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Auto merge of #108157 - scottmcm:tuple-gt-via-partialcmp, r=dtolnay
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>.
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‎library/core/benches/lib.rs‎

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@@ -20,6 +20,7 @@ mod ops;
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mod pattern;
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mod slice;
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mod str;
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mod tuple;
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/// Returns a `rand::Rng` seeded with a consistent seed.
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///

‎library/core/benches/tuple.rs‎

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use rand::prelude::*;
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use test::{black_box, Bencher};
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#[bench]
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fn bench_tuple_comparison(b: &mut Bencher) {
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let mut rng = black_box(super::bench_rng());
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let data = black_box([
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("core::iter::adapters::Chain", 123_usize),
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("core::iter::adapters::Clone", 456_usize),
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("core::iter::adapters::Copie", 789_usize),
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("core::iter::adapters::Cycle", 123_usize),
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("core::iter::adapters::Flatt", 456_usize),
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("core::iter::adapters::TakeN", 789_usize),
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]);
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b.iter(|| {
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let x = data.choose(&mut rng).unwrap();
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let y = data.choose(&mut rng).unwrap();
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[x < y, x <= y, x > y, x >= y]
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});
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}

‎library/core/src/tuple.rs‎

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

‎tests/codegen/comparison-operators-2-tuple.rs‎

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// compile-flags: -C opt-level=1 -Z merge-functions=disabled
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// min-llvm-version: 15.0
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// only-x86_64
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#![crate_type = "lib"]
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use std::cmp::Ordering;
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type TwoTuple = (i16, u16);
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//
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// The operators are all overridden directly, so should optimize easily.
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//
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// Yes, the `s[lg]t` is correct for the `[lg]e` version because it's only used
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// in the side of the select where we know the values are *not* equal.
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//
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// CHECK-LABEL: @check_lt_direct
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// CHECK-SAME: (i16 noundef %[[A0:.+]], i16 noundef %[[A1:.+]], i16 noundef %[[B0:.+]], i16 noundef %[[B1:.+]])
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#[no_mangle]
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pub fn check_lt_direct(a: TwoTuple, b: TwoTuple) -> bool {
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// CHECK-DAG: %[[EQ:.+]] = icmp eq i16 %[[A0]], %[[B0]]
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// CHECK-DAG: %[[CMP0:.+]] = icmp slt i16 %[[A0]], %[[B0]]
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// CHECK-DAG: %[[CMP1:.+]] = icmp ult i16 %[[A1]], %[[B1]]
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// CHECK: %[[R:.+]] = select i1 %[[EQ]], i1 %[[CMP1]], i1 %[[CMP0]]
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// CHECK: ret i1 %[[R]]
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a < b
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}
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// CHECK-LABEL: @check_le_direct
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// CHECK-SAME: (i16 noundef %[[A0:.+]], i16 noundef %[[A1:.+]], i16 noundef %[[B0:.+]], i16 noundef %[[B1:.+]])
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#[no_mangle]
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pub fn check_le_direct(a: TwoTuple, b: TwoTuple) -> bool {
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// CHECK-DAG: %[[EQ:.+]] = icmp eq i16 %[[A0]], %[[B0]]
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// CHECK-DAG: %[[CMP0:.+]] = icmp slt i16 %[[A0]], %[[B0]]
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// CHECK-DAG: %[[CMP1:.+]] = icmp ule i16 %[[A1]], %[[B1]]
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// CHECK: %[[R:.+]] = select i1 %[[EQ]], i1 %[[CMP1]], i1 %[[CMP0]]
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// CHECK: ret i1 %[[R]]
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a <= b
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}
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// CHECK-LABEL: @check_gt_direct
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// CHECK-SAME: (i16 noundef %[[A0:.+]], i16 noundef %[[A1:.+]], i16 noundef %[[B0:.+]], i16 noundef %[[B1:.+]])
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#[no_mangle]
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pub fn check_gt_direct(a: TwoTuple, b: TwoTuple) -> bool {
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// CHECK-DAG: %[[EQ:.+]] = icmp eq i16 %[[A0]], %[[B0]]
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// CHECK-DAG: %[[CMP0:.+]] = icmp sgt i16 %[[A0]], %[[B0]]
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// CHECK-DAG: %[[CMP1:.+]] = icmp ugt i16 %[[A1]], %[[B1]]
