wasm-linker: Implement updateDeclExports

We now correctly implement exporting decls. This means it is possible to export
a decl with a different name than the decl that is doing the export.
This also sets the symbols with the correct flags, so when we emit a relocatable
object file, a linker can correctly resolve symbols and/or export the symbol to the host environment.

This commit also includes fixes to ensure relocations have the correct offset to how other
linkers will expect the offset, rather than what we use internally.
Other linkers accept the offset, relative to the section.
Internally we use an offset relative to the atom.

src/Module.zig

@@ -4516,6 +4516,9 @@ fn deleteDeclExports(mod: *Module, decl: *Decl) void {
         if (mod.comp.bin_file.cast(link.File.MachO)) |macho| {
             macho.deleteExport(exp.link.macho);
         }
+        if (mod.comp.bin_file.cast(link.File.Wasm)) |wasm| {
+            wasm.deleteExport(exp.link.wasm);
+        }
         if (mod.failed_exports.fetchSwapRemove(exp)) |failed_kv| {
             failed_kv.value.destroy(mod.gpa);
         }

src/Sema.zig

@@ -3919,7 +3919,7 @@ pub fn analyzeExport(
             .macho => .{ .macho = .{} },
             .plan9 => .{ .plan9 = null },
             .c => .{ .c = {} },
-            .wasm => .{ .wasm = {} },
+            .wasm => .{ .wasm = .{} },
             .spirv => .{ .spirv = {} },
             .nvptx => .{ .nvptx = {} },
         },

src/arch/wasm/CodeGen.zig

@@ -1320,14 +1320,18 @@ pub const DeclGen = struct {
         }
 
         decl.markAlive();
-        try writer.writeIntLittle(u32, try self.bin_file.getDeclVAddr(
-            self.decl, // The decl containing the source symbol index
-            decl.ty, // type we generate the address of
-            self.symbol_index, // source symbol index
-            decl.link.wasm.sym_index, // target symbol index
-            @intCast(u32, self.code.items.len), // offset
-            @intCast(u32, offset), // addend
-        ));
+        if (decl.link.wasm.sym_index == 0) {
+            try writer.writeIntLittle(u32, 0);
+        } else {
+            try writer.writeIntLittle(u32, try self.bin_file.getDeclVAddr(
+                self.decl, // The decl containing the source symbol index
+                decl.ty, // type we generate the address of
+                self.symbol_index, // source symbol index
+                decl.link.wasm.sym_index, // target symbol index
+                @intCast(u32, self.code.items.len), // offset
+                @intCast(u32, offset), // addend
+            ));
+        }
         return Result{ .appended = {} };
     }
 };

src/arch/wasm/Emit.zig

@@ -242,6 +242,7 @@ fn emitGlobal(emit: *Emit, tag: Mir.Inst.Tag, inst: Mir.Inst.Index) !void {
     const global_offset = emit.offset();
     try emit.code.appendSlice(&buf);
 
+    // globals can have index 0 as it represents the stack pointer
     try emit.decl.link.wasm.relocs.append(emit.bin_file.allocator, .{
         .index = label,
         .offset = global_offset,
@@ -294,11 +295,13 @@ fn emitCall(emit: *Emit, inst: Mir.Inst.Index) !void {
     leb128.writeUnsignedFixed(5, &buf, label);
     try emit.code.appendSlice(&buf);
 
-    try emit.decl.link.wasm.relocs.append(emit.bin_file.allocator, .{
-        .offset = call_offset,
-        .index = label,
-        .relocation_type = .R_WASM_FUNCTION_INDEX_LEB,
-    });
+    if (label != 0) {
+        try emit.decl.link.wasm.relocs.append(emit.bin_file.allocator, .{
+            .offset = call_offset,
+            .index = label,
+            .relocation_type = .R_WASM_FUNCTION_INDEX_LEB,
+        });
+    }
 }
 
 fn emitCallIndirect(emit: *Emit, inst: Mir.Inst.Index) !void {
@@ -318,11 +321,13 @@ fn emitFunctionIndex(emit: *Emit, inst: Mir.Inst.Index) !void {
     leb128.writeUnsignedFixed(5, &buf, symbol_index);
     try emit.code.appendSlice(&buf);
 
