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Auto merge of #75621 - TimDiekmann:no-fast-realloc, r=Amanieu

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Remove fast path in reallocation for same layout sizes

r? @Amanieu

Before merging a perf-run should be done.

Closes https://github.com/rust-lang/wg-allocators/issues/70
This commit is contained in:
bors 2020-08-18 05:42:05 +00:00
commit 515c9fa505
3 changed files with 20 additions and 52 deletions
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27
library/std/src/alloc.rs
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@ -181,16 +181,14 @@ unsafe impl AllocRef for System {
);
// SAFETY: `new_size` must be non-zero, which is checked in the match expression.
// If `new_size` is zero, then `old_size` has to be zero as well.
// Other conditions must be upheld by the caller
unsafe {
match layout.size() {
old_size if old_size == new_size => {
Ok(NonNull::slice_from_raw_parts(ptr, new_size))
}
0 => self.alloc(Layout::from_size_align_unchecked(new_size, layout.align())),
old_size => {
// `realloc` probably checks for `new_size > size` or something similar.
intrinsics::assume(new_size > old_size);
// `realloc` probably checks for `new_size >= size` or something similar.
intrinsics::assume(new_size >= old_size);
let raw_ptr = GlobalAlloc::realloc(&System, ptr.as_ptr(), layout, new_size);
let ptr = NonNull::new(raw_ptr).ok_or(AllocErr)?;
Ok(NonNull::slice_from_raw_parts(ptr, new_size))
@ -212,16 +210,14 @@ unsafe impl AllocRef for System {
);
// SAFETY: `new_size` must be non-zero, which is checked in the match expression.
// If `new_size` is zero, then `old_size` has to be zero as well.
// Other conditions must be upheld by the caller
unsafe {
match layout.size() {
old_size if old_size == new_size => {
Ok(NonNull::slice_from_raw_parts(ptr, new_size))
}
0 => self.alloc_zeroed(Layout::from_size_align_unchecked(new_size, layout.align())),
old_size => {
// `realloc` probably checks for `new_size > size` or something similar.
intrinsics::assume(new_size > old_size);
// `realloc` probably checks for `new_size >= size` or something similar.
intrinsics::assume(new_size >= old_size);
let raw_ptr = GlobalAlloc::realloc(&System, ptr.as_ptr(), layout, new_size);
raw_ptr.add(old_size).write_bytes(0, new_size - old_size);
let ptr = NonNull::new(raw_ptr).ok_or(AllocErr)?;
@ -244,11 +240,8 @@ unsafe impl AllocRef for System {
"`new_size` must be smaller than or equal to `layout.size()`"
);
let ptr = if new_size == old_size {
ptr
} else if new_size == 0 {
// SAFETY: `layout` is non-zero in size as `old_size` != `new_size`
// Other conditions must be upheld by the caller
let ptr = if new_size == 0 {
// SAFETY: conditions must be upheld by the caller
unsafe {
self.dealloc(ptr, layout);
}
@ -257,8 +250,8 @@ unsafe impl AllocRef for System {
// SAFETY: new_size is not zero,
// Other conditions must be upheld by the caller
let raw_ptr = unsafe {
// `realloc` probably checks for `new_size < old_size` or something similar.
intrinsics::assume(new_size < old_size);
// `realloc` probably checks for `new_size <= old_size` or something similar.
intrinsics::assume(new_size <= old_size);
GlobalAlloc::realloc(&System, ptr.as_ptr(), layout, new_size)
};
NonNull::new(raw_ptr).ok_or(AllocErr)?