wasm-linker: Allow for creation of local symbols

The backend can create annonymous local symbols. This can be used for constants
that will be passed by reference so it will not have to be lowered to the stack, and then
stored into the data section. This also means it's valid to return a pointer to a constant array.

Those local symbols that are created, will be managed by the parent decl. Free'ing the parent decl,
will also free all of its locals.

When a local symbol was created, the index of said symbol will be returned and saved in the `memory`
tag of a `WValue` which is then memoized. This means that each 'emit' of this WValue will create a relocation
for that constant/symbol and the actual pointer value will be set after relocation phase.

1 files changed, 58 insertions, 12 deletions

diff --git a/src/link/Wasm.zig b/src/link/Wasm.zig
index e9ebb669fe..23d0368d77 100644
--- a/src/link/Wasm.zig
+++ b/src/link/Wasm.zig
@@ -19,7 +19,7 @@ const trace = @import("../tracy.zig").trace;
 const build_options = @import("build_options");
 const wasi_libc = @import("../wasi_libc.zig");
 const Cache = @import("../Cache.zig");
-const TypedValue = @import("../TypedValue.zig");
+const Type = @import("../type.zig").Type;
 const LlvmObject = @import("../codegen/llvm.zig").Object;
 const Air = @import("../Air.zig");
 const Liveness = @import("../Liveness.zig");
@@ -306,9 +306,41 @@ fn finishUpdateDecl(self: *Wasm, decl: *Module.Decl, result: CodeGen.Result, cod
     if (code.len == 0) return;
     const atom: *Atom = &decl.link.wasm;
     atom.size = @intCast(u32, code.len);
+    atom.alignment = decl.ty.abiAlignment(self.base.options.target);
+    self.symbols.items[atom.sym_index].name = decl.name;
     try atom.code.appendSlice(self.base.allocator, code);
 }
 
+/// Creates a new local symbol for a given type (and its bytes it's represented by)
+/// and then append it as a 'contained' atom onto the Decl.
+pub fn createLocalSymbol(self: *Wasm, decl: *Module.Decl, ty: Type, code: []const u8) !u32 {
+    assert(ty.zigTypeTag() != .Fn); // cannot create local symbols for functions
+    var symbol: Symbol = .{
+        .name = "unnamed_local",
+        .flags = 0,
+        .tag = .data,
+        .index = undefined,
+    };
+    symbol.setFlag(.WASM_SYM_BINDING_LOCAL);
+    symbol.setFlag(.WASM_SYM_VISIBILITY_HIDDEN);
+
+    var atom = Atom.empty;
+    atom.size = @intCast(u32, code.len);
+    atom.alignment = ty.abiAlignment(self.base.options.target);
+    try atom.code.appendSlice(self.base.allocator, code);
+
+    if (self.symbols_free_list.popOrNull()) |index| {
+        atom.sym_index = index;
+        self.symbols.items[index] = symbol;
+    } else {
+        atom.sym_index = @intCast(u32, self.symbols.items.len);
+        self.symbols.appendAssumeCapacity(symbol);
+    }
+
+    try decl.link.wasm.locals.append(self.base.allocator, atom);
+    return atom.sym_index;
+}
+
 pub fn updateDeclExports(
     self: *Wasm,
     module: *Module,
@@ -329,9 +361,12 @@ pub fn freeDecl(self: *Wasm, decl: *Module.Decl) void {
     }
     const atom = &decl.link.wasm;
     self.symbols_free_list.append(self.base.allocator, atom.sym_index) catch {};
-    atom.deinit(self.base.allocator);
     _ = self.decls.remove(decl);
     self.symbols.items[atom.sym_index].tag = .dead; // to ensure it does not end in the names section
+    for (atom.locals.items) |local_atom| {
+        self.symbols.items[local_atom.sym_index].tag = .dead; // also for any local symbol
+    }
+    atom.deinit(self.base.allocator);
 
     if (decl.isExtern()) {
         const import = self.imports.fetchRemove(decl.link.wasm.sym_index).?.value;
@@ -377,14 +412,16 @@ fn addOrUpdateImport(self: *Wasm, decl: *Module.Decl) !void {
     }
 }
 
-fn parseDeclIntoAtom(self: *Wasm, decl: *Module.Decl) !void {
-    const atom: *Atom = &decl.link.wasm;
+const Kind = union(enum) {
+    data: void,
+    function: FnData,
+};
+
+/// Parses an Atom and inserts its metadata into the corresponding sections.
+fn parseAtom(self: *Wasm, atom: *Atom, kind: Kind) !void {
     const symbol: *Symbol = &self.symbols.items[atom.sym_index];
-    symbol.name = decl.name;
-    atom.alignment = decl.ty.abiAlignment(self.base.options.target);
-    const final_index: u32 = switch (decl.ty.zigTypeTag()) {
-        .Fn => result: {
-            const fn_data = decl.fn_link.wasm;
+    const final_index: u32 = switch (kind) {
+        .function => |fn_data| result: {
             const type_index = fn_data.type_index;
             const index = @intCast(u32, self.functions.items.len + self.imported_functions_count);
             try self.functions.append(self.base.allocator, .{ .type_index = type_index });
@@ -402,7 +439,7 @@ fn parseDeclIntoAtom(self: *Wasm, decl: *Module.Decl) !void {
 
             break :result self.code_section_index.?;
         },
-        else => result: {
+        .data => result: {
             const gop = try self.data_segments.getOrPut(self.base.allocator, ".rodata");
             const atom_index = if (gop.found_existing) blk: {
                 self.segments.items[gop.value_ptr.*].size += atom.size;
@@ -430,7 +467,6 @@ fn parseDeclIntoAtom(self: *Wasm, decl: *Module.Decl) !void {
             });
             symbol.tag = .data;
             symbol.index = info_index;
-            atom.alignment = decl.ty.abiAlignment(self.base.options.target);
 
             break :result atom_index;
         },
@@ -617,7 +653,17 @@ pub fn flushModule(self: *Wasm, comp: *Compilation) !void {
     var decl_it = self.decls.keyIterator();
     while (decl_it.next()) |decl| {
         if (decl.*.isExtern()) continue;
-        try self.parseDeclIntoAtom(decl.*);
+        const atom = &decl.*.link.wasm;
+        if (decl.*.ty.zigTypeTag() == .Fn) {
+            try self.parseAtom(atom, .{ .function = decl.*.fn_link.wasm });
+        } else {
+            try self.parseAtom(atom, .data);
+        }
+
+        // also parse atoms for a decl's locals
+        for (atom.locals.items) |*local_atom| {
+            try self.parseAtom(local_atom, .data);
+        }
     }
 
     try self.setupMemory();