From 2d157e8fe1109aa27307ae15e27fc6bec4c959ce Mon Sep 17 00:00:00 2001
From: Ben Doherty <bendoherty@google.com>
Date: Mon, 6 May 2024 12:12:38 -0700
Subject: [PATCH] Add preferredShaderLanguage option to Engine::Config (#7816)

---
 NEW_RELEASE_NOTES.md               |  1 +
 filament/include/filament/Engine.h | 22 ++++++++++++++++++++++
 filament/src/details/Engine.h      |  4 ++++
 3 files changed, 27 insertions(+)

diff --git a/NEW_RELEASE_NOTES.md b/NEW_RELEASE_NOTES.md
index 188a51f3662..e4728cb677a 100644
--- a/NEW_RELEASE_NOTES.md
+++ b/NEW_RELEASE_NOTES.md
@@ -9,3 +9,4 @@ appropriate header in [RELEASE_NOTES.md](./RELEASE_NOTES.md).
 ## Release notes for next branch cut
 
 - filagui: Fix regression which broke WebGL
+- Add a new Engine::Config setting to control preferred shader language
diff --git a/filament/include/filament/Engine.h b/filament/include/filament/Engine.h
index e5b142c3147..a737469b768 100644
--- a/filament/include/filament/Engine.h
+++ b/filament/include/filament/Engine.h
@@ -340,6 +340,28 @@ class UTILS_PUBLIC Engine {
          * Disable backend handles use-after-free checks.
          */
         bool disableHandleUseAfterFreeCheck = false;
+
+        /*
+         * Sets a preferred shader language for Filament to use.
+         *
+         * The Metal backend supports two shader languages: MSL (Metal Shading Language) and
+         * METAL_LIBRARY (precompiled .metallib). This option controls which shader language is
+         * used when materials contain both.
+         *
+         * By default, when preferredShaderLanguage is unset, Filament will prefer METAL_LIBRARY
+         * shaders if present within a material, falling back to MSL. Setting
+         * preferredShaderLanguage to ShaderLanguage::MSL will instead instruct Filament to check
+         * for the presence of MSL in a material first, falling back to METAL_LIBRARY if MSL is not
+         * present.
+         *
+         * When using a non-Metal backend, setting this has no effect.
+         */
+        enum class ShaderLanguage {
+            DEFAULT = 0,
+            MSL = 1,
+            METAL_LIBRARY = 2,
+        };
+        ShaderLanguage preferredShaderLanguage = ShaderLanguage::DEFAULT;
     };
 
 
diff --git a/filament/src/details/Engine.h b/filament/src/details/Engine.h
index 88434071a5e..5d72ac9c2ee 100644
--- a/filament/src/details/Engine.h
+++ b/filament/src/details/Engine.h
@@ -246,6 +246,10 @@ class FEngine : public Engine {
             case Backend::VULKAN:
                 return { backend::ShaderLanguage::SPIRV };
             case Backend::METAL:
+                const auto& lang = mConfig.preferredShaderLanguage;
+                if (lang == Config::ShaderLanguage::MSL) {
+                    return { backend::ShaderLanguage::MSL, backend::ShaderLanguage::METAL_LIBRARY };
+                }
                 return { backend::ShaderLanguage::METAL_LIBRARY, backend::ShaderLanguage::MSL };
         }
     }