-    try emit.decl.link.wasm.relocs.append(emit.bin_file.allocator, .{
-        .offset = index_offset,
-        .index = symbol_index,
-        .relocation_type = .R_WASM_TABLE_INDEX_SLEB,
-    });
+    if (symbol_index != 0) {
+        try emit.decl.link.wasm.relocs.append(emit.bin_file.allocator, .{
+            .offset = index_offset,
+            .index = symbol_index,
+            .relocation_type = .R_WASM_TABLE_INDEX_SLEB,
+        });
+    }
 }
 
 fn emitMemAddress(emit: *Emit, inst: Mir.Inst.Index) !void {
@@ -342,12 +347,14 @@ fn emitMemAddress(emit: *Emit, inst: Mir.Inst.Index) !void {
         try emit.code.appendSlice(&buf);
     }
 
-    try emit.decl.link.wasm.relocs.append(emit.bin_file.allocator, .{
-        .offset = mem_offset,
-        .index = mem.pointer,
-        .relocation_type = if (is_wasm32) .R_WASM_MEMORY_ADDR_LEB else .R_WASM_MEMORY_ADDR_LEB64,
-        .addend = mem.offset,
-    });
+    if (mem.pointer != 0) {
+        try emit.decl.link.wasm.relocs.append(emit.bin_file.allocator, .{
+            .offset = mem_offset,
+            .index = mem.pointer,
+            .relocation_type = if (is_wasm32) .R_WASM_MEMORY_ADDR_LEB else .R_WASM_MEMORY_ADDR_LEB64,
+            .addend = mem.offset,
+        });
+    }
 }
 
 fn emitExtended(emit: *Emit, inst: Mir.Inst.Index) !void {

src/link.zig

@@ -237,7 +237,7 @@ pub const File = struct {
         macho: MachO.Export,
         plan9: Plan9.Export,
         c: void,
-        wasm: void,
+        wasm: Wasm.Export,
         spirv: void,
         nvptx: void,
     };

src/link/Wasm.zig

@@ -114,6 +114,9 @@ resolved_symbols: std.AutoArrayHashMapUnmanaged(SymbolLoc, void) = .{},
 /// data of a symbol, such as its size, or its offset to perform a relocation.
 /// Undefined (and synthetic) symbols do not have an Atom and therefore cannot be mapped.
 symbol_atom: std.AutoHashMapUnmanaged(SymbolLoc, *Atom) = .{},
+/// Maps a symbol's location to its export name, which may differ from the decl's name
+/// which does the exporting.
+export_names: std.AutoHashMapUnmanaged(SymbolLoc, []const u8) = .{},
 
 pub const Segment = struct {
     alignment: u32,
@@ -129,6 +132,10 @@ pub const FnData = struct {
     };
 };
 
+pub const Export = struct {
+    sym_index: ?u32 = null,
+};
+
 pub const SymbolLoc = struct {
     /// The index of the symbol within the specified file
     index: u32,
@@ -191,6 +198,7 @@ pub fn openPath(allocator: Allocator, sub_path: []const u8, options: link.Option
             },
         );
     } else {
+        symbol.setFlag(.WASM_SYM_VISIBILITY_HIDDEN);
         const global = try wasm_bin.wasm_globals.addOne(allocator);
         global.* = .{
             .global_type = .{
@@ -345,6 +353,7 @@ pub fn deinit(self: *Wasm) void {
     self.resolved_symbols.deinit(gpa);
     self.discarded.deinit(gpa);
     self.symbol_atom.deinit(gpa);
+    self.export_names.deinit(gpa);
     self.atoms.deinit(gpa);
     self.managed_atoms.deinit(gpa);
     self.segments.deinit(gpa);
@@ -372,7 +381,7 @@ pub fn allocateDeclIndexes(self: *Wasm, decl: *Module.Decl) !void {
 
