wasm: store __zig_lt_errors_len in linear data

Rather than using a function call to verify if an error fits within
the global error set's length, we now store the error set' size in
the .rodata segment of the linear memory and load that value onto
the stack to check with the integer value.

src/arch/wasm/CodeGen.zig

@@ -3339,13 +3339,14 @@ fn airCmpVector(func: *CodeGen, inst: Air.Inst.Index) InnerError!void {
 fn airCmpLtErrorsLen(func: *CodeGen, inst: Air.Inst.Index) InnerError!void {
     const un_op = func.air.instructions.items(.data)[inst].un_op;
     const operand = try func.resolveInst(un_op);
-    const sym_index = try func.bin_file.getGlobalSymbol("__zig_lt_errors_len", null);
+    const sym_index = try func.bin_file.getGlobalSymbol("__zig_errors_len", null);
+    const errors_len = WValue{ .memory = sym_index };
 
     try func.emitWValue(operand);
-    try func.addLabel(.call, sym_index);
-    const result = try func.allocLocal(Type.bool);
-    try func.addLabel(.local_set, result.local.value);
-    return func.finishAir(inst, result, &.{un_op});
+    const errors_len_val = try func.load(errors_len, Type.err_int, 0);
+    const result = try func.cmp(.stack, errors_len_val, Type.err_int, .lt);
+
+    return func.finishAir(inst, try result.toLocal(func, Type.bool), &.{un_op});
 }
 
 fn airBr(func: *CodeGen, inst: Air.Inst.Index) InnerError!void {
@@ -6518,7 +6519,6 @@ fn getTagNameFunction(func: *CodeGen, enum_ty: Type) InnerError!u32 {
 
 fn airErrorSetHasValue(func: *CodeGen, inst: Air.Inst.Index) InnerError!void {
     const ty_op = func.air.instructions.items(.data)[inst].ty_op;
-    if (func.liveness.isUnused(inst)) return func.finishAir(inst, .none, &.{ty_op.operand});
 
     const operand = try func.resolveInst(ty_op.operand);
     const error_set_ty = func.air.getRefType(ty_op.ty);

src/link/Wasm.zig

@@ -1209,9 +1209,10 @@ fn resolveLazySymbols(wasm: *Wasm) !void {
             try wasm.discarded.putNoClobber(wasm.base.allocator, kv.value, loc);
         }
     }
-    if (wasm.undefs.fetchSwapRemove("__zig_lt_errors_len")) |kv| {
-        const loc = try wasm.createSyntheticSymbol("__zig_lt_errors_len", .function);
+    if (wasm.undefs.fetchSwapRemove("__zig_errors_len")) |kv| {
+        const loc = try wasm.createSyntheticSymbol("__zig_errors_len", .data);
         try wasm.discarded.putNoClobber(wasm.base.allocator, kv.value, loc);
+        _ = wasm.resolved_symbols.swapRemove(kv.value);
     }
 }
 
@@ -2189,44 +2190,41 @@ fn setupInitFunctions(wasm: *Wasm) !void {
     std.sort.sort(InitFuncLoc, wasm.init_funcs.items, {}, InitFuncLoc.lessThan);
 }
 
-/// Generates the function which verifies if an integer value is less than the
-/// amount of error values. This will only be generated if the symbol exists.
-fn setupLtErrorsLenFunction(wasm: *Wasm) !void {
-    if (wasm.findGlobalSymbol("__zig_lt_errors_len") == null) return;
+/// Generates an atom containing the global error set' size.
+/// This will only be generated if the symbol exists.
+fn setupErrorsLen(wasm: *Wasm) !void {
+    const loc = wasm.findGlobalSymbol("__zig_errors_len") orelse return;
+
     const errors_len = wasm.base.options.module.?.global_error_set.count();
+    // overwrite existing atom if it already exists (maybe the error set has increased)
+    // if not, allcoate a new atom.
+    const atom_index = if (wasm.symbol_atom.get(loc)) |index| blk: {
+        const atom = wasm.getAtomPtr(index);
+        if (atom.next) |next_atom_index| {
+            const next_atom = wasm.getAtomPtr(next_atom_index);
+            next_atom.prev = atom.prev;
+            atom.next = null;
+        }
+        if (atom.prev) |prev_index| {
+            const prev_atom = wasm.getAtomPtr(prev_index);
+            prev_atom.next = atom.next;
+            atom.prev = null;
+        }
+        atom.deinit(wasm);
+        break :blk index;
+    } else new_atom: {
+        const atom_index = @intCast(Atom.Index, wasm.managed_atoms.items.len);
+        try wasm.symbol_atom.put(wasm.base.allocator, loc, atom_index);
+        try wasm.managed_atoms.append(wasm.base.allocator, undefined);
+        break :new_atom atom_index;
+    };
+    const atom = wasm.getAtomPtr(atom_index);
+    atom.* = Atom.empty;
+    atom.sym_index = loc.index;
+    atom.size = 2;
+    try atom.code.writer(wasm.base.allocator).writeIntLittle(u16, @intCast(u16, errors_len));
 
-    var body_list = std.ArrayList(u8).init(wasm.base.allocator);
-    defer body_list.deinit();
-    const writer = body_list.writer();
-
-    {
-        // generates bytecode for the following function:
-        // fn (index: u16) bool {
-        //     return index < errors_len;
-        // }
-
-        // no locals
-        try leb.writeULEB128(writer, @as(u32, 0));
-
-        // get argument
-        try writer.writeByte(std.wasm.opcode(.local_get));
-        try leb.writeULEB128(writer, @as(u32, 0));
-
-        // get error length
-        try writer.writeByte(std.wasm.opcode(.i32_const));
-        try leb.writeULEB128(writer, @intCast(u32, errors_len));
-
-        try writer.writeByte(std.wasm.opcode(.i32_lt_u));
-
-        // stack values are implicit return values so keep the value
-        // on the stack and end the function.
-
-        // end function
-        try writer.writeByte(std.wasm.opcode(.end));
-    }
-
-    const func_type: std.wasm.Type = .{ .params = &.{std.wasm.Valtype.i32}, .returns = &.{std.wasm.Valtype.i32} };
-    try wasm.createSyntheticFunction("__zig_lt_errors_len", func_type, &body_list);
+    try wasm.parseAtom(atom_index, .{ .data = .read_only });
 }
 
 /// Creates a function body for the `__wasm_call_ctors` symbol.
@@ -3361,6 +3359,7 @@ pub fn flushModule(wasm: *Wasm, comp: *Compilation, prog_node: *std.Progress.Nod
     // So we can rebuild the binary file on each incremental update
     defer wasm.resetState();
     try wasm.setupInitFunctions();
+    try wasm.setupErrorsLen();
     try wasm.setupStart();
     try wasm.setupImports();
     if (wasm.base.options.module) |mod| {
@@ -3423,7 +3422,6 @@ pub fn flushModule(wasm: *Wasm, comp: *Compilation, prog_node: *std.Progress.Nod
     try wasm.setupInitMemoryFunction();
     try wasm.setupTLSRelocationsFunction();
     try wasm.initializeTLSFunction();
-    try wasm.setupLtErrorsLenFunction();
     try wasm.setupExports();
     try wasm.writeToFile(enabled_features, emit_features_count, arena);
 }