Auto merge of rust-lang#115515 - the8472:zip-for-arrays, r=scottmcm

optimize zipping over array iterators

Fixes rust-lang#115339 (somewhat)

the new assembly:

```asm
zip_arrays:
        .cfi_startproc
        vmovups (%rdx), %ymm0
        leaq    32(%rsi), %rcx
        vxorps  %xmm1, %xmm1, %xmm1
        vmovups %xmm1, -24(%rsp)
        movq    $0, -8(%rsp)
        movq    %rsi, -88(%rsp)
        movq    %rdi, %rax
        movq    %rcx, -80(%rsp)
        vmovups %ymm0, -72(%rsp)
        movq    $0, -40(%rsp)
        movq    $32, -32(%rsp)
        movq    -24(%rsp), %rcx
        vmovups (%rsi,%rcx), %ymm0
        vorps   -72(%rsp,%rcx), %ymm0, %ymm0
        vmovups %ymm0, (%rsi,%rcx)
        vmovups (%rsi), %ymm0
        vmovups %ymm0, (%rdi)
        vzeroupper
        retq
```

This is still longer than the slice version given in the issue but at least it eliminates the terrible  `vpextrb`/`orb` chain. I guess this is due to excessive memcpys again (haven't looked at the llvmir)?

The `TrustedLen` specialization is a drive-by change since I had to do something for the default impl anyway to be able to specialize the `TrustedRandomAccessNoCoerce` impl.

Author: bors

library/core/src/array/iter.rs

@@ -4,7 +4,7 @@ use crate::num::NonZeroUsize;
 use crate::{
     fmt,
     intrinsics::transmute_unchecked,
-    iter::{self, ExactSizeIterator, FusedIterator, TrustedLen},
+    iter::{self, ExactSizeIterator, FusedIterator, TrustedLen, TrustedRandomAccessNoCoerce},
     mem::MaybeUninit,
     ops::{IndexRange, Range},
     ptr,
@@ -294,6 +294,12 @@ impl<T, const N: usize> Iterator for IntoIter<T, N> {
 
         NonZeroUsize::new(remaining).map_or(Ok(()), Err)
     }
+
+    #[inline]
+    unsafe fn __iterator_get_unchecked(&mut self, idx: usize) -> Self::Item {
+        // SAFETY: The caller must provide an idx that is in bound of the remainder.
+        unsafe { self.data.as_ptr().add(self.alive.start()).add(idx).cast::<T>().read() }
+    }
 }
 
 #[stable(feature = "array_value_iter_impls", since = "1.40.0")]
@@ -375,6 +381,25 @@ impl<T, const N: usize> FusedIterator for IntoIter<T, N> {}
 #[stable(feature = "array_value_iter_impls", since = "1.40.0")]
 unsafe impl<T, const N: usize> TrustedLen for IntoIter<T, N> {}
 
+#[doc(hidden)]
+#[unstable(issue = "none", feature = "std_internals")]
+#[rustc_unsafe_specialization_marker]
+pub trait NonDrop {}
+
+// T: Copy as approximation for !Drop since get_unchecked does not advance self.alive
+// and thus we can't implement drop-handling
+#[unstable(issue = "none", feature = "std_internals")]
+impl<T: Copy> NonDrop for T {}
+
+#[doc(hidden)]
+#[unstable(issue = "none", feature = "std_internals")]
+unsafe impl<T, const N: usize> TrustedRandomAccessNoCoerce for IntoIter<T, N>
+where
+    T: NonDrop,
+{
+    const MAY_HAVE_SIDE_EFFECT: bool = false;
+}
+
 #[stable(feature = "array_value_iter_impls", since = "1.40.0")]
 impl<T: Clone, const N: usize> Clone for IntoIter<T, N> {
     fn clone(&self) -> Self {

library/core/src/iter/adapters/zip.rs

@@ -94,6 +94,14 @@ where
         ZipImpl::nth(self, n)
     }
 
+    #[inline]
+    fn fold<Acc, F>(self, init: Acc, f: F) -> Acc
+    where
+        F: FnMut(Acc, Self::Item) -> Acc,
+    {
+        ZipImpl::fold(self, init, f)
+    }
+
     #[inline]
     unsafe fn __iterator_get_unchecked(&mut self, idx: usize) -> Self::Item
     where
@@ -129,6 +137,9 @@ trait ZipImpl<A, B> {
     where
         A: DoubleEndedIterator + ExactSizeIterator,
         B: DoubleEndedIterator + ExactSizeIterator;
+    fn fold<Acc, F>(self, init: Acc, f: F) -> Acc
+    where
+        F: FnMut(Acc, Self::Item) -> Acc;
     // This has the same safety requirements as `Iterator::__iterator_get_unchecked`
     unsafe fn get_unchecked(&mut self, idx: usize) -> <Self as Iterator>::Item
     where
@@ -228,6 +239,14 @@ where
     {
         unreachable!("Always specialized");
     }
+
+    #[inline]
+    default fn fold<Acc, F>(self, init: Acc, f: F) -> Acc
+    where
+        F: FnMut(Acc, Self::Item) -> Acc,
+    {
+        SpecFold::spec_fold(self, init, f)
+    }
 }
 
