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Auto merge of #108157 - scottmcm:tuple-gt-via-partialcmp, r=dtolnay

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Use `partial_cmp` to implement tuple `lt`/`le`/`ge`/`gt`

In today's implementation, `(A, B)::gt` contains calls to *both* `A::eq` *and* `A::gt`.

That's fine for primitives, but for things like `String`s it's kinda weird -- `(String, usize)::gt` has a call to both `bcmp` and `memcmp` (<https://rust.godbolt.org/z/7jbbPMesf>) because when `bcmp` says the `String`s aren't equal, it turns around and calls `memcmp` to find out which one's bigger.

This PR changes the implementation to instead implement `(A, …, C, Z)::gt` using `A::partial_cmp`, `…::partial_cmp`, `C::partial_cmp`, and `Z::gt`.  (And analogously for `lt`, `le`, and `ge`.)  That way expensive comparisons don't need to be repeated.

Technically this is an observable change on stable, so I've marked it `needs-fcp` + `T-libs-api` and will
r? rust-lang/libs-api

I'm hoping that this will be non-controversial, however, since it's very similar to the observable changes that were made to the derives (#81384 #98655) -- like those, this only changes behaviour if a type overrode behaviour in a way inconsistent with the rules for the various traits involved.

(The first commit here is #108156, adding the codegen test, which I used to make sure this doesn't regress behaviour for primitives.)

Zulip conversation about this change: <https://rust-lang.zulipchat.com/#narrow/stream/219381-t-libs/topic/.60.3E.60.20on.20Tuples/near/328392927>.
This commit is contained in:
bors 2023-03-05 22:02:26 +00:00
commit 816f958ac3
4 changed files with 179 additions and 13 deletions
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1
library/core/benches/lib.rs
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@ -20,6 +20,7 @@ mod ops;
mod pattern; mod pattern;
mod slice; mod slice;
mod str; mod str;
mod tuple;
/// Returns a `rand::Rng` seeded with a consistent seed. /// Returns a `rand::Rng` seeded with a consistent seed.
/// ///

22
library/core/benches/tuple.rs Normal file
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@ -0,0 +1,22 @@
use rand::prelude::*;
use test::{black_box, Bencher};
#[bench]
fn bench_tuple_comparison(b: &mut Bencher) {
let mut rng = black_box(super::bench_rng());
let data = black_box([
("core::iter::adapters::Chain", 123_usize),
("core::iter::adapters::Clone", 456_usize),
("core::iter::adapters::Copie", 789_usize),
("core::iter::adapters::Cycle", 123_usize),
("core::iter::adapters::Flatt", 456_usize),
("core::iter::adapters::TakeN", 789_usize),
]);
b.iter(|| {
let x = data.choose(&mut rng).unwrap();
let y = data.choose(&mut rng).unwrap();
[x < y, x <= y, x > y, x >= y]
});
}

