Add start location transformation to allow more weakly aligned heaps

[davenpcj5542009]

In this example, allocation fails despite the heap being large enough due to the weak alignment of the start location.

The init/add_to_heap function doesn't find any 4KB-aligned 4KB blocks within the 6KB region.

use std::alloc::{GlobalAlloc, Layout};
use buddy_system_allocator::{LockedHeap};

fn main() {
    unsafe {
        HEAP_ALLOCATOR.lock().init(HEAP.0.as_ptr() as usize, HEAP.0.len());
        println!("heap: {:?}", *HEAP_ALLOCATOR.lock());
        let ptr = HEAP_ALLOCATOR.alloc(Layout::from_size_align_unchecked(0xb88, 4));
        println!("ptr: {:x}", ptr as usize);
    }
}
pub static mut HEAP_ALLOCATOR: LockedHeap<16> = LockedHeap::empty();

//#[repr(align(4096))]  // works when aligned to 4k boundary
//#[repr(align(256))]    // does not work with weaker alignment
struct Aligned<T>(T);

static mut HEAP:Aligned = Aligned([0u8;0x1800]);

output:

heap: Heap { user: 0, allocated: 0, total: 6144 }
ptr: 0

After the change, the allocation can succeed, as a 4096 block is available at a non-4096-byte-aligned address, and the 6KB heap is decomposed into a 4KB and 2KB block.

heap: Heap { user: 0, allocated: 0, total: 6144 }
{32768: [], 16384: [], 8192: [], 4096: [0x7ff65ca14180], 2048: [0x7ff65ca15180], 1024: [], 
512: [], 256: [], 128: [], 64: [], 32: [], 16: [], 8: [], 4: [], 2: [], 1: []}
ptr: 7ff65ca14180

[jiegec]

Although the 6KB region is split into 4KB and 2KB ones, the 4KB region is not aligned to 4KB boundary. This is an assumption for alloc() to satisfy Layout requiremenets.

[davenpcj5542009]

Although the 6KB region is split into 4KB and 2KB ones, the 4KB region is not aligned to 4KB boundary. This is an assumption for alloc() to satisfy Layout requiremenets.

The basic pointer-alignment guarantees are still met, which suffices for most usage; But I do see the Layout's required alignment is unmet during allocation, especially during over-aligned allocations.