     var symbol: Symbol = .{
         .name = undefined, // will be set after updateDecl
-        .flags = 0,
+        .flags = @enumToInt(Symbol.Flag.WASM_SYM_BINDING_LOCAL),
         .tag = undefined, // will be set after updateDecl
         .index = undefined, // will be set after updateDecl
     };
@@ -485,7 +494,6 @@ fn finishUpdateDecl(self: *Wasm, decl: *Module.Decl, code: []const u8) !void {
     atom.alignment = decl.ty.abiAlignment(self.base.options.target);
     const symbol = &self.symbols.items[atom.sym_index];
     symbol.name = decl.name;
-    symbol.setFlag(.WASM_SYM_BINDING_LOCAL);
     try atom.code.appendSlice(self.base.allocator, code);
 }
 
@@ -541,6 +549,7 @@ pub fn getDeclVAddr(
     offset: u32,
     addend: u32,
 ) !u32 {
+    assert(target_symbol_index != 0);
     const atom = decl.link.wasm.symbolAtom(symbol_index);
     const is_wasm32 = self.base.options.target.cpu.arch == .wasm32;
     if (ty.zigTypeTag() == .Fn) {
@@ -569,6 +578,20 @@ pub fn getDeclVAddr(
     return target_symbol_index;
 }
 
+pub fn deleteExport(self: *Wasm, exp: Export) void {
+    if (self.llvm_object) |_| return;
+    const sym_index = exp.sym_index orelse return;
+    const loc: SymbolLoc = .{ .file = null, .index = sym_index };
+    const symbol = loc.getSymbol(self);
+    const symbol_name = mem.sliceTo(symbol.name, 0);
+    log.debug("Deleting export for decl '{s}'", .{symbol_name});
+    if (self.export_names.fetchRemove(loc)) |kv| {
+        assert(self.globals.remove(kv.value));
+    } else {
+        assert(self.globals.remove(symbol_name));
+    }
+}
+
 pub fn updateDeclExports(
     self: *Wasm,
     module: *Module,
@@ -581,6 +604,82 @@ pub fn updateDeclExports(
     if (build_options.have_llvm) {
         if (self.llvm_object) |llvm_object| return llvm_object.updateDeclExports(module, decl, exports);
     }
+
+    for (exports) |exp| {
+        if (exp.options.section) |section| {
+            try module.failed_exports.putNoClobber(module.gpa, exp, try Module.ErrorMsg.create(
+                module.gpa,
+                decl.srcLoc(),
+                "Unimplemented: ExportOptions.section '{s}'",
+                .{section},
+            ));
+            continue;
+        }
+        if (self.globals.getPtr(exp.options.name)) |existing_loc| {
+            if (existing_loc.index == decl.link.wasm.sym_index) continue;
+            const existing_sym: Symbol = existing_loc.getSymbol(self).*;
+
+            const exp_is_weak = exp.options.linkage == .Internal or exp.options.linkage == .Weak;
+            // When both the to-bo-exported symbol and the already existing symbol
+            // are strong symbols, we have a linker error.
+            // In the other case we replace one with the other.
+            if (!exp_is_weak and !existing_sym.isWeak()) {
+                try module.failed_exports.putNoClobber(module.gpa, exp, try Module.ErrorMsg.create(
+                    module.gpa,
+                    decl.srcLoc(),
+                    \\LinkError: symbol '{s}' defined multiple times
+                    \\  first definition in '{s}'
+                    \\  next definition in '{s}'
+                ,
+                    .{ exp.options.name, self.name, self.name },
+                ));
+            } else if (exp_is_weak) {
+                continue; // to-be-exported symbol is weak, so we keep the existing symbol
+            } else {
+                existing_loc.index = decl.link.wasm.sym_index;
+                existing_loc.file = null;
+                exp.link.wasm.sym_index = existing_loc.index;
+            }
+        }
+
+        const sym_index = exp.exported_decl.link.wasm.sym_index;
+        const sym_loc = exp.exported_decl.link.wasm.symbolLoc();
+        const symbol = sym_loc.getSymbol(self);
+        switch (exp.options.linkage) {
+            .Internal => {
+                symbol.setFlag(.WASM_SYM_VISIBILITY_HIDDEN);
+                symbol.setFlag(.WASM_SYM_BINDING_WEAK);
+            },
+            .Weak => {
+                symbol.setFlag(.WASM_SYM_BINDING_WEAK);
+            },
+            .Strong => {}, // symbols are strong by default
+            .LinkOnce => {
+                try module.failed_exports.putNoClobber(module.gpa, exp, try Module.ErrorMsg.create(
+                    module.gpa,
+                    decl.srcLoc(),
+                    "Unimplemented: LinkOnce",
+                    .{},
+                ));
+                continue;
+            },
+        }
+        // Ensure the symbol will be exported using the given name
+        if (!mem.eql(u8, exp.options.name, mem.sliceTo(exp.exported_decl.name, 0))) {
+            try self.export_names.put(self.base.allocator, sym_loc, exp.options.name);
+        }
+
+        symbol.setGlobal(true);
+        try self.globals.put(
+            self.base.allocator,
+            exp.options.name,
+            sym_loc,
+        );
+
+        // if the symbol was previously undefined, remove it as an import
+        _ = self.imports.remove(sym_loc);
+        exp.link.wasm.sym_index = sym_index;
+    }
 }
 