48
library/core/src/tuple.rs
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@ -1,7 +1,7 @@
// See src/libstd/primitive_docs.rs for documentation. // See src/libstd/primitive_docs.rs for documentation.
use crate::cmp::Ordering::*; use crate::cmp::Ordering::{self, *};
use crate::cmp::*; use crate::mem::transmute;
// Recursive macro for implementing n-ary tuple functions and operations // Recursive macro for implementing n-ary tuple functions and operations
// //
@ -61,19 +61,19 @@ macro_rules! tuple_impls {
} }
#[inline] #[inline]
fn lt(&self, other: &($($T,)+)) -> bool { fn lt(&self, other: &($($T,)+)) -> bool {
lexical_ord!(lt, $( ${ignore(T)} self.${index()}, other.${index()} ),+) lexical_ord!(lt, Less, $( ${ignore(T)} self.${index()}, other.${index()} ),+)
} }
#[inline] #[inline]
fn le(&self, other: &($($T,)+)) -> bool { fn le(&self, other: &($($T,)+)) -> bool {
lexical_ord!(le, $( ${ignore(T)} self.${index()}, other.${index()} ),+) lexical_ord!(le, Less, $( ${ignore(T)} self.${index()}, other.${index()} ),+)
} }
#[inline] #[inline]
fn ge(&self, other: &($($T,)+)) -> bool { fn ge(&self, other: &($($T,)+)) -> bool {
lexical_ord!(ge, $( ${ignore(T)} self.${index()}, other.${index()} ),+) lexical_ord!(ge, Greater, $( ${ignore(T)} self.${index()}, other.${index()} ),+)
} }
#[inline] #[inline]
fn gt(&self, other: &($($T,)+)) -> bool { fn gt(&self, other: &($($T,)+)) -> bool {
lexical_ord!(gt, $( ${ignore(T)} self.${index()}, other.${index()} ),+) lexical_ord!(gt, Greater, $( ${ignore(T)} self.${index()}, other.${index()} ),+)
} }
} }
} }
@ -123,16 +123,38 @@ macro_rules! maybe_tuple_doc {
}; };
} }
// Constructs an expression that performs a lexical ordering using method $rel. #[inline]
const fn ordering_is_some(c: Option<Ordering>, x: Ordering) -> bool {
// FIXME: Just use `==` once that's const-stable on `Option`s.
// This isn't using `match` because that optimizes worse due to
// making a two-step check (`Some` *then* the inner value).
// SAFETY: There's no public guarantee for `Option<Ordering>`,
// but we're core so we know that it's definitely a byte.
unsafe {
let c: i8 = transmute(c);
let x: i8 = transmute(Some(x));
c == x
}
}
// Constructs an expression that performs a lexical ordering using method `$rel`.
// The values are interleaved, so the macro invocation for // The values are interleaved, so the macro invocation for
// `(a1, a2, a3) < (b1, b2, b3)` would be `lexical_ord!(lt, a1, b1, a2, b2, // `(a1, a2, a3) < (b1, b2, b3)` would be `lexical_ord!(lt, opt_is_lt, a1, b1,
// a3, b3)` (and similarly for `lexical_cmp`) // a2, b2, a3, b3)` (and similarly for `lexical_cmp`)
//
// `$ne_rel` is only used to determine the result after checking that they're
// not equal, so `lt` and `le` can both just use `Less`.
macro_rules! lexical_ord { macro_rules! lexical_ord {
($rel: ident, $a:expr, $b:expr, $($rest_a:expr, $rest_b:expr),+) => { ($rel: ident, $ne_rel: ident, $a:expr, $b:expr, $($rest_a:expr, $rest_b:expr),+) => {{
if $a != $b { lexical_ord!($rel, $a, $b) } let c = PartialOrd::partial_cmp(&$a, &$b);
else { lexical_ord!($rel, $($rest_a, $rest_b),+) } if !ordering_is_some(c, Equal) { ordering_is_some(c, $ne_rel) }
else { lexical_ord!($rel, $ne_rel, $($rest_a, $rest_b),+) }
}};
($rel: ident, $ne_rel: ident, $a:expr, $b:expr) => {
// Use the specific method for the last element
PartialOrd::$rel(&$a, &$b)
}; };
($rel: ident, $a:expr, $b:expr) => { ($a) . $rel (& $b) };
} }
macro_rules! lexical_partial_cmp { macro_rules! lexical_partial_cmp {

