codegen: fix access to byte-aligned nested packed struct elems

When acessing a packed struct member via a byte aligned ptr (from the optimisation in Sema.structFieldPtrByIndex())
the codegen must apply the parent ptr packed_offset in addition to the field offset itself.

resolves https://github.com/ziglang/zig/issues/16609

1 files changed, 2 insertions, 1 deletions

diff --git a/src/codegen/llvm.zig b/src/codegen/llvm.zig
index 4af42bfa9b..129a769007 100644
--- a/src/codegen/llvm.zig
+++ b/src/codegen/llvm.zig
@@ -10611,6 +10611,7 @@ pub const FuncGen = struct {
                 .Packed => {
                     const result_ty = self.typeOfIndex(inst);
                     const result_ty_info = result_ty.ptrInfo(mod);
+                    const struct_ptr_ty_info = struct_ptr_ty.ptrInfo(mod);
 
                     if (result_ty_info.packed_offset.host_size != 0) {
                         // From LLVM's perspective, a pointer to a packed struct and a pointer
@@ -10622,7 +10623,7 @@ pub const FuncGen = struct {
 
                     // We have a pointer to a packed struct field that happens to be byte-aligned.
                     // Offset our operand pointer by the correct number of bytes.
-                    const byte_offset = struct_ty.packedStructFieldByteOffset(field_index, mod);
+                    const byte_offset = struct_ty.packedStructFieldByteOffset(field_index, mod) + @divExact(struct_ptr_ty_info.packed_offset.bit_offset, 8);
                     if (byte_offset == 0) return struct_ptr;
                     const usize_ty = try o.lowerType(Type.usize);
                     const llvm_index = try o.builder.intValue(usize_ty, byte_offset);