 #[doc(hidden)]
@@ -251,6 +270,24 @@ where
         // `Iterator::__iterator_get_unchecked`.
         unsafe { (self.a.__iterator_get_unchecked(idx), self.b.__iterator_get_unchecked(idx)) }
     }
+
+    #[inline]
+    fn fold<Acc, F>(mut self, init: Acc, mut f: F) -> Acc
+    where
+        F: FnMut(Acc, Self::Item) -> Acc,
+    {
+        let mut accum = init;
+        let len = ZipImpl::size_hint(&self).0;
+        for i in 0..len {
+            // SAFETY: since Self: TrustedRandomAccessNoCoerce we can trust the size-hint to
+            // calculate the length and then use that to do unchecked iteration.
+            // fold consumes the iterator so we don't need to fixup any state.
+            unsafe {
+                accum = f(accum, self.get_unchecked(i));
+            }
+        }
+        accum
+    }
 }
 
 #[doc(hidden)]
@@ -590,3 +627,56 @@ unsafe impl<I: Iterator + TrustedRandomAccessNoCoerce> SpecTrustedRandomAccess f
         unsafe { self.__iterator_get_unchecked(index) }
     }
 }
+
+trait SpecFold: Iterator {
+    fn spec_fold<B, F>(self, init: B, f: F) -> B
+    where
+        Self: Sized,
+        F: FnMut(B, Self::Item) -> B;
+}
+
+impl<A: Iterator, B: Iterator> SpecFold for Zip<A, B> {
+    // Adapted from default impl from the Iterator trait
+    #[inline]
+    default fn spec_fold<Acc, F>(mut self, init: Acc, mut f: F) -> Acc
+    where
+        F: FnMut(Acc, Self::Item) -> Acc,
+    {
+        let mut accum = init;
+        while let Some(x) = ZipImpl::next(&mut self) {
+            accum = f(accum, x);
+        }
+        accum
+    }
+}
+
+impl<A: TrustedLen, B: TrustedLen> SpecFold for Zip<A, B> {
+    #[inline]
+    fn spec_fold<Acc, F>(mut self, init: Acc, mut f: F) -> Acc
+    where
+        F: FnMut(Acc, Self::Item) -> Acc,
+    {
+        let mut accum = init;
+        loop {
+            let (upper, more) = if let Some(upper) = ZipImpl::size_hint(&self).1 {
+                (upper, false)
+            } else {
+                // Per TrustedLen contract a None upper bound means more than usize::MAX items
+                (usize::MAX, true)
+            };
+
+            for _ in 0..upper {
+                let pair =
+                    // SAFETY: TrustedLen guarantees that at least `upper` many items are available
+                    // therefore we know they can't be None
+                    unsafe { (self.a.next().unwrap_unchecked(), self.b.next().unwrap_unchecked()) };
+                accum = f(accum, pair);
+            }
+
+            if !more {
+                break;
+            }
+        }
+        accum
+    }
+}

library/core/tests/iter/adapters/zip.rs

@@ -184,7 +184,11 @@ fn test_zip_nested_sideffectful() {
         let it = xs.iter_mut().map(|x| *x = 1).enumerate().zip(&ys);
         it.count();
     }
-    assert_eq!(&xs, &[1, 1, 1, 1, 1, 0]);
+    let length_aware =  &xs == &[1, 1, 1, 1, 0, 0];
+    let probe_first = &xs == &[1, 1, 1, 1, 1, 0];
+
+    // either implementation is valid according to zip documentation
+    assert!(length_aware || probe_first);
 }
 
 #[test]

tests/assembly/libs/issue-115339-zip-arrays.rs

@@ -0,0 +1,25 @@
+// assembly-output: emit-asm
+// # zen3 previously exhibited odd vectorization
+// compile-flags: --crate-type=lib -Ctarget-cpu=znver3 -O
+// only-x86_64
+// ignore-sgx
+
+use std::iter;
+
+// previously this produced a long chain of
+// 56:  vpextrb $6, %xmm0, %ecx
+// 57:  orb %cl, 22(%rsi)
+// 58:  vpextrb $7, %xmm0, %ecx
+// 59:  orb %cl, 23(%rsi)
+// [...]
+
+// CHECK-LABEL: zip_arrays:
+#[no_mangle]
+pub fn zip_arrays(mut a: [u8; 32], b: [u8; 32]) -> [u8; 32] {
+    // CHECK-NOT: vpextrb
+    // CHECK-NOT: orb %cl
+    // CHECK: vorps
+    iter::zip(&mut a, b).for_each(|(a, b)| *a |= b);
+    // CHECK: retq
+    a
+}