feat(idl): add execute_instruction builtin for dynamic instruction di…

backends/cpp_hart_gen/templates/hart.hxx.erb

@@ -408,6 +408,20 @@ namespace udb {
     bool _decode(const XReg& pc, const Bits<<%= cfg_arch.largest_encoding %>>& encoding, InstBase* obj);
 
     uint64_t fetch() override { return _fetch().get(); }
+
+    void execute_instruction(const Bits<32>& encoding) {
+      InstBase* inst = reinterpret_cast<InstBase*>(m_execute_instruction_storage.data());
+      if (!_decode(m_pc, encoding, inst)) {
+        try {
+          raise(ExceptionCode{ExceptionCode::IllegalInstruction}, mode(), encoding);
+        } catch (const AbortInstruction&) {
+          throw; // propagate to outer instruction handler
+        }
+        return;
+      }
+      inst->execute();
+      // Note: PC is NOT advanced here; the calling instruction manages PC.
+    }
     PossiblyUnknownBits<INSTR_ENC_SIZE.get()> _fetch();
     void ifence() override {
       m_bb_cache.invalidate();
@@ -501,6 +515,7 @@ namespace udb {
       std::map<std::string, CsrBase*> m_csr_name_map;
 
       std::array<uint8_t, __MAX_INST_CPP_SIZE> m_run_one_inst_storage;
+      std::array<uint8_t, __MAX_INST_CPP_SIZE> m_execute_instruction_storage;
       BasicBlockCache<__MAX_INST_CPP_SIZE> m_bb_cache;
   };
 }

spec/std/isa/isa/builtin_functions.idl

@@ -280,6 +280,23 @@ builtin function ebreak {
   }
 }
 
+builtin function execute_instruction {
+  arguments
+    Bits<32> encoding
+  description {
+    Fetch and execute the instruction with the given 32-bit encoding in the current hart
+    context. The instruction executes as if it were a normal instruction, with the
+    following differences:
+
+    * The PC is not advanced after execution (the calling instruction is responsible
+      for PC management).
+    * If the instruction raises an exception, the exception propagates normally.
+
+    This builtin is intended for use by instructions that dynamically dispatch
+    sub-instructions fetched from memory (e.g., instruction lookup table instructions).
+  }
+}
+
 builtin function prefetch_instruction {
   arguments
     XReg virtual_address     # virtual cache block address