Overhaul of the AllocRef trait to match allocator-wg's latest consens

Author: TimDiekmann

src/liballoc/alloc.rs

@@ -2,7 +2,7 @@
 
 #![stable(feature = "alloc_module", since = "1.28.0")]
 
-use core::intrinsics::{min_align_of_val, size_of_val};
+use core::intrinsics::{self, min_align_of_val, size_of_val};
 use core::ptr::{NonNull, Unique};
 use core::usize;
 
@@ -165,11 +165,19 @@ pub unsafe fn alloc_zeroed(layout: Layout) -> *mut u8 {
 #[unstable(feature = "allocator_api", issue = "32838")]
 unsafe impl AllocRef for Global {
     #[inline]
-    fn alloc(&mut self, layout: Layout) -> Result<(NonNull<u8>, usize), AllocErr> {
-        if layout.size() == 0 {
+    fn alloc(&mut self, layout: Layout, init: AllocInit) -> Result<(NonNull<u8>, usize), AllocErr> {
+        let new_size = layout.size();
+        if new_size == 0 {
             Ok((layout.dangling(), 0))
         } else {
-            unsafe { NonNull::new(alloc(layout)).ok_or(AllocErr).map(|p| (p, layout.size())) }
+            unsafe {
+                let raw_ptr = match init {
+                    AllocInit::Uninitialized => alloc(layout),
+                    AllocInit::Zeroed => alloc_zeroed(layout),
+                };
+                let ptr = NonNull::new(raw_ptr).ok_or(AllocErr)?;
+                Ok((ptr, new_size))
+            }
         }
     }
 
@@ -181,33 +189,77 @@ unsafe impl AllocRef for Global {
     }
 
     #[inline]
-    unsafe fn realloc(
+    unsafe fn grow(
         &mut self,
         ptr: NonNull<u8>,
         layout: Layout,
         new_size: usize,
+        placement: ReallocPlacement,
+        init: AllocInit,
     ) -> Result<(NonNull<u8>, usize), AllocErr> {
-        match (layout.size(), new_size) {
-            (0, 0) => Ok((layout.dangling(), 0)),
-            (0, _) => self.alloc(Layout::from_size_align_unchecked(new_size, layout.align())),
-            (_, 0) => {
-                self.dealloc(ptr, layout);
-                Ok((layout.dangling(), 0))
+        let old_size = layout.size();
+        debug_assert!(
+            new_size >= old_size,
+            "`new_size` must be greater than or equal to `layout.size()`"
+        );
+
+        if old_size == new_size {
+            return Ok((ptr, new_size));
+        }
+
+        match placement {
+            ReallocPlacement::MayMove => {
+                if old_size == 0 {
+                    self.alloc(Layout::from_size_align_unchecked(new_size, layout.align()), init)
+                } else {
+                    // `realloc` probably checks for `new_size > old_size` or something similar.
+                    // `new_size` must be greater than or equal to `old_size` due to the safety constraint,
+                    // and `new_size` == `old_size` was caught before
+                    intrinsics::assume(new_size > old_size);
+                    let ptr =
+                        NonNull::new(realloc(ptr.as_ptr(), layout, new_size)).ok_or(AllocErr)?;
+                    let new_layout = Layout::from_size_align_unchecked(new_size, layout.align());
+                    init.initialize_offset(ptr, new_layout, old_size);
+                    Ok((ptr, new_size))
+                }
             }
-            (_, _) => NonNull::new(realloc(ptr.as_ptr(), layout, new_size))
-                .ok_or(AllocErr)
-                .map(|p| (p, new_size)),
+            ReallocPlacement::InPlace => Err(AllocErr),
         }
     }
 
     #[inline]
-    fn alloc_zeroed(&mut self, layout: Layout) -> Result<(NonNull<u8>, usize), AllocErr> {
-        if layout.size() == 0 {
-            Ok((layout.dangling(), 0))
-        } else {
-            unsafe {
-                NonNull::new(alloc_zeroed(layout)).ok_or(AllocErr).map(|p| (p, layout.size()))
+    unsafe fn shrink(
+        &mut self,
+        ptr: NonNull<u8>,
+        layout: Layout,
+        new_size: usize,
+        placement: ReallocPlacement,
+    ) -> Result<(NonNull<u8>, usize), AllocErr> {
+        let old_size = layout.size();
+        debug_assert!(
+            new_size <= old_size,
+            "`new_size` must be smaller than or equal to `layout.size()`"
+        );
+
+        if old_size == new_size {
+            return Ok((ptr, new_size));
+        }
+
+        match placement {
+            ReallocPlacement::MayMove => {
+                let ptr = if new_size == 0 {
+                    self.dealloc(ptr, layout);
+                    layout.dangling()
+                } else {
+                    // `realloc` probably checks for `new_size > old_size` or something similar.
+                    // `new_size` must be smaller than or equal to `old_size` due to the safety constraint,
+                    // and `new_size` == `old_size` was caught before
+                    intrinsics::assume(new_size < old_size);
+                    NonNull::new(realloc(ptr.as_ptr(), layout, new_size)).ok_or(AllocErr)?
+                };
+                Ok((ptr, new_size))
             }
+            ReallocPlacement::InPlace => Err(AllocErr),
         }
     }
 }
@@ -218,14 +270,10 @@ unsafe impl AllocRef for Global {
 #[lang = "exchange_malloc"]
 #[inline]
 unsafe fn exchange_malloc(size: usize, align: usize) -> *mut u8 {
-    if size == 0 {
-        align as *mut u8
-    } else {
-        let layout = Layout::from_size_align_unchecked(size, align);
-        match Global.alloc(layout) {
-            Ok((ptr, _)) => ptr.as_ptr(),
-            Err(_) => handle_alloc_error(layout),
-        }
+    let layout = Layout::from_size_align_unchecked(size, align);
+    match Global.alloc(layout, AllocInit::Uninitialized) {
+        Ok((ptr, _)) => ptr.as_ptr(),
+        Err(_) => handle_alloc_error(layout),
     }
 }
 
