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.

1 files changed, 37 insertions, 39 deletions

diff --git a/src/link/Wasm.zig b/src/link/Wasm.zig
index 1b7c43a6a0..0fe9ec5e3b 100644
--- a/src/link/Wasm.zig
+++ b/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;
-    const errors_len = wasm.base.options.module.?.global_error_set.count();
-
-    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.
+/// 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;
 
-        // end function
-        try writer.writeByte(std.wasm.opcode(.end));
-    }
+    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));
 
-    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);
 }