 pub fn freeDecl(self: *Wasm, decl: *Module.Decl) void {
@@ -596,12 +695,13 @@ pub fn freeDecl(self: *Wasm, decl: *Module.Decl) void {
         local_symbol.tag = .dead; // also for any local symbol
         self.base.allocator.free(mem.sliceTo(local_symbol.name, 0));
         self.symbols_free_list.append(self.base.allocator, local_atom.sym_index) catch {};
+        assert(self.resolved_symbols.swapRemove(local_atom.symbolLoc()));
     }
 
     if (decl.isExtern()) {
-        assert(self.imports.remove(.{ .file = null, .index = atom.sym_index }));
+        assert(self.imports.remove(atom.symbolLoc()));
     }
-    assert(self.resolved_symbols.swapRemove(.{ .index = atom.sym_index, .file = null }));
+    assert(self.resolved_symbols.swapRemove(atom.symbolLoc()));
     atom.deinit(self.base.allocator);
 }
 
@@ -627,6 +727,7 @@ fn addOrUpdateImport(self: *Wasm, decl: *Module.Decl) !void {
     const symbol: *Symbol = &self.symbols.items[symbol_index];
     symbol.name = decl.name;
     symbol.setUndefined(true);
+    symbol.setGlobal(true);
     try self.globals.putNoClobber(
         self.base.allocator,
         mem.sliceTo(symbol.name, 0),
@@ -778,6 +879,7 @@ fn setupImports(self: *Wasm) !void {
             }
         }
     }
+
     for (self.resolved_symbols.keys()) |symbol_loc| {
         if (symbol_loc.file == null) {
             // imports generated by Zig code are already in the `import` section
@@ -916,23 +1018,20 @@ fn mergeTypes(self: *Wasm) !void {
 }
 
 fn setupExports(self: *Wasm) !void {
+    if (self.base.options.output_mode == .Obj) return;
     log.debug("Building exports from symbols", .{});
 
-    // When importing memory option if false, we export it instead
-    if (!self.base.options.import_memory) {
-        try self.exports.append(self.base.allocator, .{ .name = "memory", .kind = .memory, .index = 0 });
-    }
-
     for (self.resolved_symbols.keys()) |sym_loc| {
         const symbol = sym_loc.getSymbol(self);
         if (!symbol.isExported()) continue;
 