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// CHECK: %[[R:.+]] = select i1 %[[EQ]], i1 %[[CMP1]], i1 %[[CMP0]]
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// CHECK: ret i1 %[[R]]
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a > b
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}
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// CHECK-LABEL: @check_ge_direct
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// CHECK-SAME: (i16 noundef %[[A0:.+]], i16 noundef %[[A1:.+]], i16 noundef %[[B0:.+]], i16 noundef %[[B1:.+]])
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#[no_mangle]
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pub fn check_ge_direct(a: TwoTuple, b: TwoTuple) -> bool {
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// CHECK-DAG: %[[EQ:.+]] = icmp eq i16 %[[A0]], %[[B0]]
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// CHECK-DAG: %[[CMP0:.+]] = icmp sgt i16 %[[A0]], %[[B0]]
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// CHECK-DAG: %[[CMP1:.+]] = icmp uge i16 %[[A1]], %[[B1]]
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// CHECK: %[[R:.+]] = select i1 %[[EQ]], i1 %[[CMP1]], i1 %[[CMP0]]
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// CHECK: ret i1 %[[R]]
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a >= b
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}
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//
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// These ones are harder, since there are more intermediate values to remove.
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//
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// `<` seems to be getting lucky right now, so test that doesn't regress.
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//
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// The others, however, aren't managing to optimize away the extra `select`s yet.
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// See <https://github.com/rust-lang/rust/issues/106107> for more about this.
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//
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// CHECK-LABEL: @check_lt_via_cmp
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// CHECK-SAME: (i16 noundef %[[A0:.+]], i16 noundef %[[A1:.+]], i16 noundef %[[B0:.+]], i16 noundef %[[B1:.+]])
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#[no_mangle]
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pub fn check_lt_via_cmp(a: TwoTuple, b: TwoTuple) -> bool {
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// CHECK-DAG: %[[EQ:.+]] = icmp eq i16 %[[A0]], %[[B0]]
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// CHECK-DAG: %[[CMP0:.+]] = icmp slt i16 %[[A0]], %[[B0]]
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// CHECK-DAG: %[[CMP1:.+]] = icmp ult i16 %[[A1]], %[[B1]]
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// CHECK: %[[R:.+]] = select i1 %[[EQ]], i1 %[[CMP1]], i1 %[[CMP0]]
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// CHECK: ret i1 %[[R]]
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Ord::cmp(&a, &b).is_lt()
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}
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// CHECK-LABEL: @check_le_via_cmp
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// CHECK-SAME: (i16 noundef %[[A0:.+]], i16 noundef %[[A1:.+]], i16 noundef %[[B0:.+]], i16 noundef %[[B1:.+]])
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#[no_mangle]
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pub fn check_le_via_cmp(a: TwoTuple, b: TwoTuple) -> bool {
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// FIXME-CHECK-DAG: %[[EQ:.+]] = icmp eq i16 %[[A0]], %[[B0]]
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// FIXME-CHECK-DAG: %[[CMP0:.+]] = icmp sle i16 %[[A0]], %[[B0]]
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// FIXME-CHECK-DAG: %[[CMP1:.+]] = icmp ule i16 %[[A1]], %[[B1]]
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// FIXME-CHECK: %[[R:.+]] = select i1 %[[EQ]], i1 %[[CMP1]], i1 %[[CMP0]]
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// FIXME-CHECK: ret i1 %[[R]]
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Ord::cmp(&a, &b).is_le()
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}
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// CHECK-LABEL: @check_gt_via_cmp
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// CHECK-SAME: (i16 noundef %[[A0:.+]], i16 noundef %[[A1:.+]], i16 noundef %[[B0:.+]], i16 noundef %[[B1:.+]])
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#[no_mangle]
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pub fn check_gt_via_cmp(a: TwoTuple, b: TwoTuple) -> bool {
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// FIXME-CHECK-DAG: %[[EQ:.+]] = icmp eq i16 %[[A0]], %[[B0]]
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// FIXME-CHECK-DAG: %[[CMP0:.+]] = icmp sgt i16 %[[A0]], %[[B0]]
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// FIXME-CHECK-DAG: %[[CMP1:.+]] = icmp ugt i16 %[[A1]], %[[B1]]
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// FIXME-CHECK: %[[R:.+]] = select i1 %[[EQ]], i1 %[[CMP1]], i1 %[[CMP0]]
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// FIXME-CHECK: ret i1 %[[R]]
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Ord::cmp(&a, &b).is_gt()
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}
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// CHECK-LABEL: @check_ge_via_cmp
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// CHECK-SAME: (i16 noundef %[[A0:.+]], i16 noundef %[[A1:.+]], i16 noundef %[[B0:.+]], i16 noundef %[[B1:.+]])
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#[no_mangle]
114+
pub fn check_ge_via_cmp(a: TwoTuple, b: TwoTuple) -> bool {
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// FIXME-CHECK-DAG: %[[EQ:.+]] = icmp eq i16 %[[A0]], %[[B0]]
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// FIXME-CHECK-DAG: %[[CMP0:.+]] = icmp sge i16 %[[A0]], %[[B0]]
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// FIXME-CHECK-DAG: %[[CMP1:.+]] = icmp uge i16 %[[A1]], %[[B1]]
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// FIXME-CHECK: %[[R:.+]] = select i1 %[[EQ]], i1 %[[CMP1]], i1 %[[CMP0]]
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// FIXME-CHECK: ret i1 %[[R]]
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Ord::cmp(&a, &b).is_ge()
121+
}

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