well I tried.

impeller/renderer/backend/gles/BUILD.gn

@@ -20,6 +20,7 @@ impeller_component("gles_unittests") {
     "test/mock_gles.cc",
     "test/mock_gles.h",
     "test/mock_gles_unittests.cc",
+    "test/specialization_constants_unittests.cc",
   ]
   deps = [
     ":gles",

impeller/renderer/backend/gles/proc_table_gles.cc

@@ -150,13 +150,29 @@ void ProcTableGLES::ShaderSourceMapping(
     ShaderSource(shader, 1u, sources, lengths);
     return;
   }
+  const auto& shader_source = ComputeShaderWithDefines(mapping, defines);
+  if (!shader_source.has_value()) {
+    VALIDATION_LOG << "Failed to append constant data to shader";
+    return;
+  }
+
+  const GLchar* sources[] = {
+      reinterpret_cast<const GLchar*>(shader_source->c_str())};
+  const GLint lengths[] = {static_cast<GLint>(shader_source->size())};
+  ShaderSource(shader, 1u, sources, lengths);
+}
+
+// Visible For testing.
+std::optional<std::string> ProcTableGLES::ComputeShaderWithDefines(
+    const fml::Mapping& mapping,
+    const std::vector<int32_t>& defines) const {
   auto shader_source = std::string{
       reinterpret_cast<const char*>(mapping.GetMapping()), mapping.GetSize()};
 
   auto index = shader_source.find('\n');
   if (index == std::string::npos) {
     VALIDATION_LOG << "Failed to append constant data to shader";
-    return;
+    return std::nullopt;
   }
 
   std::stringstream ss;
@@ -165,11 +181,7 @@ void ProcTableGLES::ShaderSourceMapping(
   }
   auto define_string = ss.str();
   shader_source.insert(index + 1, define_string);
-
-  const GLchar* sources[] = {
-      reinterpret_cast<const GLchar*>(shader_source.c_str())};
-  const GLint lengths[] = {static_cast<GLint>(shader_source.size())};
-  ShaderSource(shader, 1u, sources, lengths);
+  return shader_source;
 }
 
 const DescriptionGLES* ProcTableGLES::GetDescription() const {

impeller/renderer/backend/gles/proc_table_gles.h

@@ -260,6 +260,11 @@ class ProcTableGLES {
 
   void PopDebugGroup() const;
 
+  // Visible For testing.
+  std::optional<std::string> ComputeShaderWithDefines(
+      const fml::Mapping& mapping,
+      const std::vector<int32_t>& defines) const;
+
  private:
   bool is_valid_ = false;
   std::unique_ptr<DescriptionGLES> description_;

impeller/renderer/backend/gles/test/specialization_constants_unittests.cc

@@ -0,0 +1,75 @@
+// Copyright 2013 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "flutter/testing/testing.h"  // IWYU pragma: keep
+#include "fml/mapping.h"
+#include "gtest/gtest.h"
+#include "impeller/renderer/backend/gles/proc_table_gles.h"
+#include "impeller/renderer/backend/gles/test/mock_gles.h"
+
+namespace impeller {
+namespace testing {
+
+TEST(SpecConstant, CanCreateShaderWithSpecializationConstant) {
+  auto mock_gles = MockGLES::Init();
+  auto& proc_table = mock_gles->GetProcTable();
+  auto shader_source =
+      "#version 100\n"
+      "#ifndef SPIRV_CROSS_CONSTANT_ID_0\n"
+      "#define SPIRV_CROSS_CONSTANT_ID_0 1\n"
+      "#endif\n"
+      "void main() { return vec4(0.0); }";
+  auto test_shader = std::make_shared<fml::DataMapping>(shader_source);
+
+  auto result = proc_table.ComputeShaderWithDefines(*test_shader, {0});
+
+  auto expected_shader_source =
+      "#version 100\n"
+      "#define SPIRV_CROSS_CONSTANT_ID_0 0\n"
+      "#ifndef SPIRV_CROSS_CONSTANT_ID_0\n"
+      "#define SPIRV_CROSS_CONSTANT_ID_0 1\n"
+      "#endif\n"
+      "void main() { return vec4(0.0); }";
+
+  if (!result.has_value()) {
+    GTEST_FAIL() << "Expected shader source";
+  }
+  ASSERT_EQ(result.value(), expected_shader_source);
+}
+
+TEST(SpecConstant, CanCreateShaderWithSpecializationConstantMultipleValues) {
+  auto mock_gles = MockGLES::Init();
+  auto& proc_table = mock_gles->GetProcTable();
+  auto shader_source =
+      "#version 100\n"
+      "#ifndef SPIRV_CROSS_CONSTANT_ID_0\n"
+      "#define SPIRV_CROSS_CONSTANT_ID_0 1\n"
+      "#endif\n"
+      "void main() { return vec4(0.0); }";
+  auto test_shader = std::make_shared<fml::DataMapping>(shader_source);
+
+  auto result =
+      proc_table.ComputeShaderWithDefines(*test_shader, {0, 1, 2, 3, 4, 5});
+
+  auto expected_shader_source =
+      "#version 100\n"
+      "#define SPIRV_CROSS_CONSTANT_ID_0 0\n"
+      "#define SPIRV_CROSS_CONSTANT_ID_1 1\n"
+      "#define SPIRV_CROSS_CONSTANT_ID_2 2\n"
+      "#define SPIRV_CROSS_CONSTANT_ID_3 3\n"
+      "#define SPIRV_CROSS_CONSTANT_ID_4 4\n"
+      "#define SPIRV_CROSS_CONSTANT_ID_5 5\n"
+      "#ifndef SPIRV_CROSS_CONSTANT_ID_0\n"
+      "#define SPIRV_CROSS_CONSTANT_ID_0 1\n"
+      "#endif\n"
+      "void main() { return vec4(0.0); }";
+
+  if (!result.has_value()) {
+    GTEST_FAIL() << "Expected shader source";
+  }
+  ASSERT_EQ(result.value(), expected_shader_source);
+}
+
+}  // namespace testing
+}  // namespace impeller