+        const export_name = if (self.export_names.get(sym_loc)) |name| name else mem.sliceTo(symbol.name, 0);
         const exp: wasm.Export = .{
-            .name = mem.sliceTo(symbol.name, 0),
+            .name = export_name,
             .kind = symbol.tag.externalType(),
             .index = symbol.index,
         };
-        log.debug("Appending export for symbol '{s}' at index: ({d})", .{ exp.name, exp.index });
+        log.debug("Exporting symbol '{s}' as '{s}' at index: ({d})", .{ symbol.name, exp.name, exp.index });
         try self.exports.append(self.base.allocator, exp);
     }
 
@@ -959,7 +1058,9 @@ fn setupStart(self: *Wasm) !void {
     }
 
     // Ensure the symbol is exported so host environment can access it
-    symbol.setFlag(.WASM_SYM_EXPORTED);
+    if (self.base.options.output_mode != .Obj) {
+        symbol.setFlag(.WASM_SYM_EXPORTED);
+    }
 }
 
 /// Sets up the memory section of the wasm module, as well as the stack.
@@ -1093,10 +1194,12 @@ fn resetState(self: *Wasm) void {
         atom.prev = null;
     }
     self.functions.clearRetainingCapacity();
+    self.exports.clearRetainingCapacity();
     self.segments.clearRetainingCapacity();
     self.segment_info.clearRetainingCapacity();
     self.data_segments.clearRetainingCapacity();
     self.atoms.clearRetainingCapacity();
+    self.symbol_atom.clearRetainingCapacity();
     self.code_section_index = null;
 }
 
@@ -1121,6 +1224,13 @@ pub fn flushModule(self: *Wasm, comp: *Compilation) !void {
     const tracy = trace(@src());
     defer tracy.end();
 
+    // The amount of sections that will be written
+    var section_count: u32 = 0;
+    // Index of the code section. Used to tell relocation table where the section lives.
+    var code_section_index: ?u32 = null;
+    // Index of the data section. Used to tell relocation table where the section lives.
+    var data_section_index: ?u32 = null;
+
     // Used for all temporary memory allocated during flushin
     var arena_instance = std.heap.ArenaAllocator.init(self.base.allocator);
     defer arena_instance.deinit();
@@ -1148,6 +1258,7 @@ pub fn flushModule(self: *Wasm, comp: *Compilation) !void {
     // When we finish/error we reset the state of the linker
     // So we can rebuild the binary file on each incremental update
     defer self.resetState();
+    try self.setupStart();
     try self.setupImports();
     var decl_it = self.decls.keyIterator();
     while (decl_it.next()) |decl| {
@@ -1186,7 +1297,7 @@ pub fn flushModule(self: *Wasm, comp: *Compilation) !void {
     try file.seekTo(@sizeOf(@TypeOf(wasm.magic ++ wasm.version)));
 
     // Type section
-    {
+    if (self.func_types.items.len != 0) {
         const header_offset = try reserveVecSectionHeader(file);
         const writer = file.writer();
         log.debug("Writing type section. Count: ({d})", .{self.func_types.items.len});
@@ -1205,11 +1316,12 @@ pub fn flushModule(self: *Wasm, comp: *Compilation) !void {
             @intCast(u32, (try file.getPos()) - header_offset - header_size),
             @intCast(u32, self.func_types.items.len),
         );
+        section_count += 1;
     }
 
     // Import section
-    const import_memory = self.base.options.import_memory;
-    const import_table = self.base.options.import_table;
+    const import_memory = self.base.options.import_memory or is_obj;
+    const import_table = self.base.options.import_table or is_obj;
     if (self.imports.count() != 0 or import_memory or import_table) {
         const header_offset = try reserveVecSectionHeader(file);
         const writer = file.writer();
@@ -1242,7 +1354,7 @@ pub fn flushModule(self: *Wasm, comp: *Compilation) !void {
         if (import_memory) {
             const mem_imp: wasm.Import = .{
                 .module_name = self.host_name,
-                .name = "memory",
+                .name = "__linear_memory",
                 .kind = .{ .memory = self.memories.limits },
             };
             try emitImport(writer, mem_imp);
@@ -1255,10 +1367,11 @@ pub fn flushModule(self: *Wasm, comp: *Compilation) !void {
             @intCast(u32, (try file.getPos()) - header_offset - header_size),
             @intCast(u32, self.imports.count() + @boolToInt(import_memory) + @boolToInt(import_table)),
         );
+        section_count += 1;
     }
 
