GCC 8.1: alloc-size-larger-than warnings during operator new[]

[Paul Smith <paul@xxxxxxxxxxxxxxxxx>]

Hi all.

I'm testing my codebase with GCC 8.1 (built myself) and am running into
a couple if issues.  Note I was able to compile and run OK with GCC 7.3
FYI.  This is on a GNU/Linux x86_64 system and I'm using binutils 2.30,
and compiling with -O2 -g -Wall -Werror (and a few other things).

The first problem is that I have three instances (in a very large
codebase) where I'm seeing errors like this:

  error: argument 1 value '18446744073709551615' exceeds maximum object
size 9223372036854775807 [-Werror=alloc-size-larger-than=]

These instances all have a similar invocation model either directly or
indirectly (through inline functions for example) which involves a
size_t value as an argument to operator new[] which is overridden as an
inline function that calls malloc(), and the value is involved in some
condition operation regarding its value.

Here's a simple pseudo-example:

  SortRecord::sortRecord(size_t numberValues)
  {
      values = (numberValues == 0 ? nullptr : new Value[numberValues]);
    ...

Clearly if numberValues is large enough this allocation must fail but I
don't see how or why GCC is warning me about this, or what I'm supposed
to do about it.

If I (just to test) remove the condition then it compiles fine:

  SortRecord::sortRecord(size_t numberValues)
  {
      values = new Value[numberValues];
    ...

Also, if I don't have the inline operator new[] then it seems to
compile OK as well.  The inline version is pretty straightforward:

  operator new[](size_t sz) {
      void* p = malloc(sz);
      if (!p) MemFail();
      return p;
  }

(basically).

I can disable this warning but I wonder if this is a bug in GCC or this
warning is just too aggressive.  Unfortunately I can't point to the
real code and I don't have a cut-down example right now.