[Bug c++/109895] New: -Walloc-size-larger-than complains about code it generated itself under -flto -fno-exceptions

["thiago at kde dot org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109895

            Bug ID: 109895
           Summary: -Walloc-size-larger-than complains about code it
                    generated itself under -flto -fno-exceptions
           Product: gcc
           Version: 13.1.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: thiago at kde dot org
  Target Milestone: ---

Reference: https://bugreports.qt.io/browse/QTBUG-113603

Code in question:

    const auto paramCount = mysql_stmt_param_count(d->stmt);
    if (paramCount > 0) // allocate memory for outvalues
        d->outBinds = new MYSQL_BIND[paramCount]();

mysql_stmt_param_count returns unsigned long.

GCC 13.1 with -flto -fno-exceptions produced:

src/plugins/sqldrivers/mysql/qsql_mysql.cpp: In member function ‘prepare’:
src/plugins/sqldrivers/mysql/qsql_mysql.cpp:891:50: warning: argument 1 value
‘18446744073709551615’ exceeds maximum object size 9223372036854775807
[-Walloc-size-larger-than=]
  891 |         d->outBinds = new MYSQL_BIND[paramCount]();
      |                                                  ^
/usr/include/c++/13/new:128:26: note: in a call to allocation function
‘operator new []’ declared here
  128 | _GLIBCXX_NODISCARD void* operator new[](std::size_t) _GLIBCXX_THROW
(std::bad_alloc)
      |                          ^

Disassembling the code shows it looks similar to this:
https://godbolt.org/z/9eKPxbEMY
        movq    $-1, %rdi
        cmpq    %rbx, %rax
        jb      .L2
...
.L2:
        call    operator new[](unsigned long)@PLT

So that 18446744073709551615 value is the -1 inserted by GCC itself to deal
with the multiplication overflow.