Stop using mutable in Const.

Now that we only call `bitvectorize()` in non-const methods, we can move the casting-away-const to only happen
in `bitvectorize()`, which is deprecated so only some plugins (maybe) are using it.

This means `const` `Const` methods don't change the underlying data, which means
they'll be safe to use from multiple threads if/when we want to do that.

Author: rocallahan

kernel/rtlil.cc

@@ -204,12 +204,22 @@ const pool<IdString> &RTLIL::builtin_ff_cell_types() {
 
 #define check(condition) log_assert(condition && "malformed Const union")
 
-Const::bitvectype& Const::get_bits() const {
+const Const::bitvectype& Const::get_bits() const {
 	check(is_bits());
 	return *get_if_bits();
 }
 
-std::string& Const::get_str() const {
+const std::string& Const::get_str() const {
+	check(is_str());
+	return *get_if_str();
+}
+
+Const::bitvectype& Const::get_bits() {
+	check(is_bits());
+	return *get_if_bits();
+}
+
+std::string& Const::get_str() {
 	check(is_str());
 	return *get_if_str();
 }
@@ -395,7 +405,7 @@ bool RTLIL::Const::as_bool() const
 		return false;
 	}
 
-	bitvectype& bv = get_bits();
+	const bitvectype& bv = get_bits();
 	for (size_t i = 0; i < bv.size(); i++)
 		if (bv[i] == State::S1)
 			return true;
@@ -542,7 +552,7 @@ std::string RTLIL::Const::decode_string() const
 	if (auto str = get_if_str())
 		return *str;
 
-	bitvectype& bv = get_bits();
+	const bitvectype& bv = get_bits();
 	const int n = GetSize(bv);
 	const int n_over_8 = n / 8;
 	std::string s;
@@ -590,7 +600,7 @@ bool RTLIL::Const::empty() const {
 	}
 }
 
-void RTLIL::Const::bitvectorize_internal() const {
+void RTLIL::Const::bitvectorize_internal() {
 	if (tag == backing_tag::bits)
 		return;
 
@@ -641,7 +651,7 @@ bool RTLIL::Const::is_fully_zero() const
 		return true;
 	}
 
-	bitvectype& bv = get_bits();
+	const bitvectype& bv = get_bits();
 
 	for (const auto &bit : bv)
 		if (bit != RTLIL::State::S0)
@@ -661,7 +671,7 @@ bool RTLIL::Const::is_fully_ones() const
 		return true;
 	}
 
-	bitvectype& bv = get_bits();
+	const bitvectype& bv = get_bits();
 	for (const auto &bit : bv)
 		if (bit != RTLIL::State::S1)
 			return false;
@@ -676,7 +686,7 @@ bool RTLIL::Const::is_fully_def() const
 	if (is_str())
 		return true;
 
-	bitvectype& bv = get_bits();
+	const bitvectype& bv = get_bits();
 	for (const auto &bit : bv)
 		if (bit != RTLIL::State::S0 && bit != RTLIL::State::S1)
 			return false;
@@ -691,7 +701,7 @@ bool RTLIL::Const::is_fully_undef() const
 	if (auto str = get_if_str())
 		return str->empty();
 
-	bitvectype& bv = get_bits();
+	const bitvectype& bv = get_bits();
 	for (const auto &bit : bv)
 		if (bit != RTLIL::State::Sx && bit != RTLIL::State::Sz)
 			return false;
@@ -706,7 +716,7 @@ bool RTLIL::Const::is_fully_undef_x_only() const
 	if (auto str = get_if_str())
 		return str->empty();
 
-	bitvectype& bv = get_bits();
+	const bitvectype& bv = get_bits();
 	for (const auto &bit : bv)
 		if (bit != RTLIL::State::Sx)
 			return false;
@@ -742,7 +752,7 @@ Hasher RTLIL::Const::hash_into(Hasher h) const
 
 	// If the bits are all 0/1, hash packed bits using the string hash.
 	// Otherwise hash the leading packed bits with the rest of the bits individually.
-	bitvectype &bv = get_bits();
+	const bitvectype &bv = get_bits();
 	int size = GetSize(bv);
 	std::string packed;
 	int packed_size = (size + 7) >> 3;

kernel/rtlil.h

@@ -723,23 +723,27 @@ struct RTLIL::Const
 	using bitvectype = std::vector<RTLIL::State>;
 	enum class backing_tag: bool { bits, string };
 	// Do not access the union or tag even in Const methods unless necessary
-	mutable backing_tag tag;
+	backing_tag tag;
 	union {
-		mutable bitvectype bits_;
-		mutable std::string str_;
+		bitvectype bits_;
+		std::string str_;
 	};
 
 	// Use these private utilities instead
 	bool is_bits() const { return tag == backing_tag::bits; }
 	bool is_str() const { return tag == backing_tag::string; }
 
-	bitvectype* get_if_bits() const { return is_bits() ? &bits_ : NULL; }
-	std::string* get_if_str() const { return is_str() ? &str_ : NULL; }
+	bitvectype* get_if_bits() { return is_bits() ? &bits_ : NULL; }
+	std::string* get_if_str() { return is_str() ? &str_ : NULL; }
+	const bitvectype* get_if_bits() const { return is_bits() ? &bits_ : NULL; }
+	const std::string* get_if_str() const { return is_str() ? &str_ : NULL; }
 
-	bitvectype& get_bits() const;
-	std::string& get_str() const;
+	bitvectype& get_bits();
+	std::string& get_str();
+	const bitvectype& get_bits() const;
+	const std::string& get_str() const;
 	std::vector<RTLIL::State>& bits_internal();
-	void bitvectorize_internal() const;
+	void bitvectorize_internal();
 
 public:
 	Const() : flags(RTLIL::CONST_FLAG_NONE), tag(backing_tag::bits), bits_(std::vector<RTLIL::State>()) {}
@@ -772,7 +776,7 @@ struct RTLIL::Const
 	[[deprecated("Don't use direct access to the internal std::vector<State>, that's an implementation detail.")]]
 	std::vector<RTLIL::State>& bits() { return bits_internal(); }
 	[[deprecated("Don't call bitvectorize() directly, it's an implementation detail.")]]
-	void bitvectorize() const { bitvectorize_internal(); }
+	void bitvectorize() const { const_cast<Const*>(this)->bitvectorize_internal(); }
 
 	bool as_bool() const;