Enable preprocessor optimization of ESSL 1.0

Since #7358 is blocked by an upstream spirv-cross issue, we can at least do a
bit of preprocessor optimization for ESSL 1.0 code in the meantime and introduce
the FILAMENT_EFFECTIVE_VERSION preprocessor definitions.

filament/src/materials/blitLow.mat

@@ -35,7 +35,7 @@ vertex {
 
 fragment {
     void postProcess(inout PostProcessInputs postProcess) {
-#if __VERSION__ == 100
+#if FILAMENT_EFFECTIVE_VERSION == 100
         postProcess.color = texture2D(materialParams_color, variable_vertex.xy);
 #else
         postProcess.color = textureLod(materialParams_color, variable_vertex.xy, 0.0);

libs/filamat/src/GLSLPostProcessor.cpp

@@ -23,6 +23,7 @@
 #include <spirv_glsl.hpp>
 #include <spirv_msl.hpp>
 
+#include "backend/DriverEnums.h"
 #include "sca/builtinResource.h"
 #include "sca/GLSLTools.h"
 
@@ -395,7 +396,11 @@ bool GLSLPostProcessor::process(const std::string& inputShader, Config const& co
             break;
         case MaterialBuilder::Optimization::SIZE:
         case MaterialBuilder::Optimization::PERFORMANCE:
-            fullOptimization(tShader, config, internalConfig);
+            if (config.featureLevel == filament::backend::FeatureLevel::FEATURE_LEVEL_0) {
+                preprocessOptimization(tShader, config, internalConfig);
+            } else {
+                fullOptimization(tShader, config, internalConfig);
+            }
             break;
     }
 

libs/filamat/src/shaders/CodeGenerator.cpp

@@ -151,6 +151,36 @@ utils::io::sstream& CodeGenerator::generateProlog(utils::io::sstream& out, Shade
         out << "#define FILAMENT_HAS_FEATURE_INSTANCING\n";
     }
 
+    // During compilation and optimization, __VERSION__ reflects the shader language version of the
+    // intermediate code, not the version of the final code. spirv-cross automatically adapts
+    // certain language features (e.g. fragment output) but leaves others untouched (e.g. sampler
+    // functions, bit shift operations). Client code may have to make decisions based on this
+    // information, so define a FILAMENT_EFFECTIVE_VERSION constant.
+    const char *effective_version;
+    if (mTargetLanguage == TargetLanguage::GLSL) {
+        effective_version = "__VERSION__";
+    } else {
+        switch (mShaderModel) {
+            case ShaderModel::MOBILE:
+                if (mFeatureLevel >= FeatureLevel::FEATURE_LEVEL_1) {
+                    effective_version = "300";
+                } else {
+                    effective_version = "100";
+                }
+                break;
+            case ShaderModel::DESKTOP:
+                if (mFeatureLevel >= FeatureLevel::FEATURE_LEVEL_2) {
+                    effective_version = "450";
+                } else {
+                    effective_version = "410";
+                }
+                break;
+            default:
+                assert(false);
+        }
+    }
+    generateDefine(out, "FILAMENT_EFFECTIVE_VERSION", effective_version);
+
     if (stage == ShaderStage::VERTEX) {
         CodeGenerator::generateDefine(out, "FLIP_UV_ATTRIBUTE", material.flipUV);
         CodeGenerator::generateDefine(out, "LEGACY_MORPHING", material.useLegacyMorphing);

shaders/src/depth_main.fs

@@ -57,7 +57,7 @@ void main() {
     fragColor.xy = computeDepthMomentsVSM(depth);
     fragColor.zw = computeDepthMomentsVSM(-1.0 / depth); // requires at least RGBA16F
 #elif defined(VARIANT_HAS_PICKING)
-#if MATERIAL_FEATURE_LEVEL == 0
+#if FILAMENT_EFFECTIVE_VERSION == 0
     outPicking.a = mod(float(object_uniforms_objectId / 65536), 256.0) / 255.0;
     outPicking.b = mod(float(object_uniforms_objectId /   256), 256.0) / 255.0;
     outPicking.g = mod(float(object_uniforms_objectId)        , 256.0) / 255.0;