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[libc++] Add test to ensure that the mangling of types stays the same #143556

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Jun 23, 2025
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[libc++] Add test to ensure that the mangling of types stays the same #143556

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74 changes: 74 additions & 0 deletions libcxx/test/libcxx/mangled_names.pass.cpp
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//===----------------------------------------------------------------------===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//

// We're using `string::starts_with` in this test
// UNSUPPORTED: c++03, c++11, c++14, c++17
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It seems more important that we test the manglings are consistent across dialects, rather than use starts_with.


// Make sure that the mangling of our public types stays the same
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Suggested change
// Make sure that the mangling of our public types stays the same
// This test pins down the mangling of our public types to ensure that we don't
// change it unintentionally, which is an ABI break.


// UNSUPPORTED: no-rtti, msvc

#include <cassert>
#include <charconv>
#include <iostream>
#include <map>
#include <typeinfo>
#include <string>
#include <string_view>

template <class>
struct mangling {};

struct test_struct {};

_LIBCPP_BEGIN_NAMESPACE_STD
struct ns_mangling {};
_LIBCPP_END_NAMESPACE_STD

namespace std::__name {
struct ns_mangling {};
} // namespace std::__name

namespace std::__long_name_to_make_sure_multiple_digits_work {
struct ns_mangling {};
} // namespace std::__long_name_to_make_sure_multiple_digits_work

std::string get_std_inline_namespace_mangling(const std::type_info& info) {
std::string name = info.name();
assert(name.starts_with("NSt"));
unsigned name_len;
auto res = std::from_chars(name.data() + 3, name.data() + name.size(), name_len);
assert(res.ec == std::errc{});
return std::move(name).substr(0, (res.ptr + name_len) - name.data());
}

void expect_mangling(const std::type_info& info, std::string expected_name) {
if (expected_name != info.name())
std::__libcpp_verbose_abort("Expected: '%s'\n Got: '%s'\n", expected_name.c_str(), info.name());
}

// Mangling names are really long, but splitting it up into multiple lines doesn't make it any more readable
// clang-format off
int main(int, char**) {
// self-test inline namespace recovery
assert(get_std_inline_namespace_mangling(typeid(std::__name::ns_mangling)) == "NSt6__name");
assert(get_std_inline_namespace_mangling(typeid(std::__long_name_to_make_sure_multiple_digits_work::ns_mangling)) == "NSt45__long_name_to_make_sure_multiple_digits_work");

// selftest
expect_mangling(typeid(test_struct), "11test_struct");

std::string ns_std = get_std_inline_namespace_mangling(typeid(std::ns_mangling));
std::string ptrdiff = typeid(std::ptrdiff_t).name();

// std::map
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Is the plan to manually expand this to all public types in the library? That seems like something that'll quickly get out of hand and out of sync. I thought the idea was to create an automated test for this somehow (maybe clang-tidy?)

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At some point I'd like to add an automated test, yes. I'm not quite sure how to do that yet though.

For now my plan is to just pin down manglings where we think it's easy to mess up. I don't expect we'd miss a mangling change in most places, so I don't think it makes a ton of sense to add stuff like vector::iterator.

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In the future, I think we should write this test consistently with how we do the rest of ABI list testing -- when we rework how that works. For now, the current approach seems OK to me.

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The goal here seems similar to the symbol list tests.

We want to ensure that a users program which exposes the mangled name of std::foo as part of one of their symbols continues to link when compiler before & after a libc++ change.

It seems to me that given the specification of std::type_info::name, that it's not the most suitable tool for the job. In particular:

Returns an implementation defined null-terminated character string containing the name of the type. No guarantees are given; in particular, the returned string can be identical for several types and change between invocations of the same program.

I would consider simply generating a big symbol name, containing all of the types we want to test the names of, and then ensuring code generated before a given change can link to code generated after the change.

For example:

#include <string>
#include <vector>
#include <tuple>

// Compile the definition with the headers in trunk
// RUN: %{cxx} -c -cxx-isystem %{trunk-headers} -o %t.o -DDEFINE_TARGET

// Then compile the main 
// RUN: %{cxx} -cxx-isystem %{headers-under-test}  -o a.out %t.o
// RUN: ./a.out

template <class ...> struct TypeList {};

using TypesUnderTest = 
    TypeList<
        std::vector<int>,
        std::vector<int>::iterator,
        std::tuple<int, int&>
    >;

void target(TypesUnderTest);

#ifdef DEFINE_TARGET
void target(TypesUnderTest) {}
#else
int main() {
    target({});
}
#endif

This approach sort-of avoids having golden files checked in, which is a pain since they need manual care whenever we add new configurations or to regenerate them. It instead expects that we have two versions of the headers available, and treats one of those versions as "golden" (which is sort of what we want I think).

expect_mangling(typeid(std::map<int, int>), ns_std + "3mapIiiNS_4lessIiEENS_9allocatorINS_4pairIKiiEEEEEE");
expect_mangling(typeid(std::map<int, int>::iterator), ns_std + "14__map_iteratorINS_15__tree_iteratorINS_12__value_typeIiiEEPNS_11__tree_nodeIS3_PvEE" + ptrdiff +"EEEE");
expect_mangling(typeid(std::map<int, int>::const_iterator), ns_std + "20__map_const_iteratorINS_21__tree_const_iteratorINS_12__value_typeIiiEEPNS_11__tree_nodeIS3_PvEE" + ptrdiff + "EEEE");

return 0;
}
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