Negative offset pointers that store array length?

[zezba9000]

Does the bdwgc support negative offset pointers? Trying to avoid ptr arithmetic by storing the length of an array at a negative offset.

void* IL2X_GC_New(size_t size)
{
	void* ptr = GC_malloc(size);
	memset(ptr, 0, size);
	return ptr;
}

void* IL2X_GC_NewArray(size_t size)
{
	char* result = IL2X_GC_New(sizeof(size_t) + size);
	*((size_t*)result) = size;
	result += sizeof(size_t);// could the GC handle this
	return result;
}

int ArrayLength(void* arrayPtr)
{
  return (int)*(((char*)arrayPtr) - size_t);
}

void Foo()
{
  int* array = IL2X_GC_NewArray(128);
  int length = ArrayLength(array);
  array[22] = 0;// trying to optimize with this approch
  (int*)(((char*)array) + size_t)[22] = 0;// really don't want to have to do this
}

[ivmai]

memset(ptr, 0, size);

Not needed. GC_malloc returns zeroed memory.

result += sizeof(size_t);// could the GC handle this?

Yes.

return (int)(((char)arrayPtr) - size_t);

A cast to size_t* is missing (you store 4 bytes but read only 1, in case of big-endian architecture it is definitely a bug).

array[22] = 0;// trying to optimize with this approach

It is OK (it should work).

PS. It is better to ask question on Stackoveflow: https://stackoverflow.com/questions/tagged/boehm-gc

[zezba9000]

return (int)*(((char*)arrayPtr) - sizeof(size_t));

Was just a type-o, just pseudocode.

result += sizeof(size_t);
Yes

So the GC wont collect the allocated memory even when nothing is directly pointing to it on the heap or stack? The GC supports checking if any ptr is pointing inside a heap object not just its ref directly? Just clarifying.

Will ask future question on stack overflow.

[ivmai]

So the GC wont collect the allocated memory even when nothing is directly pointing to it on the heap or stack? The GC supports checking if any ptr is pointing inside a heap object not just its ref directly?

Yes, but you need to instruct GC at the program start to do so either by calling GC_register_displacement(sizeof(void*)) (allow pointer to have a word-size offset) or GC_set_all_interior_pointers(1) (allow pointers to any offset inside object).