Support reallocating to a different alignment?

[SimonSapin]

The GlobalAlloc trait is stable with this method:

unsafe fn realloc(&self, ptr: *mut u8, layout: Layout, new_size: usize) -> *mut u8

(And similarly the std::alloc::realloc function.)

Note new_size: usize instead of new_layout: Layout. This is because this method does not support changing the alignment.

---

Do we want to lift this restriction in the Alloc trait?

The two parameters of realloc would be old_size: usize, new_layout: Layout. For allocators that do not support this, this would shift the responsibility to trait impls to have a fallback based on alloc + copy_nonoverlapping + dealloc.

[glandium]

What are the use cases for changing the alignment? One I can think of is if you have an allocation for, say an array of u32s, that you got from somewhere, and you want to ensure they are aligned to e.g. 16 bytes to do SIMD stuff to them. But I think most algorithms dealing with things like this just deal with the unaligned parts separately, instead of reallocating.

[SimonSapin]

IIRC this lack of known use case might have contributed to the choice of not supporting this in GlobalAlloc.

[TimDiekmann]

So we stick to the current interface for now?

Alloc:

unsafe fn realloc(
    &mut self,
    ptr: NonNull<u8>,
    layout: Layout,
    new_size: usize
) -> Result<NonNull<u8>, AllocErr>

GlobalAlloc:

unsafe fn realloc(
    &self,
    ptr: *mut u8,
    layout: Layout,
    new_size: usize
) -> *mut u8

[gnzlbg]

@glandium If you have an allocation for storing Ts, and you are done with it, but want to reuse it, e.g., for storing Us, then you might want to reallocate to fit your Us, which might require an allocation that's bigger or smaller, and has a higher or a lower alignment.

So a general purpose reallocation function might be useful.

Some allocators do support this, for example, jemalloc rallocx takes a pointer, the old size, the new size, and a flags parameter that allows specifying the alignment required for the resulting allocation: https://github.com/jemalloc/jemalloc/blob/dev/test/integration/rallocx.c#L150

Some allocators do not support this, e.g., those using a standard C API, but I don't think that using two Layouts complicates the implementation for these significantly. Right now, they can't just always call C's realloc(ptr, size) because that does not necessarily preserve the alignment (e.g. if ptr was allocated with posix_memalign to a higher alignment), so they already need to have quite a bit of logic to check the old layout alignment, and figure out what to do. Sure, adding more logic for comparing that with the new layouts alignment adds another branch in the hot path, but they are already branchy:

https://github.com/rust-lang/rust/blob/9ebf47851a357faa4cd97f4b1dc7835f6376e639/src/libstd/sys/unix/alloc.rs#L41

https://github.com/rust-lang/rust/blob/9ebf47851a357faa4cd97f4b1dc7835f6376e639/src/libstd/sys_common/alloc.rs#L24

So its basically just another check to decide whether to go to the fallback or not.

[TimDiekmann]

If we decide to lift the alignment restrictions, wouldn't it make sense to also lift the requirements for grow_in_place and shrink_in_place?

adding more logic for comparing that with the new layouts alignment adds another branch in the hot path

Branches in the hot path a never good IMO. It'd be possible to add methods for it though so we end up with these signatures (all fns are unsafe and return a reasonable Result<_, _>):

fn realloc(&mut self, ptr: NonNull<u8>, old_layout: Layout, new_layout: Layout);
fn realloc_aligned(&mut self, ptr: NonNull<u8>, layout: Layout, new_size: usize);
fn grow_in_place(&mut self, ptr: NonNull<u8>, old_layout: Layout, new_layout: Layout);
fn grow_in_place_aligned(&mut self, ptr: NonNull<u8>, layout: Layout, new_size: usize);
fn shrink_in_place(&mut self, ptr: NonNull<u8>, old_layout: Layout, new_layout: Layout);
fn shrink_in_place_aligned(&mut self, ptr: NonNull<u8>, layout: Layout, new_size: usize);

or

fn realloc_unaligned(&mut self, ptr: NonNull<u8>, old_layout: Layout, new_layout: Layout);
fn realloc(&mut self, ptr: NonNull<u8>, layout: Layout, new_size: usize);
fn grow_in_place_unaligned(&mut self, ptr: NonNull<u8>, old_layout: Layout, new_layout: Layout);
fn grow_in_place(&mut self, ptr: NonNull<u8>, layout: Layout, new_size: usize);
fn shrink_in_place_unaligned(&mut self, ptr: NonNull<u8>, old_layout: Layout, new_layout: Layout);
fn shrink_in_place(&mut self, ptr: NonNull<u8>, layout: Layout, new_size: usize);

The former has the better names, the latter is analogue to GlobalAlloc.

