Fix misaligned store in Bool to union upcasts (#14906)

The code path for `Nil` looks similar, but it is perfectly fine: it directly stores `[8 x i64] zeroinitializer` to the data field, whose default alignment naturally matches.

spec/compiler/codegen/union_type_spec.cr

@@ -215,4 +215,23 @@ describe "Code gen: union type" do
       Union(Nil, Int32).foo
       )).to_string.should eq("TupleLiteral")
   end
+
+  it "respects union payload alignment when upcasting Bool (#14898)" do
+    mod = codegen(<<-CRYSTAL)
+      x = uninitialized Bool | UInt8[64]
+      x = true
+      CRYSTAL
+
+    str = mod.to_s
+    {% if LibLLVM::IS_LT_150 %}
+      str.should contain("store i512 1, i512* %2, align 8")
+    {% else %}
+      str.should contain("store i512 1, ptr %1, align 8")
+    {% end %}
+
+    # an i512 store defaults to 16-byte alignment, which is undefined behavior
+    # as it overestimates the actual alignment of `x`'s data field (x86 in
+    # particular segfaults on misaligned 16-byte stores)
+    str.should_not contain("align 16")
+  end
 end

src/compiler/crystal/codegen/unions.cr

@@ -81,16 +81,19 @@ module Crystal
 
     def store_bool_in_union(target_type, union_pointer, value)
       struct_type = llvm_type(target_type)
+      union_value_type = struct_type.struct_element_types[1]
       store type_id(value, @program.bool), union_type_id(struct_type, union_pointer)
 
       # To store a boolean in a union
-      # we sign-extend it to the size in bits of the union
-      union_size = @llvm_typer.size_of(struct_type.struct_element_types[1])
+      # we zero-extend it to the size in bits of the union
+      union_size = @llvm_typer.size_of(union_value_type)
       int_type = llvm_context.int((union_size * 8).to_i32)
 
       bool_as_extended_int = builder.zext(value, int_type)
       casted_value_ptr = pointer_cast(union_value(struct_type, union_pointer), int_type.pointer)
-      store bool_as_extended_int, casted_value_ptr
+      inst = store bool_as_extended_int, casted_value_ptr
+      set_alignment(inst, @llvm_typer.align_of(union_value_type))
+      inst
     end
 
     def store_nil_in_union(target_type, union_pointer)