Use a custom generic tagged union Either instead of std::variant for efficiency (#1476)

* Implement custom generic tagged union `Either`

This should be more efficient than `std::variant`, while still
keeping runtime safety as it `assert`s when `get`ting values.

* Use `Either` for RPN expressions

* Use `Either` for file stack node data

* Use `Either` for `File` buffer

* Use `Either` for `STRFMT` args

* Use `Either` for RGBLINK symbol values

* Support an equivalent of `std::monostate` for `Either`

* Use `Either` for lexer tokens

* Use `Either` for symbol values

* Use `Either` for lexer mmap/buffer state

src/link/main.cpp

@@ -46,28 +46,8 @@ FILE *linkerScript;
 
 static uint32_t nbErrors = 0;
 
-std::vector<uint32_t> &FileStackNode::iters() {
-	assume(std::holds_alternative<std::vector<uint32_t>>(data));
-	return std::get<std::vector<uint32_t>>(data);
-}
-
-std::vector<uint32_t> const &FileStackNode::iters() const {
-	assume(std::holds_alternative<std::vector<uint32_t>>(data));
-	return std::get<std::vector<uint32_t>>(data);
-}
-
-std::string &FileStackNode::name() {
-	assume(std::holds_alternative<std::string>(data));
-	return std::get<std::string>(data);
-}
-
-std::string const &FileStackNode::name() const {
-	assume(std::holds_alternative<std::string>(data));
-	return std::get<std::string>(data);
-}
-
 std::string const &FileStackNode::dump(uint32_t curLineNo) const {
-	if (std::holds_alternative<std::vector<uint32_t>>(data)) {
+	if (data.holds<std::vector<uint32_t>>()) {
 		assume(parent); // REPT nodes use their parent's name
 		std::string const &lastName = parent->dump(lineNo);
 		fputs(" -> ", stderr);

src/link/object.cpp

@@ -503,7 +503,7 @@ void obj_ReadFile(char const *fileName, unsigned int fileID) {
 		// object file. It's better than nothing.
 		nodes[fileID].push_back({
 		    .type = NODE_FILE,
-		    .data = fileName,
+		    .data = Either<std::vector<uint32_t>, std::string>(fileName),
 		    .parent = nullptr,
 		    .lineNo = 0,
 		});
@@ -566,8 +566,8 @@ void obj_ReadFile(char const *fileName, unsigned int fileID) {
 
 		if (symbol.type == SYMTYPE_EXPORT)
 			sym_AddSymbol(symbol);
-		if (auto *label = std::get_if<Label>(&symbol.data); label)
-			nbSymPerSect[label->sectionID]++;
+		if (symbol.data.holds<Label>())
+			nbSymPerSect[symbol.data.get<Label>().sectionID]++;
 	}
 
 	// This file's sections, stored in a table to link symbols to them
@@ -605,12 +605,11 @@ void obj_ReadFile(char const *fileName, unsigned int fileID) {
 
 	// Give symbols' section pointers to their sections
 	for (uint32_t i = 0; i < nbSymbols; i++) {
-		if (auto *label = std::get_if<Label>(&fileSymbols[i].data); label) {
-			Section *section = fileSections[label->sectionID].get();
-
-			label->section = section;
+		if (fileSymbols[i].data.holds<Label>()) {
+			Label &label = fileSymbols[i].data.get<Label>();
+			label.section = fileSections[label.sectionID].get();
 			// Give the section a pointer to the symbol as well
-			linkSymToSect(fileSymbols[i], *section);
+			linkSymToSect(fileSymbols[i], *label.section);
 		}
 	}
 
@@ -622,13 +621,15 @@ void obj_ReadFile(char const *fileName, unsigned int fileID) {
 	// This has to run **after** all the `sect_AddSection()` calls,
 	// so that `sect_GetSection()` will work
 	for (uint32_t i = 0; i < nbSymbols; i++) {
-		if (auto *label = std::get_if<Label>(&fileSymbols[i].data); label) {
-			if (Section *section = label->section; section->modifier != SECTION_NORMAL) {
-				if (section->modifier == SECTION_FRAGMENT)
+		if (fileSymbols[i].data.holds<Label>()) {
+			Label &label = fileSymbols[i].data.get<Label>();
+			if (Section *section = label.section; section->modifier != SECTION_NORMAL) {
+				if (section->modifier == SECTION_FRAGMENT) {
 					// Add the fragment's offset to the symbol's (`section->offset` is computed by `sect_AddSection`)
-					label->offset += section->offset;
+					label.offset += section->offset;
+				}
 				// Associate the symbol with the main section, not the "component" one
-				label->section = sect_GetSection(section->name);
+				label.section = sect_GetSection(section->name);
 			}
 		}
 	}

src/link/output.cpp

@@ -564,16 +564,16 @@ static void writeSym() {
 	constants.clear();
 	sym_ForEach([](Symbol &sym) {
 		// Symbols are already limited to the exported ones
-		if (std::holds_alternative<int32_t>(sym.data))
+		if (sym.data.holds<int32_t>())
 			constants.push_back(&sym);
 	});
 	// Numeric constants are ordered by value, then by name
 	std::sort(RANGE(constants), [](Symbol *sym1, Symbol *sym2) -> bool {
-		int32_t val1 = std::get<int32_t>(sym1->data), val2 = std::get<int32_t>(sym2->data);
+		int32_t val1 = sym1->data.get<int32_t>(), val2 = sym2->data.get<int32_t>();
 		return val1 != val2 ? val1 < val2 : sym1->name < sym2->name;
 	});
 	for (Symbol *sym : constants) {
-		int32_t val = std::get<int32_t>(sym->data);
+		int32_t val = sym->data.get<int32_t>();
 		int width = val < 0x100 ? 2 : val < 0x10000 ? 4 : 8;
 		fprintf(symFile, "%0*" PRIx32 " %s\n", width, val, sym->name.c_str());
 	}

src/link/patch.cpp

@@ -5,7 +5,6 @@
 #include <deque>
 #include <inttypes.h>
 #include <stdint.h>
-#include <variant>
 #include <vector>
 
