Both "-O2" and "-O1" don't work as well. Only "-O0" works.

Could be.  What GCC version are you using?


Latest:

gcc.exe (Rev2, Built by MSYS2 project) 4.9.2


Also, when -O3 is used, does it help to declare reserved_heap_size
> volatile, like this:
>


  size_t volatile reserved_heap_size;
>


(You might need to change the extern declaration on w32heap.h as
> well.)


Unfortunately, nope.