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Auto merge of #80209 - erikdesjardins:ptrcmp, r=Mark-Simulacrum

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Remove pointer comparison from slice equality

This resurrects #71735.

Fixes #71602, helps with #80140.

r? `@Mark-Simulacrum`
This commit is contained in:
bors 2020-12-26 06:43:51 +00:00
commit 780b094d76
2 changed files with 16 additions and 27 deletions
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27
library/core/src/slice/cmp.rs
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@ -75,28 +75,6 @@ impl<A, B> SlicePartialEq<B> for [A]
} }
} }
// Use an equal-pointer optimization when types are `Eq`
// We can't make `A` and `B` the same type because `min_specialization` won't
// allow it.
impl<A, B> SlicePartialEq<B> for [A]
where
A: MarkerEq<B>,
{
default fn equal(&self, other: &[B]) -> bool {
if self.len() != other.len() {
return false;
}
// While performance would suffer if `guaranteed_eq` just returned `false`
// for all arguments, correctness and return value of this function are not affected.
if self.as_ptr().guaranteed_eq(other.as_ptr() as *const A) {
return true;
}
self.iter().zip(other.iter()).all(|(x, y)| x == y)
}
}
// Use memcmp for bytewise equality when the types allow // Use memcmp for bytewise equality when the types allow
impl<A, B> SlicePartialEq<B> for [A] impl<A, B> SlicePartialEq<B> for [A]
where where
@ -107,11 +85,6 @@ fn equal(&self, other: &[B]) -> bool {
return false; return false;
} }
// While performance would suffer if `guaranteed_eq` just returned `false`
// for all arguments, correctness and return value of this function are not affected.
if self.as_ptr().guaranteed_eq(other.as_ptr() as *const A) {
return true;
}
// SAFETY: `self` and `other` are references and are thus guaranteed to be valid. // SAFETY: `self` and `other` are references and are thus guaranteed to be valid.
// The two slices have been checked to have the same size above. // The two slices have been checked to have the same size above.
unsafe { unsafe {

16
src/test/codegen/slice-ref-equality.rs Normal file
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@ -0,0 +1,16 @@
// compile-flags: -C opt-level=3
#![crate_type = "lib"]
// #71602: check that slice equality just generates a single bcmp
// CHECK-LABEL: @is_zero_slice
#[no_mangle]
pub fn is_zero_slice(data: &[u8; 4]) -> bool {
// CHECK: start:
// CHECK-NEXT: %{{.+}} = getelementptr {{.+}}
// CHECK-NEXT: %[[BCMP:.+]] = tail call i32 @{{bcmp|memcmp}}({{.+}})
// CHECK-NEXT: %[[EQ:.+]] = icmp eq i32 %[[BCMP]], 0
// CHECK-NEXT: ret i1 %[[EQ]]
*data == [0; 4]
}