Pull size alignment to AValue

Summary:
Align object size of alignment of allocation to `AValue`.

Simplify code code and make sure alignment is not overlooked.

Reviewed By: ndmitchell

Differential Revision: D41111517

fbshipit-source-id: add1472ccc0393054109a93570d9855f86f58439

Author: stepancheg

starlark/src/values/layout/avalue.rs

@@ -163,11 +163,12 @@ pub(crate) trait AValue<'v>: StarlarkValueDyn<'v> + Sized {
             Self::offset_of_extra() % mem::align_of::<Self::ExtraElem>() == 0,
             "extra must be aligned"
         );
-        cmp::max(
+        let size = cmp::max(
             mem::size_of::<Self::StarlarkValue>(),
             // Content is not necessarily aligned to end of `A`.
             Self::offset_of_extra() + (mem::size_of::<Self::ExtraElem>() * extra_len),
-        )
+        );
+        AValueHeader::align_up(size)
     }
 
     unsafe fn heap_freeze(

starlark/src/values/layout/heap/arena.rs

@@ -132,8 +132,6 @@ impl<'c> Iterator for ChunkIter<'c> {
                 let or_forward = &*(self.chunk.as_ptr() as *const AValueOrForward);
                 let n = or_forward.alloc_size();
                 debug_assert!(n <= self.chunk.len());
-                let n = AValueHeader::align_up(n);
-                let n = cmp::min(n, self.chunk.len());
                 self.chunk = self.chunk.split_at(n).1;
                 Some(or_forward)
             }
@@ -174,6 +172,7 @@ impl Arena {
             mem::size_of::<AValueHeader>() + T::memory_size_for_extra_len(extra_len),
             MIN_ALLOC,
         );
+        debug_assert!(size % AValueHeader::ALIGN == 0);
         let layout = Layout::from_size_align(size, mem::align_of::<AValueHeader>()).unwrap();
         let p = bump.alloc_layout(layout).as_ptr();
         unsafe {

starlark/src/values/layout/heap/repr.rs

@@ -179,7 +179,9 @@ impl AValueOrForward {
             }
         };
         let n = mem::size_of::<AValueHeader>() + n;
-        cmp::max(n, MIN_ALLOC)
+        let size = cmp::max(n, MIN_ALLOC);
+        debug_assert!(size % AValueHeader::ALIGN == 0);
+        size
     }
 }
 

starlark/src/values/layout/vtable.rs

@@ -223,7 +223,9 @@ pub(crate) struct AValueDyn<'v> {
 impl<'v> AValueDyn<'v> {
     #[inline]
     pub(crate) fn memory_size(self) -> usize {
-        (self.vtable.memory_size)(self.value as *const ())
+        let size = (self.vtable.memory_size)(self.value as *const ());
+        debug_assert!(size % AValueHeader::ALIGN == 0);
+        size
     }
 
     pub(crate) fn total_memory(self) -> usize {