Question

I’m working on adapting a large Delphi code base to 64-bits. In many cases there are lines where pointers are casted to/from 32-bit values similar to this:

var
  p1,p2 : pointer;
begin
  inc(Integer(p1),10);
  p2 := Pointer(Integer(p1) + 42);

Where I can find these casts I have replaced them with NativeInt-casts instead to make them correct in 64-bit mode.

However I’m not sure I have found them all. Sometimes the casts are more subtle so just text-searching for the string “integer(” is not sufficient either.

Since the “integer(” casts will fail in 64-bit if the pointer value is above the range of integer type I have an idea: what if I could force the memory manager to allocate memory above 4gb (so the pointer values are using more than 32-bits)? Then I would get runtime errors and can more easily find the casts that are wrong. Is this possible? Or can anyone recommend some other technique?

Answer 1

There’s no magic trick to finding these casts beyond the sort of text search that you are using. It would be really nice if the compiler warned of such a cast. I find it very disappointing that it doesn’t.

When you do find such a problem, don’t change to NativeInt. Change the pointers to be typed pointers, and use pointer arithmetic.

var
  p1, p2: PByte;
....
inc(p1, 10);
p2 := p2;
inc(p2, 42);

Then your code will be safe forever.

There are still some situations where you need to cast to integers. For example when passing addresses to SendMessage. But cast these to either WPARAM or LPARAM as appropriate.

Your idea of forcing runtime errors is sound and, thankfully for you, not original! You should use the full version of FastMM and define AlwaysAllocateTopDown. This forces the calls that FastMM makes to VirtualAlloc to pass the MEM_TOP_DOWN flag. This will flush out most of your erroneous casts as runtime pointer truncation errors.

However, that will only force top down allocation for memory allocated by your memory manager. Other modules in your process will use the default policy of bottom up. You can set a machine wide setting to change that default policy. Set HKLM\System\CurrentControlSet\Control\Session Manager\Memory Management\AllocationPreference to REG_DWORD with value 0x100000 and reboot.

Note that this might cause your machine to have stability problems. Many applications cannot cope with this. In particular there are very few anti-virus products that can cope with this setting. MSE is the one that I found works with machine wide top down allocation. What’s more the 64 bit debugger does not run under top down allocation! So you have to do this kind of testing without the debugger. My QC report is still open and this problem has not been addressed, even in XE3.