@@ -239,11 +287,8 @@ unsafe fn exchange_malloc(size: usize, align: usize) -> *mut u8 {
 pub(crate) unsafe fn box_free<T: ?Sized>(ptr: Unique<T>) {
     let size = size_of_val(ptr.as_ref());
     let align = min_align_of_val(ptr.as_ref());
-    // We do not allocate for Box<T> when T is ZST, so deallocation is also not necessary.
-    if size != 0 {
-        let layout = Layout::from_size_align_unchecked(size, align);
-        Global.dealloc(ptr.cast().into(), layout);
-    }
+    let layout = Layout::from_size_align_unchecked(size, align);
+    Global.dealloc(ptr.cast().into(), layout)
 }
 
 /// Abort on memory allocation error or failure.

src/liballoc/alloc/tests.rs

@@ -8,8 +8,9 @@ use test::Bencher;
 fn allocate_zeroed() {
     unsafe {
         let layout = Layout::from_size_align(1024, 1).unwrap();
-        let (ptr, _) =
-            Global.alloc_zeroed(layout.clone()).unwrap_or_else(|_| handle_alloc_error(layout));
+        let (ptr, _) = Global
+            .alloc(layout.clone(), AllocInit::Zeroed)
+            .unwrap_or_else(|_| handle_alloc_error(layout));
 
         let mut i = ptr.cast::<u8>().as_ptr();
         let end = i.add(layout.size());

src/liballoc/boxed.rs

@@ -146,7 +146,7 @@ use core::ptr::{self, NonNull, Unique};
 use core::slice;
 use core::task::{Context, Poll};
 
-use crate::alloc::{self, AllocRef, Global};
+use crate::alloc::{self, AllocInit, AllocRef, Global};
 use crate::raw_vec::RawVec;
 use crate::str::from_boxed_utf8_unchecked;
 use crate::vec::Vec;
@@ -196,14 +196,12 @@ impl<T> Box<T> {
     #[unstable(feature = "new_uninit", issue = "63291")]
     pub fn new_uninit() -> Box<mem::MaybeUninit<T>> {
         let layout = alloc::Layout::new::<mem::MaybeUninit<T>>();
-        unsafe {
-            let ptr = if layout.size() == 0 {
-                NonNull::dangling()
-            } else {
-                Global.alloc(layout).unwrap_or_else(|_| alloc::handle_alloc_error(layout)).0.cast()
-            };
-            Box::from_raw(ptr.as_ptr())
-        }
+        let ptr = Global
+            .alloc(layout, AllocInit::Uninitialized)
+            .unwrap_or_else(|_| alloc::handle_alloc_error(layout))
+            .0
+            .cast();
+        unsafe { Box::from_raw(ptr.as_ptr()) }
     }
 
     /// Constructs a new `Box` with uninitialized contents, with the memory
@@ -226,11 +224,13 @@ impl<T> Box<T> {
     /// [zeroed]: ../../std/mem/union.MaybeUninit.html#method.zeroed
     #[unstable(feature = "new_uninit", issue = "63291")]
     pub fn new_zeroed() -> Box<mem::MaybeUninit<T>> {
-        unsafe {
-            let mut uninit = Self::new_uninit();
-            ptr::write_bytes::<T>(uninit.as_mut_ptr(), 0, 1);
-            uninit
-        }
+        let layout = alloc::Layout::new::<mem::MaybeUninit<T>>();
+        let ptr = Global
+            .alloc(layout, AllocInit::Zeroed)
+            .unwrap_or_else(|_| alloc::handle_alloc_error(layout))
+            .0
+            .cast();
+        unsafe { Box::from_raw(ptr.as_ptr()) }
     }
 
     /// Constructs a new `Pin<Box<T>>`. If `T` does not implement `Unpin`, then
@@ -266,14 +266,12 @@ impl<T> Box<[T]> {
     #[unstable(feature = "new_uninit", issue = "63291")]
     pub fn new_uninit_slice(len: usize) -> Box<[mem::MaybeUninit<T>]> {
         let layout = alloc::Layout::array::<mem::MaybeUninit<T>>(len).unwrap();
-        unsafe {
-            let ptr = if layout.size() == 0 {
-                NonNull::dangling()
-            } else {
-                Global.alloc(layout).unwrap_or_else(|_| alloc::handle_alloc_error(layout)).0.cast()
-            };
-            Box::from_raw(slice::from_raw_parts_mut(ptr.as_ptr(), len))
-        }
+        let ptr = Global
+            .alloc(layout, AllocInit::Uninitialized)
+            .unwrap_or_else(|_| alloc::handle_alloc_error(layout))
+            .0
+            .cast();
+        unsafe { Box::from_raw(slice::from_raw_parts_mut(ptr.as_ptr(), len)) }
     }
 }
 
@@ -778,7 +776,7 @@ impl<T: Copy> From<&[T]> for Box<[T]> {
         let buf = RawVec::with_capacity(len);
         unsafe {
             ptr::copy_nonoverlapping(slice.as_ptr(), buf.ptr(), len);
-            buf.into_box()
+            buf.into_box().assume_init()
         }
     }
 }