Make IdString refcounts a hashtable containing only the nonzero refcounts

This saves space and doesn't cost very much since we hardly ever have nonzero refcounts any more.

It also allows for IdStrings with negative indexes, which we're going to add.

Author: rocallahan

kernel/rtlil.cc

@@ -38,7 +38,7 @@ RTLIL::IdString::destruct_guard_t RTLIL::IdString::destruct_guard;
 std::vector<char*> RTLIL::IdString::global_id_storage_;
 std::unordered_map<std::string_view, int> RTLIL::IdString::global_id_index_;
 #ifndef YOSYS_NO_IDS_REFCNT
-std::vector<uint32_t> RTLIL::IdString::global_refcount_storage_;
+std::unordered_map<int, int> RTLIL::IdString::global_refcount_storage_;
 std::vector<int> RTLIL::IdString::global_free_idx_list_;
 #endif
 
@@ -62,7 +62,6 @@ void RTLIL::IdString::prepopulate()
 	global_id_storage_.reserve(size);
 	RTLIL::IdString::global_id_storage_.push_back(const_cast<char*>(""));
 	global_id_index_.reserve(size);
-	global_refcount_storage_.resize(size, 1);
 #define X(N) populate("\\" #N);
 #include "kernel/constids.inc"
 #undef X
@@ -179,17 +178,20 @@ struct IdStringCollector {
 void RTLIL::OwningIdString::collect_garbage()
 {
 #ifndef YOSYS_NO_IDS_REFCNT
-	int size = GetSize(global_refcount_storage_);
+	int size = GetSize(global_id_storage_);
 	IdStringCollector collector(size);
 	for (auto &[idx, design] : *RTLIL::Design::get_all_designs()) {
 		collector.trace(*design);
 	}
 	for (int i = static_cast<int>(StaticId::STATIC_ID_END); i < size; ++i) {
-		if (collector.live[i] || global_refcount_storage_[i] > 0)
+		if (collector.live[i])
 			continue;
 		char *&storage = global_id_storage_.at(i);
 		if (storage == nullptr)
 			continue;
+		if (global_refcount_storage_.find(i) != global_refcount_storage_.end())
+			continue;
+
 		if (yosys_xtrace) {
 			log("#X# Removed IdString '%s' with index %d.\n", storage, i);
 			log_backtrace("-X- ", yosys_xtrace-1);

kernel/rtlil.h

@@ -153,16 +153,18 @@ struct RTLIL::IdString
 	static std::vector<char*> global_id_storage_;
 	static std::unordered_map<std::string_view, int> global_id_index_;
 #ifndef YOSYS_NO_IDS_REFCNT
-	// For prepopulated IdStrings, the refcount is meaningless since they
-	// are never freed even if the refcount is zero. For code efficiency
-	// we increment the refcount of prepopulated IdStrings like any other string,
-	// but we never decrement the refcount or check whether it's zero.
-	// So, make this unsigned because refcounts of preopulated IdStrings may overflow
-	// and overflow of signed integers is undefined behavior.
-	static std::vector<uint32_t> global_refcount_storage_;
+	// All (index, refcount) pairs in this map have refcount > 0.
+	static std::unordered_map<int, int> global_refcount_storage_;
 	static std::vector<int> global_free_idx_list_;
 #endif
 
+	static int refcount(int idx) {
+		auto it = global_refcount_storage_.find(idx);
+		if (it == global_refcount_storage_.end())
+			return 0;
+		return it->second;
+	}
+
 	static inline void xtrace_db_dump()
 	{
 	#ifdef YOSYS_XTRACE_GET_PUT
@@ -171,7 +173,7 @@ struct RTLIL::IdString
 			if (global_id_storage_.at(idx) == nullptr)
 				log("#X# DB-DUMP index %d: FREE\n", idx);
 			else
-				log("#X# DB-DUMP index %d: '%s' (ref %u)\n", idx, global_id_storage_.at(idx), global_refcount_storage_.at(idx));
+				log("#X# DB-DUMP index %d: '%s' (ref %u)\n", idx, refcount(idx), refcount);
 		}
 	#endif
 	}
@@ -191,7 +193,7 @@ struct RTLIL::IdString
 		if (it != global_id_index_.end()) {
 	#ifdef YOSYS_XTRACE_GET_PUT
 			if (yosys_xtrace)
-				log("#X# GET-BY-NAME '%s' (index %d, refcount %u)\n", global_id_storage_.at(it->second), it->second, global_refcount_storage_.at(it->second));
+				log("#X# GET-BY-NAME '%s' (index %d, refcount %u)\n", global_id_storage_.at(it->second), it->second, refcount(it->second));
 	#endif
 			return it->second;
 		}
@@ -211,7 +213,6 @@ struct RTLIL::IdString
 			log_assert(global_id_storage_.size() < 0x40000000);
 			global_free_idx_list_.push_back(global_id_storage_.size());
 			global_id_storage_.push_back(nullptr);
-			global_refcount_storage_.push_back(0);
 		}
 
 		int idx = global_free_idx_list_.back();
@@ -234,7 +235,7 @@ struct RTLIL::IdString
 
 	#ifdef YOSYS_XTRACE_GET_PUT
 		if (yosys_xtrace)
-			log("#X# GET-BY-NAME '%s' (index %d, refcount %u)\n", global_id_storage_.at(idx), idx, global_refcount_storage_.at(idx));
+			log("#X# GET-BY-NAME '%s' (index %d, refcount %u)\n", global_id_storage_.at(idx), idx, refcount(idx));
 	#endif
 
 		return idx;
@@ -436,11 +437,17 @@ struct RTLIL::OwningIdString : public RTLIL::IdString {
 	static void get_reference(int idx)
 	{
 	#ifndef YOSYS_NO_IDS_REFCNT
-		global_refcount_storage_[idx]++;
+		if (idx < static_cast<short>(StaticId::STATIC_ID_END))
+			return;
+		auto it = global_refcount_storage_.find(idx);
+		if (it == global_refcount_storage_.end())
+			global_refcount_storage_.insert(it, {idx, 1});
+		else
+			++it->second;
 	#endif
 	#ifdef YOSYS_XTRACE_GET_PUT
 		if (yosys_xtrace && idx >= static_cast<short>(StaticId::STATIC_ID_END))
-			log("#X# GET-BY-INDEX '%s' (index %d, refcount %u)\n", global_id_storage_.at(idx), idx, global_refcount_storage_.at(idx));
+			log("#X# GET-BY-INDEX '%s' (index %d, refcount %u)\n", global_id_storage_.at(idx), idx, refcount(idx));
 	#endif
 	}
 
@@ -452,11 +459,14 @@ struct RTLIL::OwningIdString : public RTLIL::IdString {
 		if (index_ < static_cast<short>(StaticId::STATIC_ID_END) || !destruct_guard_ok)
 			return;
 	#ifdef YOSYS_XTRACE_GET_PUT
-		if (yosys_xtrace) {
-			log("#X# PUT '%s' (index %d, refcount %u)\n", global_id_storage_.at(index_), index_, global_refcount_storage_.at(index_));
-		}
+		if (yosys_xtrace)
+			log("#X# PUT '%s' (index %d, refcount %u)\n", global_id_storage_.at(index_), index_, refcount(index_));
 	#endif
-		--global_refcount_storage_[index_];
+		auto it = global_refcount_storage_.find(index_);
+		log_assert(it != global_refcount_storage_.end() && it->second >= 1);
+		if (--it->second == 0) {
+			global_refcount_storage_.erase(it);
+		}
 	#endif
 	}
 };