121
tests/codegen/comparison-operators-2-tuple.rs Normal file
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@ -0,0 +1,121 @@
// compile-flags: -C opt-level=1 -Z merge-functions=disabled
// min-llvm-version: 15.0
// only-x86_64
#![crate_type = "lib"]
use std::cmp::Ordering;
type TwoTuple = (i16, u16);
//
// The operators are all overridden directly, so should optimize easily.
//
// Yes, the `s[lg]t` is correct for the `[lg]e` version because it's only used
// in the side of the select where we know the values are *not* equal.
//
// CHECK-LABEL: @check_lt_direct
// CHECK-SAME: (i16 noundef %[[A0:.+]], i16 noundef %[[A1:.+]], i16 noundef %[[B0:.+]], i16 noundef %[[B1:.+]])
#[no_mangle]
pub fn check_lt_direct(a: TwoTuple, b: TwoTuple) -> bool {
// CHECK-DAG: %[[EQ:.+]] = icmp eq i16 %[[A0]], %[[B0]]
// CHECK-DAG: %[[CMP0:.+]] = icmp slt i16 %[[A0]], %[[B0]]
// CHECK-DAG: %[[CMP1:.+]] = icmp ult i16 %[[A1]], %[[B1]]
// CHECK: %[[R:.+]] = select i1 %[[EQ]], i1 %[[CMP1]], i1 %[[CMP0]]
// CHECK: ret i1 %[[R]]
a < b
}
// CHECK-LABEL: @check_le_direct
// CHECK-SAME: (i16 noundef %[[A0:.+]], i16 noundef %[[A1:.+]], i16 noundef %[[B0:.+]], i16 noundef %[[B1:.+]])
#[no_mangle]
pub fn check_le_direct(a: TwoTuple, b: TwoTuple) -> bool {
// CHECK-DAG: %[[EQ:.+]] = icmp eq i16 %[[A0]], %[[B0]]
// CHECK-DAG: %[[CMP0:.+]] = icmp slt i16 %[[A0]], %[[B0]]
// CHECK-DAG: %[[CMP1:.+]] = icmp ule i16 %[[A1]], %[[B1]]
// CHECK: %[[R:.+]] = select i1 %[[EQ]], i1 %[[CMP1]], i1 %[[CMP0]]
// CHECK: ret i1 %[[R]]
a <= b
}
// CHECK-LABEL: @check_gt_direct
// CHECK-SAME: (i16 noundef %[[A0:.+]], i16 noundef %[[A1:.+]], i16 noundef %[[B0:.+]], i16 noundef %[[B1:.+]])
#[no_mangle]
pub fn check_gt_direct(a: TwoTuple, b: TwoTuple) -> bool {
// CHECK-DAG: %[[EQ:.+]] = icmp eq i16 %[[A0]], %[[B0]]
// CHECK-DAG: %[[CMP0:.+]] = icmp sgt i16 %[[A0]], %[[B0]]
// CHECK-DAG: %[[CMP1:.+]] = icmp ugt i16 %[[A1]], %[[B1]]
// CHECK: %[[R:.+]] = select i1 %[[EQ]], i1 %[[CMP1]], i1 %[[CMP0]]
// CHECK: ret i1 %[[R]]
a > b
}
// CHECK-LABEL: @check_ge_direct
// CHECK-SAME: (i16 noundef %[[A0:.+]], i16 noundef %[[A1:.+]], i16 noundef %[[B0:.+]], i16 noundef %[[B1:.+]])
#[no_mangle]
pub fn check_ge_direct(a: TwoTuple, b: TwoTuple) -> bool {
// CHECK-DAG: %[[EQ:.+]] = icmp eq i16 %[[A0]], %[[B0]]
// CHECK-DAG: %[[CMP0:.+]] = icmp sgt i16 %[[A0]], %[[B0]]
// CHECK-DAG: %[[CMP1:.+]] = icmp uge i16 %[[A1]], %[[B1]]
// CHECK: %[[R:.+]] = select i1 %[[EQ]], i1 %[[CMP1]], i1 %[[CMP0]]