 #include "helpers.hpp" // assume, clz, ctz
@@ -230,8 +229,8 @@ static int32_t computeRPNExpr(Patch const &patch, std::vector<Symbol> const &fil
 				);
 				isError = true;
 				value = 1;
-			} else if (auto *label = std::get_if<Label>(&symbol->data); label) {
-				value = label->section->bank;
+			} else if (symbol->data.holds<Label>()) {
+				value = symbol->data.get<Label>().section->bank;
 			} else {
 				error(
 				    patch.src,
@@ -390,11 +389,11 @@ static int32_t computeRPNExpr(Patch const &patch, std::vector<Symbol> const &fil
 					    fileSymbols[value].name.c_str()
 					);
 					isError = true;
-				} else if (auto *label = std::get_if<Label>(&symbol->data); label) {
-					value = label->section->org + label->offset;
+				} else if (symbol->data.holds<Label>()) {
+					Label const &label = symbol->data.get<Label>();
+					value = label.section->org + label.offset;
 				} else {
-					assume(std::holds_alternative<int32_t>(symbol->data));
-					value = std::get<int32_t>(symbol->data);
+					value = symbol->data.get<int32_t>();
 				}
 			}
 			break;

src/link/sdas_obj.cpp

@@ -8,7 +8,6 @@
 #include <stdint.h>
 #include <string.h>
 #include <tuple>
-#include <variant>
 
 #include "helpers.hpp" // assume
 #include "linkdefs.hpp"
@@ -395,10 +394,11 @@ void sdobj_ReadFile(FileStackNode const &where, FILE *file, std::vector<Symbol>
 					// The same symbol can only be defined twice if neither
 					// definition is in a floating section
 					auto checkSymbol = [](Symbol const &sym) -> std::tuple<Section *, int32_t> {
-						if (auto *label = std::get_if<Label>(&sym.data); label)
-							return {label->section, label->offset};
-						assume(std::holds_alternative<int32_t>(sym.data));
-						return {nullptr, std::get<int32_t>(sym.data)};
+						if (sym.data.holds<Label>()) {
+							Label const &label = sym.data.get<Label>();
+							return {label.section, label.offset};
+						}
+						return {nullptr, sym.data.get<int32_t>()};
 					};
 					auto [symbolSection, symbolValue] = checkSymbol(symbol);
 					auto [otherSection, otherValue] = checkSymbol(*other);
@@ -876,13 +876,15 @@ void sdobj_ReadFile(FileStackNode const &where, FILE *file, std::vector<Symbol>
 	// This has to run **after** all the `sect_AddSection()` calls,
 	// so that `sect_GetSection()` will work
 	for (Symbol &sym : fileSymbols) {
-		if (auto *label = std::get_if<Label>(&sym.data); label) {
-			if (Section *section = label->section; section->modifier != SECTION_NORMAL) {
-				if (section->modifier == SECTION_FRAGMENT)
+		if (sym.data.holds<Label>()) {
+			Label &label = sym.data.get<Label>();
+			if (Section *section = label.section; section->modifier != SECTION_NORMAL) {
+				if (section->modifier == SECTION_FRAGMENT) {
 					// Add the fragment's offset to the symbol's (`section->offset` is computed by `sect_AddSection`)
-					label->offset += section->offset;
+					label.offset += section->offset;
+				}
 				// Associate the symbol with the main section, not the "component" one
-				label->section = sect_GetSection(section->name);
+				label.section = sect_GetSection(section->name);
 			}
 		}
 	}

src/link/symbol.cpp

@@ -12,16 +12,6 @@
 
 std::unordered_map<std::string, Symbol *> symbols;
 
-Label &Symbol::label() {
-	assume(std::holds_alternative<Label>(data));
-	return std::get<Label>(data);
-}
-
-Label const &Symbol::label() const {
-	assume(std::holds_alternative<Label>(data));
-	return std::get<Label>(data);
-}
-
 void sym_ForEach(void (*callback)(Symbol &)) {
 	for (auto &it : symbols)
 		callback(*it.second);
@@ -29,8 +19,9 @@ void sym_ForEach(void (*callback)(Symbol &)) {
 
 void sym_AddSymbol(Symbol &symbol) {
 	Symbol *other = sym_GetSymbol(symbol.name);
-	auto *symValue = std::get_if<int32_t>(&symbol.data);
-	auto *otherValue = other ? std::get_if<int32_t>(&other->data) : nullptr;
+	int32_t *symValue = symbol.data.holds<int32_t>() ? &symbol.data.get<int32_t>() : nullptr;
+	int32_t *otherValue =
+	    other && other->data.holds<int32_t>() ? &other->data.get<int32_t>() : nullptr;
 
 	// Check if the symbol already exists with a different value
 	if (other && !(symValue && otherValue && *symValue == *otherValue)) {