[libcxx-commits] [libcxx] [libc++] Avoid -Wzero-as-null-pointer-constant in operator<=> (PR #79465)
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Thu Jan 25 08:11:12 PST 2024
llvmbot wrote:
<!--LLVM PR SUMMARY COMMENT-->
@llvm/pr-subscribers-libcxx
Author: Louis Dionne (ldionne)
<details>
<summary>Changes</summary>
Issue #<!-- -->43670 describes a situation where the following comparison will issue a warning when -Wzero-as-null-pointer-constant is enabled:
#include <compare>
auto b = (1 <=> 2) < 0;
This code uses operator<(strong_ordering, Unspecified), which is specified by the Standard to only work with a literal 0. In the library, this is achieved by constructing Unspecified from a pointer, which works but has the downside of triggering the warning.
This patch uses an alternative implementation where we require that the operator is used exactly with an int of value 0 (known at compile-time), however that value can technically be an expression like `1 - 1`, which makes us a bit less strict than what's specified in the Standard.
Fixes #<!-- -->43670
---
Full diff: https://github.com/llvm/llvm-project/pull/79465.diff
3 Files Affected:
- (modified) libcxx/include/__compare/ordering.h (+12-8)
- (added) libcxx/test/std/language.support/cmp/cmp.categories.pre/reject-other-than-literal-zero.verify.cpp (+70)
- (removed) libcxx/test/std/language.support/cmp/cmp.categories.pre/zero_type.verify.cpp (-57)
``````````diff
diff --git a/libcxx/include/__compare/ordering.h b/libcxx/include/__compare/ordering.h
index 2995d381304f0e4..778cf5dab4ce567 100644
--- a/libcxx/include/__compare/ordering.h
+++ b/libcxx/include/__compare/ordering.h
@@ -30,14 +30,17 @@ class partial_ordering;
class weak_ordering;
class strong_ordering;
-template <class _Tp, class... _Args>
-inline constexpr bool __one_of_v = (is_same_v<_Tp, _Args> || ...);
-
struct _CmpUnspecifiedParam {
- _LIBCPP_HIDE_FROM_ABI constexpr _CmpUnspecifiedParam(int _CmpUnspecifiedParam::*) noexcept {}
-
- template <class _Tp, class = enable_if_t<!__one_of_v<_Tp, int, partial_ordering, weak_ordering, strong_ordering>>>
- _CmpUnspecifiedParam(_Tp) = delete;
+ template <class _Tp, class = __enable_if_t<is_same_v<_Tp, int>>>
+ _LIBCPP_HIDE_FROM_ABI consteval _CmpUnspecifiedParam(_Tp __zero) noexcept {
+ // If anything other than 0 is provided, the behavior is undefined.
+ // We catch this case by making this function consteval and making the program ill-formed if
+ // a value other than 0 is provided. This technically also accepts things that are not
+ // literal 0s like `1 - 1`. The alternative is to use the fact that a pointer can be
+ // constructed from literal 0, but this conflicts with `-Wzero-as-null-pointer-constant`.
+ if (__zero != 0)
+ __builtin_trap();
+ }
};
class partial_ordering {
@@ -269,7 +272,8 @@ inline constexpr strong_ordering strong_ordering::greater(_OrdResult::__greater)
/// The types partial_ordering, weak_ordering, and strong_ordering are
/// collectively termed the comparison category types.
template <class _Tp>
-concept __comparison_category = __one_of_v<_Tp, partial_ordering, weak_ordering, strong_ordering>;
+concept __comparison_category =
+ is_same_v<_Tp, partial_ordering> || is_same_v<_Tp, weak_ordering> || is_same_v<_Tp, strong_ordering>;
#endif // _LIBCPP_STD_VER >= 20
diff --git a/libcxx/test/std/language.support/cmp/cmp.categories.pre/reject-other-than-literal-zero.verify.cpp b/libcxx/test/std/language.support/cmp/cmp.categories.pre/reject-other-than-literal-zero.verify.cpp
new file mode 100644
index 000000000000000..f2b30284d42a60c
--- /dev/null
+++ b/libcxx/test/std/language.support/cmp/cmp.categories.pre/reject-other-than-literal-zero.verify.cpp
@@ -0,0 +1,70 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++03, c++11, c++14, c++17
+
+// In MSVC mode, there's a slightly different number of errors printed for
+// each of these, so it doesn't add up to the exact expected count of 18.