     // Function section
-    {
+    if (self.functions.items.len != 0) {
         const header_offset = try reserveVecSectionHeader(file);
         const writer = file.writer();
         for (self.functions.items) |function| {
@@ -1272,11 +1385,12 @@ pub fn flushModule(self: *Wasm, comp: *Compilation) !void {
             @intCast(u32, (try file.getPos()) - header_offset - header_size),
             @intCast(u32, self.functions.items.len),
         );
+        section_count += 1;
     }
 
     // Table section
     const export_table = self.base.options.export_table;
-    if (!import_table) {
+    if (!import_table and self.function_table.count() != 0) {
         const header_offset = try reserveVecSectionHeader(file);
         const writer = file.writer();
 
@@ -1293,10 +1407,11 @@ pub fn flushModule(self: *Wasm, comp: *Compilation) !void {
             @intCast(u32, (try file.getPos()) - header_offset - header_size),
             @as(u32, 1),
         );
+        section_count += 1;
     }
 
     // Memory section
-    if (!self.base.options.import_memory) {
+    if (!import_memory) {
         const header_offset = try reserveVecSectionHeader(file);
         const writer = file.writer();
 
@@ -1308,6 +1423,7 @@ pub fn flushModule(self: *Wasm, comp: *Compilation) !void {
             @intCast(u32, (try file.getPos()) - header_offset - header_size),
             @as(u32, 1), // wasm currently only supports 1 linear memory segment
         );
+        section_count += 1;
     }
 
     // Global section (used to emit stack pointer)
@@ -1328,43 +1444,18 @@ pub fn flushModule(self: *Wasm, comp: *Compilation) !void {
             @intCast(u32, (try file.getPos()) - header_offset - header_size),
             @intCast(u32, self.wasm_globals.items.len),
         );
+        section_count += 1;
     }
 
     // Export section
-    if (self.base.options.module) |module| {
+    if (self.exports.items.len != 0 or export_table or !import_memory) {
         const header_offset = try reserveVecSectionHeader(file);
         const writer = file.writer();
-        var count: u32 = 0;
-        for (module.decl_exports.values()) |exports| {
-            for (exports) |exprt| {
-                // Export name length + name
-                try leb.writeULEB128(writer, @intCast(u32, exprt.options.name.len));
-                try writer.writeAll(exprt.options.name);
-
-                switch (exprt.exported_decl.ty.zigTypeTag()) {
-                    .Fn => {
-                        const target = exprt.exported_decl.link.wasm.sym_index;
-                        const target_symbol = self.symbols.items[target];
-                        assert(target_symbol.tag == .function);
-                        // Type of the export
-                        try writer.writeByte(wasm.externalKind(.function));
-                        // Exported function index
-                        try leb.writeULEB128(writer, target_symbol.index);
-                    },
-                    else => return error.TODOImplementNonFnDeclsForWasm,
-                }
-
-                count += 1;
-            }
-        }
-
-        // export memory if size is not 0
-        if (!import_memory) {
-            try leb.writeULEB128(writer, @intCast(u32, "memory".len));
-            try writer.writeAll("memory");
-            try writer.writeByte(wasm.externalKind(.memory));
-            try leb.writeULEB128(writer, @as(u32, 0)); // only 1 memory 'object' can exist
-            count += 1;
+        for (self.exports.items) |exp| {
+            try leb.writeULEB128(writer, @intCast(u32, exp.name.len));
+            try writer.writeAll(exp.name);
+            try leb.writeULEB128(writer, @enumToInt(exp.kind));
+            try leb.writeULEB128(writer, exp.index);
         }
 
