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Auto merge of #103659 - clubby789:improve-partialord-derive, r=nagisa

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Special-case deriving `PartialOrd` for enums with dataless variants

I was able to get slightly better codegen by flipping the derived `PartialOrd` logic for two-variant enums.  I also tried to document the implementation of the derive macro to make the special-case logic a little clearer.
```rs
#[derive(PartialEq, PartialOrd)]
pub enum A<T> {
    A,
    B(T)
}
```
```diff
impl<T: ::core::cmp::PartialOrd> ::core::cmp::PartialOrd for A<T> {
   #[inline]
   fn partial_cmp(
       &self,
       other: &A<T>,
   ) -> ::core::option::Option<::core::cmp::Ordering> {
       let __self_tag = ::core::intrinsics::discriminant_value(self);
       let __arg1_tag = ::core::intrinsics::discriminant_value(other);
-      match ::core::cmp::PartialOrd::partial_cmp(&__self_tag, &__arg1_tag) {
-          ::core::option::Option::Some(::core::cmp::Ordering::Equal) => {
-              match (self, other) {
-                  (A::B(__self_0), A::B(__arg1_0)) => {
-                      ::core::cmp::PartialOrd::partial_cmp(__self_0, __arg1_0)
-                  }
-                  _ => ::core::option::Option::Some(::core::cmp::Ordering::Equal),
-              }
+      match (self, other) {
+          (A::B(__self_0), A::B(__arg1_0)) => {
+              ::core::cmp::PartialOrd::partial_cmp(__self_0, __arg1_0)
           }
-          cmp => cmp,
+          _ => ::core::cmp::PartialOrd::partial_cmp(&__self_tag, &__arg1_tag),
       }
   }
}
```
Godbolt: [Current](https://godbolt.org/z/GYjEzG1T8), [New](https://godbolt.org/z/GoK78qx15)
I'm not sure how common a case comparing two enums like this (such as `Option`) is, and if it's worth the slowdown of adding a special case to the derive. If it causes overall regressions it might be worth just manually implementing this for `Option`.
This commit is contained in:
bors 2023-01-28 22:11:11 +00:00
commit 9f82651a5f
2 changed files with 96 additions and 37 deletions
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82
compiler/rustc_builtin_macros/src/deriving/cmp/partial_ord.rs
View File

@ -1,7 +1,7 @@
use crate::deriving::generic::ty::*;
use crate::deriving::generic::*;
use crate::deriving::{path_std, pathvec_std};
use rustc_ast::MetaItem;
use rustc_ast::{ExprKind, ItemKind, MetaItem, PatKind};
use rustc_expand::base::{Annotatable, ExtCtxt};
use rustc_span::symbol::{sym, Ident};
use rustc_span::Span;
@ -21,6 +21,27 @@ pub fn expand_deriving_partial_ord(
let attrs = thin_vec![cx.attr_word(sym::inline, span)];
// Order in which to perform matching
let tag_then_data = if let Annotatable::Item(item) = item
&& let ItemKind::Enum(def, _) = &item.kind {
let dataful: Vec<bool> = def.variants.iter().map(|v| !v.data.fields().is_empty()).collect();
match dataful.iter().filter(|&&b| b).count() {
// No data, placing the tag check first makes codegen simpler
0 => true,
1..=2 => false,
_ => {
(0..dataful.len()-1).any(|i| {
if dataful[i] && let Some(idx) = dataful[i+1..].iter().position(|v| *v) {
idx >= 2
} else {
false
}
})
}
}
} else {
true
};
let partial_cmp_def = MethodDef {
name: sym::partial_cmp,
generics: Bounds::empty(),
@ -30,7 +51,7 @@ pub fn expand_deriving_partial_ord(
attributes: attrs,
fieldless_variants_strategy: FieldlessVariantsStrategy::Unify,
combine_substructure: combine_substructure(Box::new(|cx, span, substr| {
cs_partial_cmp(cx, span, substr)
cs_partial_cmp(cx, span, substr, tag_then_data)
})),
};
@ -47,7 +68,12 @@ pub fn expand_deriving_partial_ord(
trait_def.expand(cx, mitem, item, push)
}
pub fn cs_partial_cmp(cx: &mut ExtCtxt<'_>, span: Span, substr: &Substructure<'_>) -> BlockOrExpr {
fn cs_partial_cmp(
cx: &mut ExtCtxt<'_>,
span: Span,
substr: &Substructure<'_>,
tag_then_data: bool,
) -> BlockOrExpr {
let test_id = Ident::new(sym::cmp, span);
let equal_path = cx.path_global(span, cx.std_path(&[sym::cmp, sym::Ordering, sym::Equal]));
let partial_cmp_path = cx.std_path(&[sym::cmp, sym::PartialOrd, sym::partial_cmp]);
@ -74,12 +100,50 @@ pub fn cs_partial_cmp(cx: &mut ExtCtxt<'_>, span: Span, substr: &Substructure<'_
let args = vec![field.self_expr.clone(), other_expr.clone()];
cx.expr_call_global(field.span, partial_cmp_path.clone(), args)
}
CsFold::Combine(span, expr1, expr2) => {
let eq_arm =
cx.arm(span, cx.pat_some(span, cx.pat_path(span, equal_path.clone())), expr1);
let neq_arm =
cx.arm(span, cx.pat_ident(span, test_id), cx.expr_ident(span, test_id));
cx.expr_match(span, expr2, vec![eq_arm, neq_arm])
CsFold::Combine(span, mut expr1, expr2) => {
// When the item is an enum, this expands to
// ```
// match (expr2) {
// Some(Ordering::Equal) => expr1,
// cmp => cmp
// }
// ```
// where `expr2` is `partial_cmp(self_tag, other_tag)`, and `expr1` is a `match`
// against the enum variants. This means that we begin by comparing the enum tags,
// before either inspecting their contents (if they match), or returning
// the `cmp::Ordering` of comparing the enum tags.
// ```
// match partial_cmp(self_tag, other_tag) {
// Some(Ordering::Equal) => match (self, other) {
// (Self::A(self_0), Self::A(other_0)) => partial_cmp(self_0, other_0),
// (Self::B(self_0), Self::B(other_0)) => partial_cmp(self_0, other_0),
// _ => Some(Ordering::Equal)
// }
// cmp => cmp
// }
// ```
// If we have any certain enum layouts, flipping this results in better codegen
// ```
// match (self, other) {
// (Self::A(self_0), Self::A(other_0)) => partial_cmp(self_0, other_0),
// _ => partial_cmp(self_tag, other_tag)
// }
// ```
// Reference: https://github.com/rust-lang/rust/pull/103659#issuecomment-1328126354
if !tag_then_data
&& let ExprKind::Match(_, arms) = &mut expr1.kind
&& let Some(last) = arms.last_mut()
&& let PatKind::Wild = last.pat.kind {
last.body = expr2;
expr1
} else {
let eq_arm =
cx.arm(span, cx.pat_some(span, cx.pat_path(span, equal_path.clone())), expr1);
let neq_arm =
cx.arm(span, cx.pat_ident(span, test_id), cx.expr_ident(span, test_id));
cx.expr_match(span, expr2, vec![eq_arm, neq_arm])
}
}
CsFold::Fieldless => cx.expr_some(span, cx.expr_path(equal_path.clone())),
},

