[libcxx-commits] [libcxx] [libc++] std::cmp_less and other integer comparison functions could be improved (PR #151332)

[via libcxx-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-libcxx

Author: None (kisuhorikka)

<details>
<summary>Changes</summary>

Fix #<!-- -->65136 

---
Full diff: https://github.com/llvm/llvm-project/pull/151332.diff


1 Files Affected:

- (modified) libcxx/include/__utility/cmp.h (+14-2) 


``````````diff
diff --git a/libcxx/include/__utility/cmp.h b/libcxx/include/__utility/cmp.h
index 14dc0c154c040..4c29f09628095 100644
--- a/libcxx/include/__utility/cmp.h
+++ b/libcxx/include/__utility/cmp.h
@@ -28,7 +28,13 @@ _LIBCPP_BEGIN_NAMESPACE_STD
 
 template <__signed_or_unsigned_integer _Tp, __signed_or_unsigned_integer _Up>
 _LIBCPP_HIDE_FROM_ABI constexpr bool cmp_equal(_Tp __t, _Up __u) noexcept {
-  if constexpr (is_signed_v<_Tp> == is_signed_v<_Up>)
+  if constexpr (sizeof(_Tp) < sizeof(int) && sizeof(_Up) < sizeof(int)) {
+    __builtin_assume(__t < numeric_limits<int>::max() && __u < numeric_limits<int>::max());
+    return static_cast<int>(__t) == static_cast<int>(__u);
+  } else if constexpr (sizeof(_Tp) < sizeof(long long) && sizeof(_Up) < sizeof(long long)) {
+    __builtin_assume(__t < numeric_limits<long long>::max() && __u < numeric_limits<long long>::max());
+    return static_cast<long long>(__t) == static_cast<long long>(__u);
+  } else if constexpr (is_signed_v<_Tp> == is_signed_v<_Up>)
     return __t == __u;
   else if constexpr (is_signed_v<_Tp>)
     return __t < 0 ? false : make_unsigned_t<_Tp>(__t) == __u;
@@ -43,7 +49,13 @@ _LIBCPP_HIDE_FROM_ABI constexpr bool cmp_not_equal(_Tp __t, _Up __u) noexcept {
 
 template <__signed_or_unsigned_integer _Tp, __signed_or_unsigned_integer _Up>
 _LIBCPP_HIDE_FROM_ABI constexpr bool cmp_less(_Tp __t, _Up __u) noexcept {
-  if constexpr (is_signed_v<_Tp> == is_signed_v<_Up>)
+  if constexpr (sizeof(_Tp) < sizeof(int) && sizeof(_Up) < sizeof(int)) {
+    __builtin_assume(__t < numeric_limits<int>::max() && __u < numeric_limits<int>::max());
+    return static_cast<int>(__t) < static_cast<int>(__u);
+  } else if constexpr (sizeof(_Tp) < sizeof(long long) && sizeof(_Up) < sizeof(long long)) {
+    __builtin_assume(__t < numeric_limits<long long>::max() && __u < numeric_limits<long long>::max());
+    return static_cast<long long>(__t) < static_cast<long long>(__u);
+  } else if constexpr (is_signed_v<_Tp> == is_signed_v<_Up>)
     return __t < __u;
   else if constexpr (is_signed_v<_Tp>)
     return __t < 0 ? true : make_unsigned_t<_Tp>(__t) < __u;

``````````

</details>


https://github.com/llvm/llvm-project/pull/151332