From 710e95360f1e2cf3852f9131fbaeafeb9cdd64d1 Mon Sep 17 00:00:00 2001
From: Chinmay Garde <chinmaygarde@google.com>
Date: Thu, 9 Feb 2023 21:31:00 -0800
Subject: [PATCH] [Impeller] Generate a Metal library symbols file for shader
 debugging.

Towards fixing https://github.com/flutter/flutter/issues/120326. This allows you
to specify the sources for the Impeller Metal Library. The user is meant to
navigate to `out/<engine_variant>/shaders/entity.metallib` when Xcode prompts
for sources.

Shader debugging with line by line profiling is still not a turnkey operation
though. This works fine on macOS. But on iOS, I had to bump up the min iOS
version level to iOS 12 at least and specify `-g` to the `metal` compiler for
the sources to show up with profiling. The performance of the shaders was
identical after the changes though so I suspect engine developers will patch in
these flags locally to develop and debug shaders.
---
 impeller/renderer/backend/metal/context_mtl.mm | 3 +++
 impeller/tools/build_metal_library.py          | 4 +++-
 impeller/tools/impeller.gni                    | 7 ++++++-
 3 files changed, 12 insertions(+), 2 deletions(-)

diff --git a/impeller/renderer/backend/metal/context_mtl.mm b/impeller/renderer/backend/metal/context_mtl.mm
index faedec8e5bf2b..b4effef0a3a2e 100644
--- a/impeller/renderer/backend/metal/context_mtl.mm
+++ b/impeller/renderer/backend/metal/context_mtl.mm
@@ -149,6 +149,9 @@
                      << shader_library_error.localizedDescription.UTF8String;
       return nil;
     }
+    if (!label.empty()) {
+      library.label = @(label.c_str());
+    }
     [found_libraries addObject:library];
   }
   return found_libraries;
diff --git a/impeller/tools/build_metal_library.py b/impeller/tools/build_metal_library.py
index 8556042e5c186..90d5fbf78451d 100644
--- a/impeller/tools/build_metal_library.py
+++ b/impeller/tools/build_metal_library.py
@@ -81,6 +81,8 @@ def main():
       '-Oz',
       # Allow aggressive, lossy floating-point optimizations.
       '-ffast-math',
+      # Record symbols in a separate *.metallibsym file.
+      '-frecord-sources=flat',
       '-MF',
       args.depfile,
       '-o',
@@ -97,7 +99,7 @@ def main():
   elif args.platform == 'ios':
     command += [
         '--std=ios-metal1.2',
-        '-mios-version-min=10.0',
+        '-mios-version-min=11.0',
     ]
   elif args.platform == 'ios-simulator':
     command += [
diff --git a/impeller/tools/impeller.gni b/impeller/tools/impeller.gni
index 793d9f10dc814..2db7cb37877c4 100644
--- a/impeller/tools/impeller.gni
+++ b/impeller/tools/impeller.gni
@@ -130,8 +130,13 @@ template("metal_library") {
     inputs = invoker.sources
 
     metal_library_path = "$root_out_dir/shaders/$metal_library_name.metallib"
+    metal_library_symbols_path =
+        "$root_out_dir/shaders/$metal_library_name.metallibsym"
 
-    outputs = [ metal_library_path ]
+    outputs = [
+      metal_library_path,
+      metal_library_symbols_path,
+    ]
 
     script = "//flutter/impeller/tools/build_metal_library.py"