wasm: implement error_set_has_value

This implements the safety check for error casts. The instruction
generates a jump table with 2 possibilities. The operand is used
as an index into the jump table. For cases where the value does
not exist within the error set, it will generate a jump to the
'false' block. For cases where it does exist, it will generate
a jump to the 'true' block. By calculating the highest and lowest
value we can keep the jump table smaller, as it doesn't need to
contain an index into the entire error set.

src/Module.zig

@@ -6626,7 +6626,7 @@ pub fn backendSupportsFeature(mod: Module, feature: Feature) bool {
         .safety_check_formatted => mod.comp.bin_file.options.use_llvm,
         .error_return_trace => mod.comp.bin_file.options.use_llvm,
         .is_named_enum_value => mod.comp.bin_file.options.use_llvm,
-        .error_set_has_value => mod.comp.bin_file.options.use_llvm,
+        .error_set_has_value => mod.comp.bin_file.options.use_llvm or mod.comp.bin_file.options.target.isWasm(),
         .field_reordering => mod.comp.bin_file.options.use_llvm,
     };
 }

src/arch/wasm/CodeGen.zig

@@ -1946,6 +1946,8 @@ fn genInst(func: *CodeGen, inst: Air.Inst.Index) InnerError!void {
         .ret_addr => func.airRetAddr(inst),
         .tag_name => func.airTagName(inst),
 
+        .error_set_has_value => func.airErrorSetHasValue(inst),
+
         .mul_sat,
         .mod,
         .assembly,
@@ -1967,7 +1969,6 @@ fn genInst(func: *CodeGen, inst: Air.Inst.Index) InnerError!void {
         .set_err_return_trace,
         .save_err_return_trace_index,
         .is_named_enum_value,
-        .error_set_has_value,
         .addrspace_cast,
         .vector_store_elem,
         .c_va_arg,
@@ -6514,3 +6515,85 @@ fn getTagNameFunction(func: *CodeGen, enum_ty: Type) InnerError!u32 {
     const func_type = try genFunctype(arena, .Unspecified, &.{int_tag_ty}, slice_ty, func.target);
     return func.bin_file.createFunction(func_name, func_type, &body_list, &relocs);
 }
+
+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);
+    const result = try func.allocLocal(Type.bool);
+
+    const names = error_set_ty.errorSetNames();
+    var values = try std.ArrayList(u32).initCapacity(func.gpa, names.len);
+    defer values.deinit();
+
+    const module = func.bin_file.base.options.module.?;
+    var lowest: ?u32 = null;
+    var highest: ?u32 = null;
+    for (names) |name| {
+        const err_int = module.global_error_set.get(name).?;
+        if (lowest) |*l| {
+            if (err_int < l.*) {
+                l.* = err_int;
+            }
+        } else {
+            lowest = err_int;
+        }
+        if (highest) |*h| {
+            if (err_int > h.*) {
+                highest = err_int;
+            }
+        } else {
+            highest = err_int;
+        }
+
+        values.appendAssumeCapacity(err_int);
+    }
+
+    // start block for 'true' branch
+    try func.startBlock(.block, wasm.block_empty);
+    // start block for 'false' branch
+    try func.startBlock(.block, wasm.block_empty);
+    // block for the jump table itself
+    try func.startBlock(.block, wasm.block_empty);
+
+    // lower operand to determine jump table target
+    try func.emitWValue(operand);
+    try func.addImm32(@intCast(i32, lowest.?));
+    try func.addTag(.i32_sub);
+
+    // Account for default branch so always add '1'
+    const depth = @intCast(u32, highest.? - lowest.? + 1);
+    const jump_table: Mir.JumpTable = .{ .length = depth };
+    const table_extra_index = try func.addExtra(jump_table);
+    try func.addInst(.{ .tag = .br_table, .data = .{ .payload = table_extra_index } });
+    try func.mir_extra.ensureUnusedCapacity(func.gpa, depth);
+
+    var value: u32 = lowest.?;
+    while (value <= highest.?) : (value += 1) {
+        const idx: u32 = blk: {
+            for (values.items) |val| {
+                if (val == value) break :blk 1;
+            }
+            break :blk 0;
+        };
+        func.mir_extra.appendAssumeCapacity(idx);
+    }
+    try func.endBlock();
+
+    // 'false' branch (i.e. error set does not have value
+    // ensure we set local to 0 in case the local was re-used.
+    try func.addImm32(0);
+    try func.addLabel(.local_set, result.local.value);
+    try func.addLabel(.br, 1);
+    try func.endBlock();
+
+    // 'true' branch
+    try func.addImm32(1);
+    try func.addLabel(.local_set, result.local.value);
+    try func.addLabel(.br, 0);
+    try func.endBlock();
+
+    return func.finishAir(inst, result, &.{ty_op.operand});
+}