diff --git a/kernel/mem.cc b/kernel/mem.cc
index 01c866770..46e4331c5 100644
--- a/kernel/mem.cc
+++ b/kernel/mem.cc
@@ -1679,3 +1679,219 @@ SigSpec MemWr::decompress_en(const std::vector<int> &swizzle, SigSpec sig) {
 		res.append(sig[i]);
 	return res;
 }
+
+using addr_t = MemContents::addr_t;
+
+MemContents::MemContents(Mem *mem) :
+	MemContents(ceil_log2(mem->size), mem->width)
+{
+	for(const auto &init : mem->inits) {
+		if(init.en.is_fully_zero()) continue;
+		log_assert(init.en.size() == _data_width);
+		if(init.en.is_fully_ones())
+			insert_concatenated(init.addr.as_int(), init.data);
+		else {
+			// TODO: this case could be handled more efficiently by adding
+			// a flag to reserve_range that tells it to preserve
+			// previous contents
+			addr_t addr = init.addr.as_int();
+			addr_t words = init.data.size() / _data_width;
+			RTLIL::Const data = init.data;
+			log_assert(data.size() % _data_width == 0);
+			for(addr_t i = 0; i < words; i++) {
+				RTLIL::Const previous = (*this)[addr + i];
+				for(int j = 0; j < _data_width; j++)
+					if(init.en[j] != State::S1)
+						data[_data_width * i + j] = previous[j];
+			}
+			insert_concatenated(init.addr.as_int(), data);
+		}
+	}
+}
+
+MemContents::iterator & MemContents::iterator::operator++() {
+	auto it = _memory->_values.upper_bound(_addr);
+	if(it == _memory->_values.end()) {
+		_memory = nullptr;
+		_addr = ~(addr_t) 0;
+	} else
+		_addr = it->first;
+	return *this;
+}
+
+void MemContents::check() {
+	log_assert(_addr_width > 0 && _addr_width < (int)sizeof(addr_t) * 8);
+	log_assert(_data_width > 0);
+	log_assert(_default_value.size() == _data_width);
+	if(_values.empty()) return;
+	auto it = _values.begin();
+	for(;;) {
+		log_assert(!it->second.empty());
+		log_assert(it->second.size() % _data_width == 0);
+		auto end1 = _range_end(it);
+		log_assert(_range_begin(it) < (1<<_addr_width));
+		log_assert(end1 <= (1<<_addr_width));
+		if(++it == _values.end())
+			break;
+		// check that ranges neither overlap nor touch
+		log_assert(_range_begin(it) > end1);
+	}
+}
+
+bool MemContents::_range_contains(std::map<addr_t, RTLIL::Const>::iterator it, addr_t addr) const {
+	// if addr < begin, the subtraction will overflow, and the comparison will always fail
+	// (since we have an invariant that begin + size <= 2^(addr_t bits))
+	return it != _values.end() && addr - _range_begin(it) < _range_size(it);
+}
+
+
+bool MemContents::_range_contains(std::map<addr_t, RTLIL::Const>::iterator it, addr_t begin_addr, addr_t end_addr) const {
+	// note that we assume begin_addr <= end_addr
+	return it != _values.end() && _range_begin(it) <= begin_addr && end_addr - _range_begin(it) <= _range_size(it);
+}
+
+bool MemContents::_range_overlaps(std::map<addr_t, RTLIL::Const>::iterator it, addr_t begin_addr, addr_t end_addr) const {
+	if(it == _values.end() || begin_addr >= end_addr)
+		return false;
+	auto top1 = _range_end(it) - 1;
+	auto top2 = end_addr - 1;
+	return !(top1 < begin_addr || top2 < _range_begin(it));
+}
+
+std::map<addr_t, RTLIL::Const>::iterator MemContents::_range_at(addr_t addr) const {
+	// allow addr == 1<<_addr_width (which will just return end())
+	log_assert(addr <= 1<<_addr_width);
+	// get the first range with base > addr
+	// (we use const_cast since map::iterators are only passed around internally and not exposed to the user
+	// and using map::iterator in both the const and non-const case simplifies the code a little,
+	// at the cost of having to be a little careful when implementing const methods)
+	auto it = const_cast<std::map<addr_t, RTLIL::Const> &>(_values).upper_bound(addr);
+	// if we get the very first range, all ranges are past the addr, so return the first one
+	if(it == _values.begin())
+		return it;
+	// otherwise, go back to the previous interval
+	// this must be the last interval with base <= addr
+	auto it_prev = std::next(it, -1);
+	if(_range_contains(it_prev, addr))
+		return it_prev;
+	else
