[llvm-bugs] [Bug 51816] New: std::basic_string misbehavior when sizeof(CharT) > 8

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=51816

            Bug ID: 51816
           Summary: std::basic_string misbehavior when sizeof(CharT) > 8
           Product: libc++
           Version: unspecified
          Hardware: PC
                OS: Windows NT
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: All Bugs
          Assignee: unassignedclangbugs at nondot.org
          Reporter: georgthegreat at gmail.com
                CC: llvm-bugs at lists.llvm.org, mclow.lists at gmail.com

Created attachment 25247
  --> https://bugs.llvm.org/attachment.cgi?id=25247&action=edit
Test using gtest

An attempt to compile std::basic_string with various weird character types
would lead to a multiple issues.

So far I have found the following:

1. stack allocation size would grow due to overallocation, i. e:

if constexpr (sizeof(Char) == 12) {
    EXPECT_EQ(sizeof(s), 40u);
}

2. allocations would become power-of-two-exponential, i. e:

for (size_t i = 0; i < 47u; ++i) {
    s.push_back({});
}

if constexpr (sizeof(Char) <= 4) {
        // OK, do not request power-of-two blocks from allocator 
        EXPECT_EQ(
                s.capacity(),
                // capacity grows exponentially starting from 22 bytes
                47u
        );
} else if constexpr (sizeof(Char) == 8) {
        // Bad 
        EXPECT_EQ(
                s.capacity(),
                // capacity grows exponentially as the power-of-two
                63u
        );
}

3. Memory leak is detected by ASan when both -fsized-deallocation is enabled
and stable ABI is used (i. e. _LIBCPP_ABI_VERSION == 2):

==1260518==ERROR: AddressSanitizer: new-delete-type-mismatch on 0x6080000004a0
in thread T0:
  object passed to delete has wrong type:
  size of the allocated type:   84 bytes;
  size of the deallocated type: 72 bytes.
    #0 0x4cdc02 in void std::__y1::__libcpp_operator_delete<void*, unsigned
long>(void*, unsigned long)
/home/thegeorg/arcadia/contrib/libs/cxxsupp/libcxx/include/new:245:3
    #1 0x4cdc02 in void std::__y1::__do_deallocate_handle_size<>(void*,
unsigned long) include/new:271:10
    #2 0x4cdc02 in std::__y1::__libcpp_deallocate(void*, unsigned long,
unsigned long) include/new:285:14
    #3 0x4cdc02 in std::__y1::allocator<WeirdChar12>::deallocate(WeirdChar12*,
unsigned long) include/memory:850:13
    #4 0x4cdc02 in std::__y1::allocator_traits<Allocator<WeirdChar12>
>::deallocate(Allocator<WeirdChar12>&, WeirdChar12*, unsigned long)
include/__memory/allocator_traits.h:484:14

This is caused by incorrect alignment in __recommend() and attempt to allocate
odd number of characters. String capacity() must be even as odd bit will be
overwritten by __long_mask.

Though C++ standard does not limit sizeof(CharT), I suggest
static_assert(sizeof(CharT) <= 4) to be added into libc++.
Though this could break existing codebases, I consider the break fast to be the
better option.

I do not think SSO is anyway helpful whenever sizeof(CharT) >= 8. Maybe we
should ensure that std::basic_strin is always long is such cases.

-- 
You are receiving this mail because:
You are on the CC list for the bug.
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-bugs/attachments/20210910/66b6f569/attachment-0001.html>