From ef4dfcecd6aec7492825f6743848c7d6149eb86c Mon Sep 17 00:00:00 2001
From: Ben Doherty <bendoherty@google.com>
Date: Thu, 21 Oct 2021 11:27:05 -0700
Subject: [PATCH] Metal: support framebuffer fetch on M1 devices (#4735)

---
 filament/backend/src/metal/MetalDriver.mm  | 20 +++++++++++---------
 filament/backend/src/metal/MetalHandles.mm |  3 +--
 libs/filamat/src/GLSLPostProcessor.cpp     | 10 ++++++++--
 3 files changed, 20 insertions(+), 13 deletions(-)

diff --git a/filament/backend/src/metal/MetalDriver.mm b/filament/backend/src/metal/MetalDriver.mm
index ac30da6bb6..f111dc613a 100644
--- a/filament/backend/src/metal/MetalDriver.mm
+++ b/filament/backend/src/metal/MetalDriver.mm
@@ -64,16 +64,19 @@ MetalDriver::MetalDriver(backend::MetalPlatform* platform) noexcept
 
     initializeSupportedGpuFamilies(mContext);
 
-    utils::slog.d << "Supported GPU families: " << utils::io::endl;
+    utils::slog.v << "Supported GPU families: " << utils::io::endl;
     if (mContext->highestSupportedGpuFamily.common > 0) {
-        utils::slog.d << "  MTLGPUFamilyCommon" << (int) mContext->highestSupportedGpuFamily.common << utils::io::endl;
+        utils::slog.v << "  MTLGPUFamilyCommon" << (int) mContext->highestSupportedGpuFamily.common << utils::io::endl;
     }
     if (mContext->highestSupportedGpuFamily.apple > 0) {
-        utils::slog.d << "  MTLGPUFamilyApple" << (int) mContext->highestSupportedGpuFamily.apple << utils::io::endl;
+        utils::slog.v << "  MTLGPUFamilyApple" << (int) mContext->highestSupportedGpuFamily.apple << utils::io::endl;
     }
     if (mContext->highestSupportedGpuFamily.mac > 0) {
-        utils::slog.d << "  MTLGPUFamilyMac" << (int) mContext->highestSupportedGpuFamily.mac << utils::io::endl;
+        utils::slog.v << "  MTLGPUFamilyMac" << (int) mContext->highestSupportedGpuFamily.mac << utils::io::endl;
     }
+    utils::slog.v << "Features:" << utils::io::endl;
+    utils::slog.v << "  readWriteTextureSupport: " <<
+            (bool) mContext->device.readWriteTextureSupport << utils::io::endl;
 
     // In order to support texture swizzling, the GPU needs to support it and the system be running
     // iOS 13+.
@@ -650,11 +653,10 @@ bool MetalDriver::isRenderTargetFormatSupported(TextureFormat format) {
 }
 
 bool MetalDriver::isFrameBufferFetchSupported() {
-#if defined(IOS) && !defined(FILAMENT_IOS_SIMULATOR)
-    return true;
-#else
-    return false;
-#endif
+    // FrameBuffer fetch is achievable via "programmable blending" in Metal, and only supported on
+    // Apple GPUs with readWriteTextureSupport.
+    return mContext->highestSupportedGpuFamily.apple >= 1 &&
+            mContext->device.readWriteTextureSupport;
 }
 
 bool MetalDriver::isFrameBufferFetchMultiSampleSupported() {
diff --git a/filament/backend/src/metal/MetalHandles.mm b/filament/backend/src/metal/MetalHandles.mm
index 2e3dad5636..88133e15bb 100644
--- a/filament/backend/src/metal/MetalHandles.mm
+++ b/filament/backend/src/metal/MetalHandles.mm
@@ -352,8 +352,7 @@ MetalProgram::MetalProgram(id<MTLDevice> device, const Program& program) noexcep
                                                         length:source.size()
                                                       encoding:NSUTF8StringEncoding];
         NSError* error = nil;
-        MTLCompileOptions* options = [MTLCompileOptions new];
-        options.languageVersion = MTLLanguageVersion1_1;
+        // When options is nil, Metal uses the most recent language version available.
         id<MTLLibrary> library = [device newLibraryWithSource:objcSource
                                                       options:nil
                                                         error:&error];
diff --git a/libs/filamat/src/GLSLPostProcessor.cpp b/libs/filamat/src/GLSLPostProcessor.cpp
index 6c9de50f25..30d0f967f8 100644
--- a/libs/filamat/src/GLSLPostProcessor.cpp
+++ b/libs/filamat/src/GLSLPostProcessor.cpp
@@ -121,10 +121,16 @@ void GLSLPostProcessor::spirvToToMsl(const SpirvBlob *spirv, std::string *outMsl
 
     CompilerMSL::Options mslOptions = {};
     mslOptions.platform = platform,
-    mslOptions.msl_version = CompilerMSL::Options::make_msl_version(1, 1);
+    mslOptions.msl_version = config.shaderModel == filament::backend::ShaderModel::GL_ES_30 ?
+        CompilerMSL::Options::make_msl_version(2, 0) : CompilerMSL::Options::make_msl_version(2, 2);
 
-    if (config.shaderModel == filament::backend::ShaderModel::GL_ES_30) {
+    if (config.hasFramebufferFetch) {
         mslOptions.use_framebuffer_fetch_subpasses = true;
+        // On macOS, framebuffer fetch is only available starting with MSL 2.3. Filament will only
+        // use framebuffer fetch materials on devices that support it.
+        if (config.shaderModel == filament::backend::ShaderModel::GL_CORE_41) {
+            mslOptions.msl_version = CompilerMSL::Options::make_msl_version(2, 3);
+        }
     }
 
     mslCompiler.set_msl_options(mslOptions);