+		return it;
+}
+
+RTLIL::Const MemContents::operator[](addr_t addr) const {
+	auto it = _range_at(addr);
+	if(_range_contains(it, addr))
+		return it->second.extract(_range_offset(it, addr), _data_width);
+	else
+		return _default_value;
+}
+
+addr_t MemContents::count_range(addr_t begin_addr, addr_t end_addr) const {
+	addr_t count = 0;
+	for(auto it = _range_at(begin_addr); _range_overlaps(it, begin_addr, end_addr); it++) {
+		auto first = std::max(_range_begin(it), begin_addr);
+		auto last = std::min(_range_end(it), end_addr);
+		count += last - first;
+	}
+	return count;
+}
+
+void MemContents::clear_range(addr_t begin_addr, addr_t end_addr) {
+	if(begin_addr >= end_addr) return;
+	// identify which ranges are affected by this operation
+	// the first iterator affected is the first one containing any addr >= begin_addr
+	auto begin_it = _range_at(begin_addr);
+	// the first iterator *not* affected is the first one with base addr > end_addr - 1
+	auto end_it = _values.upper_bound(end_addr - 1);
+	if(begin_it == end_it)
+		return; // nothing to do
+	// the last iterator affected is one before the first one not affected
+	auto last_it = std::next(end_it, -1);
+	// the first and last range may need to be truncated, the rest can just be deleted
+	// to handle the begin_it == last_it case correctly, do the end case first by inserting a new range past the end
+	if(_range_contains(last_it, end_addr - 1)) {
+		auto new_begin = end_addr;
+		auto end = _range_end(last_it);
+		// if there is data past the end address, preserve it by creating a new range
+		if(new_begin != end)
+			end_it = _values.emplace_hint(last_it, new_begin, last_it->second.extract(_range_offset(last_it, new_begin), (_range_end(last_it) - new_begin) * _data_width));
+		// the original range will either be truncated in the next if() block or deleted in the erase, so we can leave it untruncated
+	}
+	if(_range_contains(begin_it, begin_addr)) {
+		auto new_end = begin_addr;
+		// if there is data before the start address, truncate but don't delete
+		if(new_end != begin_it->first) {
+			begin_it->second.extu(_range_offset(begin_it, new_end));
+			++begin_it;
+		}
+		// else: begin_it will be deleted
+	}
+	_values.erase(begin_it, end_it);
+}
+
+std::map<addr_t, RTLIL::Const>::iterator MemContents::_reserve_range(addr_t begin_addr, addr_t end_addr) {
+	if(begin_addr >= end_addr)
+		return _values.end(); // need a dummy value to return, end() is cheap
+	// find the first range containing any addr >= begin_addr - 1
+	auto lower_it = begin_addr == 0 ? _values.begin() : _range_at(begin_addr - 1);
+	// check if our range is already covered by a single range
+	// note that since ranges are not allowed to touch, if any range contains begin_addr, lower_it equals that range
+	if (_range_contains(lower_it, begin_addr, end_addr))
+		return lower_it;
+	// find the first range containing any addr >= end_addr
+	auto upper_it = _range_at(end_addr);
+	// check if either of the two ranges we just found touch our range
+	bool lower_touch = begin_addr > 0 && _range_contains(lower_it, begin_addr - 1);
+	bool upper_touch = _range_contains(upper_it, end_addr);
+	if (lower_touch && upper_touch) {
+		log_assert (lower_it != upper_it); // lower_it == upper_it should be excluded by the check above
+		// we have two different ranges touching at either end, we need to merge them
+		auto upper_end = _range_end(upper_it);
+		// make range bigger (maybe reserve here instead of resize?)
+		lower_it->second.bits.resize(_range_offset(lower_it, upper_end), State::Sx);
+		// copy only the data beyond our range
+		std::copy(_range_data(upper_it, end_addr), _range_data(upper_it, upper_end), _range_data(lower_it, end_addr));
+		// keep lower_it, but delete upper_it
+		_values.erase(std::next(lower_it), std::next(upper_it));
+		return lower_it;
+	} else if (lower_touch) {
+		// we have a range to the left, just make it bigger and delete any other that may exist.
