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replace manual ptr arithmetic with ptr_sub
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@ -6,7 +6,7 @@ mod macros;
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use crate::cmp;
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use crate::cmp::Ordering;
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use crate::fmt;
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use crate::intrinsics::{assume, exact_div, unchecked_sub};
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use crate::intrinsics::assume;
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use crate::iter::{FusedIterator, TrustedLen, TrustedRandomAccess, TrustedRandomAccessNoCoerce};
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use crate::marker::{PhantomData, Send, Sized, Sync};
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use crate::mem::{self, SizedTypeProperties};
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@ -35,12 +35,6 @@ impl<'a, T> IntoIterator for &'a mut [T] {
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}
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}
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// Macro helper functions
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#[inline(always)]
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fn size_from_ptr<T>(_: *const T) -> usize {
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mem::size_of::<T>()
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}
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/// Immutable slice iterator
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///
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/// This struct is created by the [`iter`] method on [slices].
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@ -9,30 +9,20 @@ macro_rules! is_empty {
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};
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}
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// To get rid of some bounds checks (see `position`), we compute the length in a somewhat
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// unexpected way. (Tested by `codegen/slice-position-bounds-check`.)
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macro_rules! len {
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($self: ident) => {{
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#![allow(unused_unsafe)] // we're sometimes used within an unsafe block
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let start = $self.ptr;
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let size = size_from_ptr(start.as_ptr());
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if size == 0 {
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// This _cannot_ use `unchecked_sub` because we depend on wrapping
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if T::IS_ZST {
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// This _cannot_ use `ptr_sub` because we depend on wrapping
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// to represent the length of long ZST slice iterators.
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$self.end.addr().wrapping_sub(start.as_ptr().addr())
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} else {
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// We know that `start <= end`, so can do better than `offset_from`,
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// which needs to deal in signed. By setting appropriate flags here
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// we can tell LLVM this, which helps it remove bounds checks.
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// SAFETY: By the type invariant, `start <= end`
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let diff = unsafe { unchecked_sub($self.end.addr(), start.as_ptr().addr()) };
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// By also telling LLVM that the pointers are apart by an exact
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// multiple of the type size, it can optimize `len() == 0` down to
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// `start == end` instead of `(end - start) < size`.
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// SAFETY: By the type invariant, the pointers are aligned so the
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// distance between them must be a multiple of pointee size
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unsafe { exact_div(diff, size) }
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// To get rid of some bounds checks (see `position`), we use ptr_sub instead of
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// offset_from (Tested by `codegen/slice-position-bounds-check`.)
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// SAFETY: by the type invariant pointers are aligned and `start <= end`
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unsafe { $self.end.sub_ptr(start.as_ptr()) }
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}
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}};
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}
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@ -2,6 +2,7 @@
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// prevent optimizing away bounds checks
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// compile-flags: -O
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// ignore-debug: the debug assertions get in the way
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#![crate_type="rlib"]
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