         if (export_table) {
@@ -1372,7 +1463,13 @@ pub fn flushModule(self: *Wasm, comp: *Compilation) !void {
             try writer.writeAll("__indirect_function_table");
             try writer.writeByte(wasm.externalKind(.table));
             try leb.writeULEB128(writer, @as(u32, 0)); // function table is always the first table
-            count += 1;
+        }
+
+        if (!import_memory) {
+            try leb.writeULEB128(writer, @intCast(u32, "memory".len));
+            try writer.writeAll("memory");
+            try writer.writeByte(wasm.externalKind(.memory));
+            try leb.writeULEB128(writer, @as(u32, 0));
         }
 
         try writeVecSectionHeader(
@@ -1380,8 +1477,9 @@ pub fn flushModule(self: *Wasm, comp: *Compilation) !void {
             header_offset,
             .@"export",
             @intCast(u32, (try file.getPos()) - header_offset - header_size),
-            count,
+            @intCast(u32, self.exports.items.len) + @boolToInt(export_table) + @boolToInt(!import_memory),
         );
+        section_count += 1;
     }
 
     // element section (function table)
@@ -1407,6 +1505,7 @@ pub fn flushModule(self: *Wasm, comp: *Compilation) !void {
             @intCast(u32, (try file.getPos()) - header_offset - header_size),
             @as(u32, 1),
         );
+        section_count += 1;
     }
 
     // Code section
@@ -1429,6 +1528,8 @@ pub fn flushModule(self: *Wasm, comp: *Compilation) !void {
             @intCast(u32, (try file.getPos()) - header_offset - header_size),
             @intCast(u32, self.functions.items.len),
         );
+        code_section_index = section_count;
+        section_count += 1;
     }
 
     // Data section
@@ -1493,6 +1594,8 @@ pub fn flushModule(self: *Wasm, comp: *Compilation) !void {
             @intCast(u32, (try file.getPos()) - header_offset - header_size),
             @intCast(u32, segment_count),
         );
+        data_section_index = section_count;
+        section_count += 1;
     }
 
     if (is_obj) {
@@ -1501,8 +1604,12 @@ pub fn flushModule(self: *Wasm, comp: *Compilation) !void {
         // This means that for a relocatable object file, we need to generate one and provide it to the relocation sections.
         var symbol_table = std.AutoArrayHashMap(SymbolLoc, u32).init(arena);
         try self.emitLinkSection(file, arena, &symbol_table);
-        try self.emitCodeRelocations(file, arena, 6, symbol_table);
-        try self.emitDataRelocations(file, arena, 7, symbol_table);
+        if (code_section_index) |code_index| {
+            try self.emitCodeRelocations(file, arena, code_index, symbol_table);
+        }
+        if (data_section_index) |data_index| {
+            try self.emitDataRelocations(file, arena, data_index, symbol_table);
+        }
     } else {
         try self.emitNameSection(file, arena);
     }
@@ -1563,6 +1670,7 @@ fn emitNameSubsection(self: *Wasm, section_id: std.wasm.NameSubsection, names: a
 
     try leb.writeULEB128(sub_writer, @intCast(u32, names.len));
     for (names) |name| {
+        log.debug("Emit symbol '{s}' type({s})", .{ name.name, @tagName(section_id) });
         try leb.writeULEB128(sub_writer, name.index);
         try leb.writeULEB128(sub_writer, @intCast(u32, name.name.len));
         try sub_writer.writeAll(name.name);
@@ -2194,6 +2302,18 @@ fn emitSegmentInfo(self: *Wasm, file: fs.File, arena: Allocator) !void {
     try file.writevAll(&.{iovec});
 }
 