+// XFAIL: msvc
+
+// <compare>
+
+// Ensure we reject all cases where an argument other than a literal 0 is used
+// for a comparison against a comparison category type.
+
+// Also ensure that we don't warn about providing a null pointer constant when
+// comparing an ordering type against literal 0, since one of the common
+// implementation strategies is to use a pointer as the "unspecified type".
+// ADDITIONAL_COMPILE_FLAGS: -Wzero-as-null-pointer-constant
+
+#include <compare>
+
+#include "test_macros.h"
+
+#define TEST_FAIL(v, op) \
+ do { \
+ void(v op 0L); \
+ void(0L op v); \
+ void(v op 1); \
+ void(1 op v); \
+ void(v op nullptr); \
+ void(nullptr op v); \
+ } while (false)
+
+#define TEST_PASS(v, op) \
+ do { \
+ void(v op 0); \
+ void(0 op v); \
+ LIBCPP_ONLY(void(v op(1 - 1))); \
+ LIBCPP_ONLY(void((1 - 1) op v)); \
+ } while (false)
+
+template <typename T>
+void test_category(T v) {
+ TEST_FAIL(v, ==); // expected-error 18 {{}}
+ TEST_FAIL(v, !=); // expected-error 18 {{}}
+ TEST_FAIL(v, <); // expected-error 18 {{}}
+ TEST_FAIL(v, <=); // expected-error 18 {{}}
+ TEST_FAIL(v, >); // expected-error 18 {{}}
+ TEST_FAIL(v, >=); // expected-error 18 {{}}
+ TEST_FAIL(v, <=>); // expected-error 18 {{}}
+
+ TEST_PASS(v, ==);
+ TEST_PASS(v, !=);
+ TEST_PASS(v, <);
+ TEST_PASS(v, >);
+ TEST_PASS(v, <=);
+ TEST_PASS(v, >=);
+ TEST_PASS(v, <=>);
+}
+
+void f() {
+ test_category(std::strong_ordering::equivalent);
+ test_category(std::weak_ordering::equivalent);
+ test_category(std::partial_ordering::equivalent);
+}
diff --git a/libcxx/test/std/language.support/cmp/cmp.categories.pre/zero_type.verify.cpp b/libcxx/test/std/language.support/cmp/cmp.categories.pre/zero_type.verify.cpp
deleted file mode 100644
index 8e3c793de4b92ae..000000000000000
--- a/libcxx/test/std/language.support/cmp/cmp.categories.pre/zero_type.verify.cpp
+++ /dev/null
@@ -1,57 +0,0 @@
-//===----------------------------------------------------------------------===//
-//
-// 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
-//
-//===----------------------------------------------------------------------===//
-
-// UNSUPPORTED: c++03, c++11, c++14, c++17
-
-// In MSVC mode, there's a slightly different number of errors printed for
-// each of these, so it doesn't add up to the exact expected count of 18.
-// XFAIL: msvc
-
-// <compare>
-
-// Ensure we reject all cases where an argument other than a literal 0 is used
-// for a comparison against a comparison category type.
-
-#include <compare>
-
-#define TEST_FAIL(v, op) \
- void(v op 0L); \
- void(0L op v); \
- void(v op nullptr); \
- void(nullptr op v); \
- void(v op(1 - 1)); \
- void((1 - 1) op v)
-
-#define TEST_PASS(v, op) \
- void(v op 0); \
- void(0 op v)
-
-template <typename T>
-void test_category(T v) {
- TEST_FAIL(v, ==); // expected-error 18 {{}}
- TEST_FAIL(v, !=); // expected-error 18 {{}}
- TEST_FAIL(v, <); // expected-error 18 {{}}
- TEST_FAIL(v, <=); // expected-error 18 {{}}
- TEST_FAIL(v, >); // expected-error 18 {{}}
- TEST_FAIL(v, >=); // expected-error 18 {{}}
- TEST_FAIL(v, <=>); // expected-error 18 {{}}
-
- TEST_PASS(v, ==);
- TEST_PASS(v, !=);
- TEST_PASS(v, <);
- TEST_PASS(v, >);
- TEST_PASS(v, <=);
- TEST_PASS(v, >=);
- TEST_PASS(v, <=>);
-}
-
-void f() {
- test_category(std::strong_ordering::equivalent);
- test_category(std::weak_ordering::equivalent);
- test_category(std::partial_ordering::equivalent);
-}
``````````
</details>
https://github.com/llvm/llvm-project/pull/79465
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