+		lower_it->second.bits.resize(_range_offset(lower_it, end_addr), State::Sx);
+		// keep lower_it and upper_it
+		_values.erase(std::next(lower_it), upper_it);
+		return lower_it;
+	} else if (upper_touch) {
+		// we have a range to the right, we need to expand it
+		// since we need to erase and reinsert to a new address, steal the data
+		RTLIL::Const data = std::move(upper_it->second);
+		// note that begin_addr is not in upper_it, otherwise the whole range covered check would have tripped
+		data.bits.insert(data.bits.begin(), (_range_begin(upper_it) - begin_addr) * _data_width, State::Sx);
+		// delete lower_it and upper_it, then reinsert
+		_values.erase(lower_it, std::next(upper_it));
+		return _values.emplace(begin_addr, std::move(data)).first;
+	} else {
+		// no ranges are touching, so just delete all ranges in our range and allocate a new one
+		// could try to resize an existing range but not sure if that actually helps
+		_values.erase(lower_it, upper_it);
+		return _values.emplace(begin_addr, RTLIL::Const(State::Sx, (end_addr - begin_addr) * _data_width)).first;
+	}
+}
+
+void MemContents::insert_concatenated(addr_t addr, RTLIL::Const const &values) {
+	addr_t words = (values.size() + _data_width - 1) / _data_width;
+	log_assert(addr < 1<<_addr_width);
+	log_assert(words <= (1<<_addr_width) - addr);
+	auto it = _reserve_range(addr, addr + words);
+	auto to_begin = _range_data(it, addr);
+	std::copy(values.bits.begin(), values.bits.end(), to_begin);
+	// if values is not word-aligned, fill any missing bits with 0
+	std::fill(to_begin + values.size(), to_begin + words * _data_width, State::S0);
+}
+
+std::vector<State>::iterator MemContents::_range_write(std::vector<State>::iterator it, RTLIL::Const const &word) {
+	auto from_end = word.size() <= _data_width ? word.bits.end() : word.bits.begin() + _data_width;
+	auto to_end = std::copy(word.bits.begin(), from_end, it);
+	auto it_next = std::next(it, _data_width);
+	std::fill(to_end, it_next, State::S0);
+	return it_next;
+}
\ No newline at end of file
diff --git a/kernel/mem.h b/kernel/mem.h
index 8c484274c..4be4b6864 100644
--- a/kernel/mem.h
+++ b/kernel/mem.h
@@ -224,6 +224,122 @@ struct Mem : RTLIL::AttrObject {
 	Mem(Module *module, IdString memid, int width, int start_offset, int size) : module(module), memid(memid), packed(false), mem(nullptr), cell(nullptr), width(width), start_offset(start_offset), size(size) {}
 };
 
+// this class is used for implementing operator-> on iterators that return values rather than references
+// it's necessary because in C++ operator-> is called recursively until a raw pointer is obtained
+template<class T>
+struct arrow_proxy {
+	T v;
+	explicit arrow_proxy(T const & v) : v(v) {}
+	T* operator->() { return &v; }
+};
+
+// MemContents efficiently represents the contents of a potentially sparse memory by storing only those segments that are actually defined
+class MemContents {
+public:
+	class range; class iterator;
+	using addr_t = uint32_t;
+private:
+	// we ban _addr_width == sizeof(addr_t) * 8 because it adds too many cornercases
+	int _addr_width;
+	int _data_width;
+	RTLIL::Const _default_value;
+	// for each range, store the concatenation of the words at the start address
+	// invariants:
+	// - no overlapping or adjacent ranges
+	// - no empty ranges
+	// - all Consts are a multiple of the word size
+	std::map<addr_t, RTLIL::Const> _values;
+	// returns an iterator to the range containing addr, if it exists, or the first range past addr
+	std::map<addr_t, RTLIL::Const>::iterator _range_at(addr_t addr) const;
+	addr_t _range_size(std::map<addr_t, RTLIL::Const>::iterator it) const { return it->second.size() / _data_width; }
+	addr_t _range_begin(std::map<addr_t, RTLIL::Const>::iterator it) const { return it->first; }
+	addr_t _range_end(std::map<addr_t, RTLIL::Const>::iterator it) const { return _range_begin(it) + _range_size(it); }
+	// check if the iterator points to a range containing addr
+	bool _range_contains(std::map<addr_t, RTLIL::Const>::iterator it, addr_t addr) const;
+	// check if the iterator points to a range containing [begin_addr, end_addr). assumes end_addr >= begin_addr.
