adds faster instruction decoding

src/vm/interp/vm_tgc_c.cpp

@@ -153,14 +153,22 @@ private:
     /****************************************************************************
      * start opcode definitions
      ****************************************************************************/
-    struct InstructionDesriptor {
+    struct instruction_descriptor {
         size_t length;
         uint32_t value;
         uint32_t mask;
         typename arch::traits<ARCH>::opcode_e op;
     };
-
+    struct decoding_tree_node{
-    const std::array<InstructionDesriptor, 87> instr_descr = {{
+        std::vector<instruction_descriptor> instrs;
+        std::vector<decoding_tree_node*> children;
+        uint32_t submask = std::numeric_limits<uint32_t>::max();
+        uint32_t value;
+        decoding_tree_node(uint32_t value) : value(value){}
+    };
+    
+    decoding_tree_node* root {nullptr};
+    const std::array<instruction_descriptor, 87> instr_descr = {{
          /* entries are: size, valid value, valid mask, function ptr */
         {32, 0b00000000000000000000000000110111, 0b00000000000000000000000001111111, arch::traits<ARCH>::opcode_e::LUI},
         {32, 0b00000000000000000000000000010111, 0b00000000000000000000000001111111, arch::traits<ARCH>::opcode_e::AUIPC},
@@ -251,18 +259,61 @@ private:
         {16, 0b0000000000000000, 0b1111111111111111, arch::traits<ARCH>::opcode_e::DII},
     }};
 
-    //static constexpr typename traits::addr_t upper_bits = ~traits::PGMASK;
     iss::status fetch_ins(virt_addr_t pc, uint8_t * data){
         auto phys_pc = this->core.v2p(pc);
-        //if ((pc.val & upper_bits) != ((pc.val + 2) & upper_bits)) { // we may cross a page boundary
-        //    if (this->core.read(phys_pc, 2, data) != iss::Ok) return iss::Err;
-        //    if ((data[0] & 0x3) == 0x3) // this is a 32bit instruction
-        //        if (this->core.read(this->core.v2p(pc + 2), 2, data + 2) != iss::Ok) return iss::Err;
-        //} else {
             if (this->core.read(phys_pc, 4, data) != iss::Ok)  return iss::Err;
-        //}
         return iss::Ok;
     }
+    void populate_decoding_tree(decoding_tree_node* root){
+        //create submask
+        for(auto instr: root->instrs){
+            root->submask &= instr.mask;
+        }
+        //put each instr according to submask&encoding into children
+        for(auto instr: root->instrs){
+            bool foundMatch = false;
+            for(auto child: root->children){
+                //use value as identifying trait
+                if(child->value == (instr.value&root->submask)){
+                    child->instrs.push_back(instr);
+                    foundMatch = true;
+                }
+            }
+            if(!foundMatch){
+                decoding_tree_node* child = new decoding_tree_node(instr.value&root->submask);
+                child->instrs.push_back(instr);
+                root->children.push_back(child);
+            }
+        }
+        root->instrs.clear();
+        //call populate_decoding_tree for all children
+        if(root->children.size() >1)
+            for(auto child: root->children){
+                populate_decoding_tree(child);      
+            }
+        else{
+            //sort instrs by value of the mask, this works bc we want to have the least restrictive one last
+            std::sort(root->children[0]->instrs.begin(), root->children[0]->instrs.end(), [](const instruction_descriptor& instr1, const instruction_descriptor& instr2) {
+            return instr1.mask > instr2.mask;
+            }); 
+        }
+    }
+    typename arch::traits<ARCH>::opcode_e  decodeInstr(decoding_tree_node* node, code_word_t word){
+        if(!node->children.size()){
+            if(node->instrs.size() == 1) return node->instrs[0].op;
+            for(auto instr : node->instrs){
+                if((instr.mask&word) == instr.value) return instr.op;
+            }
+        }
+        else{
+            for(auto child : node->children){
+                if (child->value == (node->submask&word)){
+                    return decodeInstr(child, word);
+                }  
+            }  
+        }
+        return arch::traits<ARCH>::opcode_e::MAX_OPCODE;
+    }
 };
 
 template <typename CODE_WORD> void debug_fn(CODE_WORD insn) {
@@ -290,15 +341,11 @@ template <typename ARCH>
 vm_impl<ARCH>::vm_impl(ARCH &core, unsigned core_id, unsigned cluster_id)
 : vm_base<ARCH>(core, core_id, cluster_id) {
     unsigned id=0;
-    for (auto instr : instr_descr) {
+    root = new decoding_tree_node(std::numeric_limits<uint32_t>::max());
-        auto quadrant = instr.value & 0x3;
+    for(auto instr:instr_descr){
-        qlut[quadrant].push_back(instruction_pattern{instr.value, instr.mask, instr.op});
+        root->instrs.push_back(instr);
-    }
-    for(auto& lut: qlut){
-        std::sort(std::begin(lut), std::end(lut), [](instruction_pattern const& a, instruction_pattern const& b){
-            return bit_count(a.mask) > bit_count(b.mask);
-        });
     }
+    populate_decoding_tree(root);
 }
 
 inline bool is_count_limit_enabled(finish_cond_e cond){
@@ -309,14 +356,6 @@ inline bool is_jump_to_self_enabled(finish_cond_e cond){
     return (cond & finish_cond_e::JUMP_TO_SELF) == finish_cond_e::JUMP_TO_SELF;
 }
 
-template <typename ARCH>
-typename arch::traits<ARCH>::opcode_e vm_impl<ARCH>::decode_inst_id(code_word_t instr){
-    for(auto& e: qlut[instr&0x3]){
-        if(!((instr&e.mask) ^ e.value )) return e.id;
-    }
-    return arch::traits<ARCH>::opcode_e::MAX_OPCODE;
-}
-
 template <typename ARCH>
 typename vm_base<ARCH>::virt_addr_t vm_impl<ARCH>::execute_inst(finish_cond_e cond, virt_addr_t start, uint64_t icount_limit){
     auto pc=start;
@@ -338,7 +377,7 @@ typename vm_base<ARCH>::virt_addr_t vm_impl<ARCH>::execute_inst(finish_cond_e co
         } else {
             if (is_jump_to_self_enabled(cond) &&
                     (instr == 0x0000006f || (instr&0xffff)==0xa001)) throw simulation_stopped(0); // 'J 0' or 'C.J 0'
-            auto inst_id = decode_inst_id(instr);
+            auto inst_id = decodeInstr(root, instr);
             // pre execution stuff
              this->core.reg.last_branch = 0;
             if(this->sync_exec && PRE_SYNC) this->do_sync(PRE_SYNC, static_cast<unsigned>(inst_id));