// CHECK: ret i1 %[[R]]
a >= b
}
//
// These ones are harder, since there are more intermediate values to remove.
//
// `<` seems to be getting lucky right now, so test that doesn't regress.
//
// The others, however, aren't managing to optimize away the extra `select`s yet.
// See <https://github.com/rust-lang/rust/issues/106107> for more about this.
//
// CHECK-LABEL: @check_lt_via_cmp
// CHECK-SAME: (i16 noundef %[[A0:.+]], i16 noundef %[[A1:.+]], i16 noundef %[[B0:.+]], i16 noundef %[[B1:.+]])
#[no_mangle]
pub fn check_lt_via_cmp(a: TwoTuple, b: TwoTuple) -> bool {
// CHECK-DAG: %[[EQ:.+]] = icmp eq i16 %[[A0]], %[[B0]]
// CHECK-DAG: %[[CMP0:.+]] = icmp slt i16 %[[A0]], %[[B0]]
// CHECK-DAG: %[[CMP1:.+]] = icmp ult i16 %[[A1]], %[[B1]]
// CHECK: %[[R:.+]] = select i1 %[[EQ]], i1 %[[CMP1]], i1 %[[CMP0]]
// CHECK: ret i1 %[[R]]
Ord::cmp(&a, &b).is_lt()
}
// CHECK-LABEL: @check_le_via_cmp
// CHECK-SAME: (i16 noundef %[[A0:.+]], i16 noundef %[[A1:.+]], i16 noundef %[[B0:.+]], i16 noundef %[[B1:.+]])
#[no_mangle]
pub fn check_le_via_cmp(a: TwoTuple, b: TwoTuple) -> bool {
// FIXME-CHECK-DAG: %[[EQ:.+]] = icmp eq i16 %[[A0]], %[[B0]]
// FIXME-CHECK-DAG: %[[CMP0:.+]] = icmp sle i16 %[[A0]], %[[B0]]
// FIXME-CHECK-DAG: %[[CMP1:.+]] = icmp ule i16 %[[A1]], %[[B1]]
// FIXME-CHECK: %[[R:.+]] = select i1 %[[EQ]], i1 %[[CMP1]], i1 %[[CMP0]]
// FIXME-CHECK: ret i1 %[[R]]
Ord::cmp(&a, &b).is_le()
}
// CHECK-LABEL: @check_gt_via_cmp
// CHECK-SAME: (i16 noundef %[[A0:.+]], i16 noundef %[[A1:.+]], i16 noundef %[[B0:.+]], i16 noundef %[[B1:.+]])
#[no_mangle]
pub fn check_gt_via_cmp(a: TwoTuple, b: TwoTuple) -> bool {
// FIXME-CHECK-DAG: %[[EQ:.+]] = icmp eq i16 %[[A0]], %[[B0]]
// FIXME-CHECK-DAG: %[[CMP0:.+]] = icmp sgt i16 %[[A0]], %[[B0]]
// FIXME-CHECK-DAG: %[[CMP1:.+]] = icmp ugt i16 %[[A1]], %[[B1]]
// FIXME-CHECK: %[[R:.+]] = select i1 %[[EQ]], i1 %[[CMP1]], i1 %[[CMP0]]
// FIXME-CHECK: ret i1 %[[R]]
Ord::cmp(&a, &b).is_gt()
}
// CHECK-LABEL: @check_ge_via_cmp
// CHECK-SAME: (i16 noundef %[[A0:.+]], i16 noundef %[[A1:.+]], i16 noundef %[[B0:.+]], i16 noundef %[[B1:.+]])
#[no_mangle]
pub fn check_ge_via_cmp(a: TwoTuple, b: TwoTuple) -> bool {
// FIXME-CHECK-DAG: %[[EQ:.+]] = icmp eq i16 %[[A0]], %[[B0]]
// FIXME-CHECK-DAG: %[[CMP0:.+]] = icmp sge i16 %[[A0]], %[[B0]]
// FIXME-CHECK-DAG: %[[CMP1:.+]] = icmp uge i16 %[[A1]], %[[B1]]
// FIXME-CHECK: %[[R:.+]] = select i1 %[[EQ]], i1 %[[CMP1]], i1 %[[CMP0]]
// FIXME-CHECK: ret i1 %[[R]]
Ord::cmp(&a, &b).is_ge()
}