Common: Add convenience function for hashing a struct

src/common/hash.h

@@ -20,4 +20,17 @@ static inline u64 ComputeHash64(const void* data, size_t len) {
     return CityHash64(static_cast<const char*>(data), len);
 }
 
+/**
+ * Computes a 64-bit hash of a struct. In addition to being POD (trivially copyable and having
+ * standard layout), it is also critical that either the struct includes no padding, or that any
+ * padding is initialized to a known value by memsetting the struct to 0 before filling it in.
+ */
+template <typename T>
+static inline u64 ComputeStructHash64(const T& data) {
+    static_assert(
+        std::is_trivially_copyable<T>::value && std::is_standard_layout<T>::value,
+        "Type passed to ComputeStructHash64 must be trivially copyable and standard layout");
+    return ComputeHash64(&data, sizeof(data));
+}
+
 } // namespace Common

src/video_core/renderer_opengl/gl_shader_gen.cpp

@@ -65,6 +65,7 @@ PicaShaderConfig PicaShaderConfig::BuildFromRegs(const Pica::Regs& regs) {
     PicaShaderConfig res;
 
     auto& state = res.state;
+    // Memset structure to zero padding bits, so that they will be deterministic when hashing
     std::memset(&state, 0, sizeof(PicaShaderConfig::State));
 
     state.scissor_test_mode = regs.rasterizer.scissor_test.mode;

src/video_core/renderer_opengl/gl_shader_gen.h

@@ -131,10 +131,6 @@ union PicaShaderConfig {
 
     } state;
 };
-#if (__GNUC__ >= 5) || defined(__clang__) || defined(_MSC_VER)
-static_assert(std::is_trivially_copyable<PicaShaderConfig::State>::value,
-              "PicaShaderConfig::State must be trivially copyable");
-#endif
 
 /**
  * Generates the GLSL vertex shader program source code for the current Pica state
@@ -156,7 +152,7 @@ namespace std {
 template <>
 struct hash<GLShader::PicaShaderConfig> {
     size_t operator()(const GLShader::PicaShaderConfig& k) const {
-        return Common::ComputeHash64(&k.state, sizeof(GLShader::PicaShaderConfig::State));
+        return Common::ComputeStructHash64(k.state);
     }
 };
 } // namespace std