+fn getULEB128Size(uint_value: anytype) u32 {
+    const T = @TypeOf(uint_value);
+    const U = if (@typeInfo(T).Int.bits < 8) u8 else T;
+    var value = @intCast(U, uint_value);
+
+    var size: u32 = 0;
+    while (value != 0) : (size += 1) {
+        value >>= 7;
+    }
+    return size;
+}
+
 /// For each relocatable section, emits a custom "relocation.<section_name>" section
 fn emitCodeRelocations(
     self: *Wasm,
@@ -2215,17 +2335,22 @@ fn emitCodeRelocations(
 
     var count: u32 = 0;
     var atom: *Atom = self.atoms.get(code_index).?.getFirst();
+    // for each atom, we calculate the uleb size and append that
+    var size_offset: u32 = 5; // account for code section size leb128
     while (true) {
+        size_offset += getULEB128Size(atom.size);
         for (atom.relocs.items) |relocation| {
             count += 1;
             const sym_loc: SymbolLoc = .{ .file = atom.file, .index = relocation.index };
             const symbol_index = symbol_table.get(sym_loc).?;
             try leb.writeULEB128(writer, @enumToInt(relocation.relocation_type));
-            try leb.writeULEB128(writer, atom.offset + relocation.offset);
+            const offset = atom.offset + relocation.offset + size_offset;
+            try leb.writeULEB128(writer, offset);
             try leb.writeULEB128(writer, symbol_index);
             if (relocation.relocation_type.addendIsPresent()) {
                 try leb.writeULEB128(writer, relocation.addend orelse 0);
             }
+            log.debug("Emit relocation: {}", .{relocation});
         }
         atom = atom.next orelse break;
     }
@@ -2262,9 +2387,12 @@ fn emitDataRelocations(
     const reloc_start = payload.items.len;
 
     var count: u32 = 0;
+    // for each atom, we calculate the uleb size and append that
+    var size_offset: u32 = 5; // account for code section size leb128
     for (self.data_segments.values()) |segment_index| {
         var atom: *Atom = self.atoms.get(segment_index).?.getFirst();
         while (true) {
+            size_offset += getULEB128Size(atom.size);
             for (atom.relocs.items) |relocation| {
                 count += 1;
                 const sym_loc: SymbolLoc = .{
@@ -2273,11 +2401,13 @@ fn emitDataRelocations(
                 };
                 const symbol_index = symbol_table.get(sym_loc).?;
                 try leb.writeULEB128(writer, @enumToInt(relocation.relocation_type));
-                try leb.writeULEB128(writer, atom.offset + relocation.offset);
+                const offset = atom.offset + relocation.offset + size_offset;
+                try leb.writeULEB128(writer, offset);
                 try leb.writeULEB128(writer, symbol_index);
                 if (relocation.relocation_type.addendIsPresent()) {
                     try leb.writeULEB128(writer, relocation.addend orelse 0);
                 }
+                log.debug("Emit relocation: {}", .{relocation});
             }
             atom = atom.next orelse break;
         }

src/link/Wasm/Atom.zig

@@ -173,7 +173,6 @@ fn relocationValue(self: Atom, relocation: types.Relocation, wasm_bin: *const Wa
             if (symbol.isUndefined() and (symbol.tag == .data or symbol.isWeak())) {
                 return 0;
             }
-
             const merge_segment = wasm_bin.base.options.output_mode != .Obj;
             const segment_name = wasm_bin.segment_info.items[symbol.index].outputName(merge_segment);
             const atom_index = wasm_bin.data_segments.get(segment_name).?;

src/link/Wasm/Symbol.zig

@@ -104,6 +104,14 @@ pub fn setUndefined(self: *Symbol, is_undefined: bool) void {
     }
 }
 
+pub fn setGlobal(self: *Symbol, is_global: bool) void {
+    if (is_global) {
+        self.flags &= ~@enumToInt(Flag.WASM_SYM_BINDING_LOCAL);
+    } else {
+        self.setFlag(.WASM_SYM_BINDING_LOCAL);
+    }
+}
+
 pub fn isDefined(self: Symbol) bool {
     return !self.isUndefined();
 }