wasm: add dead tag to WValue

This new tag is used for freed locals that are not allowed to have any
remaining references pointing to it. This new tag allows us to easily
identify liveness bugs. Previously we would set the entire region to
`undefined` which would incorrectly set the tag to `function_index`,
making codegen think it was a valid `WValue` while it wasn't.

src/arch/wasm/CodeGen.zig

@@ -29,6 +29,9 @@ const errUnionErrorOffset = codegen.errUnionErrorOffset;
 
 /// Wasm Value, created when generating an instruction
 const WValue = union(enum) {
+    /// `WValue` which has been freed and may no longer hold
+    /// any references.
+    dead: void,
     /// May be referenced but is unused
     none: void,
     /// The value lives on top of the stack
@@ -86,6 +89,7 @@ const WValue = union(enum) {
     fn offset(value: WValue) u32 {
         switch (value) {
             .stack_offset => |stack_offset| return stack_offset.value,
+            .dead => unreachable,
             else => return 0,
         }
     }
@@ -123,7 +127,7 @@ const WValue = union(enum) {
             .f64 => gen.free_locals_f64.append(gen.gpa, local_value) catch return,
             .v128 => gen.free_locals_v128.append(gen.gpa, local_value) catch return,
         }
-        value.* = undefined;
+        value.* = .dead;
     }
 };
 
@@ -832,6 +836,7 @@ const Branch = struct {
 
     fn deinit(branch: *Branch, gpa: Allocator) void {
         branch.values.deinit(gpa);
+        branch.* = undefined;
     }
 };
 
@@ -884,7 +889,7 @@ fn processDeath(func: *CodeGen, ref: Air.Inst.Ref) void {
     if (value.local.value < reserved_indexes) {
         return; // function arguments can never be re-used
     }
-    log.debug("Decreasing reference for ref: %{?d}, using local '{d}'\n", .{ Air.refToIndex(ref), value.local.value });
+    log.debug("Decreasing reference for ref: %{?d}, using local '{d}'", .{ Air.refToIndex(ref), value.local.value });
     value.local.references -= 1; // if this panics, a call to `reuseOperand` was forgotten by the developer
     if (value.local.references == 0) {
         value.free(func);
@@ -1030,6 +1035,7 @@ fn genBlockType(ty: Type, target: std.Target) u8 {
 /// Writes the bytecode depending on the given `WValue` in `val`
 fn emitWValue(func: *CodeGen, value: WValue) InnerError!void {
     switch (value) {
+        .dead => unreachable, // reference to free'd `WValue` (missing reuseOperand?)
         .none, .stack => {}, // no-op
         .local => |idx| try func.addLabel(.local_get, idx.value),
         .imm32 => |val| try func.addImm32(@bitCast(i32, val)),
@@ -1226,6 +1232,7 @@ fn genFunc(func: *CodeGen) InnerError!void {
     defer {
         var outer_branch = func.branches.pop();
         outer_branch.deinit(func.gpa);
+        assert(func.branches.items.len == 0); // missing branch merge
     }
     // Generate MIR for function body
     try func.genBody(func.air.getMainBody());