchain/compiler.gni")
 
+# Flags in this file are passed both to the compiler that's building
+# compiler-rt at build time (via normal gn cflags/ldflags), as well as to the
+# compiler building compiler-rt test programs at test time (via
+# COMPILER_RT_TEST_COMPILER_CFLAGS).
+
 target_flags = []
 target_ldflags = []
 
@@ -14,6 +19,26 @@
   if (current_cpu == "arm") {
     target_flags += [ "-march=armv7-a" ]
   }
+} else if (current_os == "ios") {
+  if (current_cpu == "arm64") {
+    target_flags += [
+      "-arch",
+      "arm64",
+    ]
+    target_ldflags += [
+      "-arch",
+      "arm64",
+    ]
+  } else if (current_cpu == "x64") {
+    target_flags += [
+      "-arch",
+      "x86_64",
+    ]
+    target_ldflags += [
+      "-arch",
+      "x86_64",
+    ]
+  }
 }
 
 if (current_cpu == "x86") {
diff --git a/llvm/utils/gn/secondary/compiler-rt/BUILD.gn b/llvm/utils/gn/secondary/compiler-rt/BUILD.gn
--- a/llvm/utils/gn/secondary/compiler-rt/BUILD.gn
+++ b/llvm/utils/gn/secondary/compiler-rt/BUILD.gn
@@ -1,3 +1,4 @@
+import("//llvm/lib/Target/targets.gni")
 import("//llvm/utils/gn/build/toolchain/compiler.gni")
 
 # In the GN build, compiler-rt is always built by just-built clang and lld.
@@ -10,10 +11,20 @@
     "//llvm/utils/gn/build/toolchain:stage2_android_arm",
   ]
 }
-
 group("compiler-rt") {
   deps = []
   foreach(toolchain, supported_toolchains) {
     deps += [ "//compiler-rt/lib($toolchain)" ]
   }
+
+  # FIXME: Do this only if a gn arg compiler_rt_enable_ios is set?
+  # That would match the cmake build.
+  if (host_os == "mac") {
+    if (llvm_build_AArch64) {
+      deps += [ "//compiler-rt/lib/builtins(//llvm/utils/gn/build/toolchain:stage2_ios_aarch64)" ]
+    }
+    if (llvm_build_X86) {
+      deps += [ "//compiler-rt/lib/builtins(//llvm/utils/gn/build/toolchain:stage2_iossim_x64)" ]
+    }
+  }
 }
diff --git a/llvm/utils/gn/secondary/compiler-rt/lib/builtins/BUILD.gn b/llvm/utils/gn/secondary/compiler-rt/lib/builtins/BUILD.gn
--- a/llvm/utils/gn/secondary/compiler-rt/lib/builtins/BUILD.gn
+++ b/llvm/utils/gn/secondary/compiler-rt/lib/builtins/BUILD.gn
@@ -13,6 +13,10 @@
   output_dir = crt_current_out_dir
   if (current_os == "mac") {
     output_name = "clang_rt.osx"
+  } else if (current_os == "ios" && current_cpu == "arm64") {
+    output_name = "clang_rt.ios"
+  } else if (current_os == "ios" && current_cpu == "x64") {
+    output_name = "clang_rt.iossim"
   } else {
     output_name = "clang_rt.builtins$crt_current_target_suffix"
   }
@@ -177,7 +181,7 @@
     sources += [ "clear_cache.c" ]
   }
 
-  if (current_os == "mac") {
+  if (current_os == "mac" || current_os == "ios") {
     sources += [
       "atomic_flag_clear.c",
       "atomic_flag_clear_explicit.c",
@@ -496,8 +500,7 @@
   }
 }
 
-# Currently unused but necessary to make the sync_source_lists_from_cmake.py
-# happy.
+# Currently unused but necessary to make sync_source_lists_from_cmake.py happy.
 source_set("_unused") {
   sources = [
     # Thumb1
diff --git a/llvm/utils/gn/secondary/compiler-rt/target.gni b/llvm/utils/gn/secondary/compiler-rt/target.gni
--- a/llvm/utils/gn/secondary/compiler-rt/target.gni
+++ b/llvm/utils/gn/secondary/compiler-rt/target.gni
@@ -26,7 +26,7 @@
   if (current_os == "android") {
     crt_current_target_suffix += "-android"
   }
-} else if (current_os == "mac") {
+} else if (current_os == "ios" || current_os == "mac") {
   crt_current_out_dir = "$clang_resource_dir/lib/darwin"
 } else {
   assert(false, "unimplemented current_os " + current_os)
diff --git a/llvm/utils/gn/secondary/llvm/triples.gni b/llvm/utils/gn/secondary/llvm/triples.gni
--- a/llvm/utils/gn/secondary/llvm/triples.gni
+++ b/llvm/utils/gn/secondary/llvm/triples.gni
@@ -7,7 +7,7 @@
     llvm_current_triple = "x86_64-unknown-freebsd"
   } else if (current_os == "linux") {
     llvm_current_triple = "x86_64-unknown-linux-gnu"
-  } else if (current_os == "mac") {
+  } else if (current_os == "ios" || current_os == "mac") {
     llvm_current_triple = "x86_64-apple-darwin"
   } else if (current_os == "win") {
     llvm_current_triple = "x86_64-pc-windows-msvc"
@@ -19,7 +19,7 @@
 } else if (current_cpu == "arm64") {
   if (current_os == "android") {
     llvm_current_triple = "aarch64-linux-android29"
-  } else if (current_os == "mac") {
+  } else if (current_os == "ios" || current_os == "mac") {
     llvm_current_triple = "arm64-apple-darwin"
   }
 } else if (current_cpu == "ppc64") {