+	bool _range_contains(std::map<addr_t, RTLIL::Const>::iterator it, addr_t begin_addr, addr_t end_addr) const;
+	// check if the iterator points to a range overlapping with [begin_addr, end_addr)
+	bool _range_overlaps(std::map<addr_t, RTLIL::Const>::iterator it, addr_t begin_addr, addr_t end_addr) const;
+	// return the offset the addr would have in the range at `it`
+	size_t _range_offset(std::map<addr_t, RTLIL::Const>::iterator it, addr_t addr) const { return (addr - it->first) * _data_width; }
+	// assuming _range_contains(it, addr), return an iterator pointing to the data at addr
+	std::vector<State>::iterator _range_data(std::map<addr_t, RTLIL::Const>::iterator it, addr_t addr) { return it->second.bits.begin() + _range_offset(it, addr); }
+	// internal version of reserve_range that returns an iterator to the range
+	std::map<addr_t, RTLIL::Const>::iterator _reserve_range(addr_t begin_addr, addr_t end_addr);
+	// write a single word at addr, return iterator to next word
+	std::vector<State>::iterator _range_write(std::vector<State>::iterator it, RTLIL::Const const &data);
+public:
+	class range {
+		int _data_width;
+		addr_t _base;
+		RTLIL::Const const &_values;
+		friend class iterator;
+		range(int data_width, addr_t base, RTLIL::Const const &values)
+		: _data_width(data_width), _base(base), _values(values) {}
+	public:
+		addr_t base() const { return _base; }
+		addr_t size() const { return ((addr_t) _values.size()) / _data_width; }
+		addr_t limit() const { return _base + size(); }
+		RTLIL::Const const &concatenated() const { return _values; }
+		RTLIL::Const operator[](addr_t addr) const {
+			log_assert(addr - _base < size());
+			return _values.extract((addr - _base) * _data_width, _data_width);
+		}
+		RTLIL::Const at_offset(addr_t offset) const { return (*this)[_base + offset]; }
+	};
+	class iterator {
+		MemContents const *_memory;
+		// storing addr instead of an iterator gives more well-defined behaviour under insertions/deletions
+		// use ~0 for end so that all end iterators compare the same
+		addr_t _addr;
+		friend class MemContents;
+		iterator(MemContents const *memory, addr_t addr) : _memory(memory), _addr(addr) {}
+	public:
+		using iterator_category = std::input_iterator_tag;
+		using value_type = range;
+		using pointer = arrow_proxy<range>;
+		using reference = range;
+		using difference_type = addr_t;
+		reference operator *() const { return range(_memory->_data_width, _addr, _memory->_values.at(_addr)); }
+		pointer operator->() const { return arrow_proxy<range>(**this); }
+		bool operator !=(iterator const &other) const { return _memory != other._memory || _addr != other._addr; }
+		iterator &operator++();
+	};
+	MemContents(int addr_width, int data_width, RTLIL::Const default_value)
+		: _addr_width(addr_width), _data_width(data_width)
+		, _default_value((default_value.extu(data_width), std::move(default_value)))
+	{ log_assert(_addr_width > 0 && _addr_width < (int)sizeof(addr_t) * 8); log_assert(_data_width > 0); }
+	MemContents(int addr_width, int data_width) : MemContents(addr_width, data_width, RTLIL::Const(State::Sx, data_width)) {}
+	explicit MemContents(Mem *mem);
+	int addr_width() const { return _addr_width; }
+	int data_width() const { return _data_width; }
+	RTLIL::Const const &default_value() const { return _default_value; }
+	// return the value at the address if it exists, the default_value of the memory otherwise. address must not exceed 2**addr_width.
+	RTLIL::Const operator [](addr_t addr) const;
+	// return the number of defined words in the range [begin_addr, end_addr)
+	addr_t count_range(addr_t begin_addr, addr_t end_addr) const;
+	// allocate memory for the range [begin_addr, end_addr), but leave the contents undefined.
+	void reserve_range(addr_t begin_addr, addr_t end_addr) { _reserve_range(begin_addr, end_addr); }
+	// insert multiple words (provided as a single concatenated RTLIL::Const) at the given address, overriding any previous assignment.
+	void insert_concatenated(addr_t addr, RTLIL::Const const &values);
+	// insert multiple words at the given address, overriding any previous assignment.
+	template<typename Iterator> void insert_range(addr_t addr, Iterator begin, Iterator end) {
+		auto words = end - begin;
+		log_assert(addr < 1<<_addr_width); log_assert(words <= (1<<_addr_width) - addr);
+		auto range = _reserve_range(addr, addr + words);
+		auto it = _range_data(range, addr);
+		for(; begin != end; ++begin)
+			it = _range_write(it, *begin);
+	}
+	// undefine all words in the range [begin_addr, end_addr)
+	void clear_range(addr_t begin_addr, addr_t end_addr);
+	// check invariants, abort if invariants failed
+	void check();
+	iterator end() const { return iterator(nullptr, ~(addr_t) 0); }
+	iterator begin() const { return _values.empty() ? end() : iterator(this, _values.begin()->first); }
+	bool empty() const { return _values.empty(); }
+};
+
 YOSYS_NAMESPACE_END
 
 #endif