Commit `832330094c`

Luuk de Gram <luuk@degram.dev>

2023-05-19 20:14:34

wasm: aggregate_init - ensure zeroed result local

When initializing a packed struct, we must ensure the result local is zero'd. Previously we would do this by ensuring a new local is allocated. Although a local is always zero by default, it meant that if such an initialization was being done inside a loop, it would re- use that very same local that could potentially still hold a different value. Because this value is `or`'d with the value, it would result in a miscompilation. By manually setting this result to 0, we guarantee the correct behavior.

master

1 parent ca870aa

Changed files (1)

src

arch

wasm

CodeGen.zig

@@ -4893,8 +4893,15 @@ fn airAggregateInit(func: *CodeGen, inst: Air.Inst.Index) InnerError!void {
                     const struct_obj = result_ty.castTag(.@"struct").?.data;
                     const fields = struct_obj.fields.values();
                     const backing_type = struct_obj.backing_int_ty;
-                    // we ensure a new local is created so it's zero-initialized
-                    const result = try func.ensureAllocLocal(backing_type);
+
+                    // ensure the result is zero'd
+                    const result = try func.allocLocal(backing_type);
+                    if (struct_obj.backing_int_ty.bitSize(func.target) <= 32)
+                        try func.addImm32(0)
+                    else
+                        try func.addImm64(0);
+                    try func.addLabel(.local_set, result.local.value);
+
                     var current_bit: u16 = 0;
                     for (elements, 0..) |elem, elem_index| {
                         const field = fields[elem_index];

Commit 832330094c

Commit `832330094c`