Holy size savings Batman!

Optimize and compress shaders for size in release modes.

impeller/compiler/compiler.cc

@@ -298,6 +298,10 @@ Compiler::Compiler(const fml::Mapping& source_mapping,
   {
     spirv_cross::CompilerMSL::Options msl_options;
     msl_options.platform = spirv_cross::CompilerMSL::Options::Platform::macOS;
+    // If this version specification changes, the GN rules that process the
+    // Metal to AIR must be updated as well.
+    msl_options.msl_version =
+        spirv_cross::CompilerMSL::Options::make_msl_version(1, 2);
     msl_compiler->set_msl_options(msl_options);
   }
 

impeller/tools/build_metal_library.py

@@ -29,18 +29,32 @@ def Main():
   parser.add_argument("--source",
                     type=str, action="append", required=True,
                     help="The source file to compile. Can be specified multiple times.")
+  parser.add_argument("--optimize", action="store_true", default=False,
+                    help="If available optimizations must be applied to the compiled Metal sources.")
+  parser.add_argument("--platform", required=True, choices=["mac", "ios"],
+                    help="Select the platform.")
 
   args = parser.parse_args()
 
   MakeDirectories(os.path.dirname(args.depfile))
 
   command = [
     "xcrun",
+  ]
+
+  if args.platform == "mac":
+    command += [
+      "-sdk",
+      "macosx",
+    ]
+  elif args.platform == "ios":
+    command += [
+      "-sdk",
+      "iphoneos",
+    ]
+
+  command += [
     "metal",
-    # TODO: Embeds both sources and driver options in the output. This aids in
-    # debugging but should be removed from release builds.
-    "-MO",
-    "-gline-tables-only",
     # These warnings are from generated code and would make no sense to the GLSL
     # author.
     "-Wno-unused-variable",
@@ -49,9 +63,38 @@ def Main():
     "-MF",
     args.depfile,
     "-o",
-    args.output
+    args.output,
   ]
 
+  # The Metal standard must match the specification in impellerc.
+  if args.platform == "mac":
+    command += [
+      "--std=macos-metal1.2",
+    ]
+  elif args.platform == "ios":
+    command += [
+      "--std=ios-metal1.2",
+    ]
+
+  if args.optimize:
+    command += [
+      # Like -Os (and thus -O2), but reduces code size further.
+      "-Oz",
+      # Allow aggressive, lossy floating-point optimizations.
+      "-ffast-math",
+    ]
+  else:
+    command += [
+      # Embeds both sources and driver options in the output. This aids in
+      # debugging but should be removed from release builds.
+      "-frecord-sources",
+      # Assist the sampling profiler.
+      "-gline-tables-only",
+      "-g",
+      # Optimize for debuggability.
+      "-Og",
+    ]
+
   command += args.source
 
   subprocess.check_call(command)

impeller/tools/impeller.gni

@@ -75,6 +75,18 @@ template("metal_library") {
       rebase_path(depfile),
     ]
 
+    if (!is_debug) {
+      args += [ "--optimize" ]
+    }
+
+    if (is_ios) {
+      args += [ "--platform=ios" ]
+    } else if (is_mac) {
+      args += [ "--platform=mac" ]
+    } else {
+      assert(false, "Unsupported platform for generating Metal shaders.")
+    }
+
     foreach(source, invoker.sources) {
       args += [
         "--source",