An example implementation for the former case:

#[inline]
fn is_aligned(ptr: NonNull<u8>, align: usize) -> bool {
    ptr.as_ptr() as usize % align == 0
}

#[inline]
unsafe fn naive_realloc<A: ?Sized + Alloc>(
    alloc: &mut A,
    ptr: NonNull<u8>,
    old_layout: Layout,
    new_layout: Layout,
) -> Result<NonNull<u8>, AllocErr> {
    let new_ptr = alloc.alloc(new_layout)?;
    ptr::copy_nonoverlapping(
        ptr.as_ptr(),
        new_ptr.as_ptr(),
        cmp::min(old_layout.size(), new_layout.size()),
    );
    alloc.dealloc(ptr, old_layout);
    Ok(new_ptr)
}

#[inline]
unsafe fn realloc_in_place_aligned<A: ?Sized + Alloc>(
    alloc: &mut A,
    ptr: NonNull<u8>,
    layout: Layout,
    new_size: usize,
) -> Result<(), CannotReallocInPlace> {
    let old_size = layout.size();

    if new_size == old_size {
        Ok(())
    } else if new_size > old_size {
        alloc.grow_in_place_aligned(ptr, layout, new_size)
    } else {
        alloc.shrink_in_place_aligned(ptr, layout, new_size)
    }
}

pub unsafe trait Alloc {
    // ...
    unsafe fn realloc(
        &mut self,
        ptr: NonNull<u8>,
        old_layout: Layout,
        new_layout: Layout,
    ) -> Result<NonNull<u8>, AllocErr> {
        if is_aligned(ptr, new_layout.align())
            && realloc_in_place_aligned(self, ptr, old_layout, new_layout.size()).is_ok()
        {
            return Ok(ptr);
        }
        naive_realloc(self, ptr, old_layout, new_layout)
    }

    unsafe fn realloc_aligned(
        &mut self,
        ptr: NonNull<u8>,
        layout: Layout,
        new_size: usize,
    ) -> Result<NonNull<u8>, AllocErr> {
        if realloc_in_place_aligned(self, ptr, layout, new_size).is_ok() {
            return Ok(ptr);
        }

        naive_realloc(
            self,
            ptr,
            layout,
            Layout::from_size_align_unchecked(new_size, layout.align()),
        )
    }

    #[inline]
    unsafe fn grow_in_place(
        &mut self,
        ptr: NonNull<u8>,
        old_layout: Layout,
        new_layout: Layout,
    ) -> Result<(), CannotReallocInPlace> {
        debug_assert!(new_layout.size() >= old_layout.size());
        if is_aligned(ptr, new_layout.align()) {
            self.grow_in_place_aligned(ptr, old_layout, new_layout.size())
        } else {
            Err(CannotReallocInPlace)
        }
    }

    unsafe fn grow_in_place_aligned(
        &mut self,
        ptr: NonNull<u8>,
        layout: Layout,
        new_size: usize,
    ) -> Result<(), CannotReallocInPlace> {
        // same as old `grow_in_place` 
    }

    #[inline]
    unsafe fn shrink_in_place(
        &mut self,
        ptr: NonNull<u8>,
        old_layout: Layout,
        new_layout: Layout,
    ) -> Result<(), CannotReallocInPlace> {
        debug_assert!(new_layout.size() <= old_layout.size());
        if is_aligned(ptr, new_layout.align()) {
            self.shrink_in_place_aligned(ptr, old_layout, new_layout.size())
        } else {
            Err(CannotReallocInPlace)
        }
    }

    unsafe fn shrink_in_place_aligned(
        &mut self,
        ptr: NonNull<u8>,
        layout: Layout,
        new_size: usize,
    ) -> Result<(), CannotReallocInPlace> {
        // same as old `shrink_in_place` 
    }
}

[gnzlbg]

Branches in the hot path a never good IMO.

Looking at the code, I think it would still be one branch, but the branch condition would be slightly more complicated, e.g., it would contain an extra && old_layout.align() == new_layout.align().

I don't know what the perf cost of that would be, and without benchmarking that it might be hard to tell (both layouts could be inlined), but I can't imagine that the perf cost is worth the API cost of adding 3 trait methods to work around that, particularly without having more concrete use cases of code that actually needs to do this, and that we could benchmark to make an informed decision.

[TimDiekmann]

This proposal is around for quite a while now. Do you think the extra branch (or the more complicated check) is worth?

[Lokathor]

Realloc is ultimately a shorthand and you can live without it.

If it doesn't support an alignment change that's fine, and a particular allocator can choose to have a non-trait method to do that if it has such an ability.