51
tests/ui/deriving/deriving-all-codegen.stdout
View File

@ -889,23 +889,20 @@ impl ::core::cmp::PartialOrd for Mixed {
-> ::core::option::Option<::core::cmp::Ordering> {
let __self_tag = ::core::intrinsics::discriminant_value(self);
let __arg1_tag = ::core::intrinsics::discriminant_value(other);
match ::core::cmp::PartialOrd::partial_cmp(&__self_tag, &__arg1_tag) {
::core::option::Option::Some(::core::cmp::Ordering::Equal) =>
match (self, other) {
(Mixed::R(__self_0), Mixed::R(__arg1_0)) =>
::core::cmp::PartialOrd::partial_cmp(__self_0, __arg1_0),
(Mixed::S { d1: __self_0, d2: __self_1 }, Mixed::S {
d1: __arg1_0, d2: __arg1_1 }) =>
match ::core::cmp::PartialOrd::partial_cmp(__self_0,
__arg1_0) {
::core::option::Option::Some(::core::cmp::Ordering::Equal)
=> ::core::cmp::PartialOrd::partial_cmp(__self_1, __arg1_1),
cmp => cmp,
},
_ =>
::core::option::Option::Some(::core::cmp::Ordering::Equal),
match (self, other) {
(Mixed::R(__self_0), Mixed::R(__arg1_0)) =>
::core::cmp::PartialOrd::partial_cmp(__self_0, __arg1_0),
(Mixed::S { d1: __self_0, d2: __self_1 }, Mixed::S {
d1: __arg1_0, d2: __arg1_1 }) =>
match ::core::cmp::PartialOrd::partial_cmp(__self_0, __arg1_0)
{
::core::option::Option::Some(::core::cmp::Ordering::Equal)
=> ::core::cmp::PartialOrd::partial_cmp(__self_1, __arg1_1),
cmp => cmp,
},
cmp => cmp,
_ =>
::core::cmp::PartialOrd::partial_cmp(&__self_tag,
&__arg1_tag),
}
}
}
@ -1019,18 +1016,16 @@ impl ::core::cmp::PartialOrd for Fielded {
-> ::core::option::Option<::core::cmp::Ordering> {
let __self_tag = ::core::intrinsics::discriminant_value(self);
let __arg1_tag = ::core::intrinsics::discriminant_value(other);
match ::core::cmp::PartialOrd::partial_cmp(&__self_tag, &__arg1_tag) {
::core::option::Option::Some(::core::cmp::Ordering::Equal) =>
match (self, other) {
(Fielded::X(__self_0), Fielded::X(__arg1_0)) =>
::core::cmp::PartialOrd::partial_cmp(__self_0, __arg1_0),
(Fielded::Y(__self_0), Fielded::Y(__arg1_0)) =>
::core::cmp::PartialOrd::partial_cmp(__self_0, __arg1_0),
(Fielded::Z(__self_0), Fielded::Z(__arg1_0)) =>
::core::cmp::PartialOrd::partial_cmp(__self_0, __arg1_0),
_ => unsafe { ::core::intrinsics::unreachable() }
},
cmp => cmp,
match (self, other) {
(Fielded::X(__self_0), Fielded::X(__arg1_0)) =>
::core::cmp::PartialOrd::partial_cmp(__self_0, __arg1_0),
(Fielded::Y(__self_0), Fielded::Y(__arg1_0)) =>
::core::cmp::PartialOrd::partial_cmp(__self_0, __arg1_0),
(Fielded::Z(__self_0), Fielded::Z(__arg1_0)) =>
::core::cmp::PartialOrd::partial_cmp(__self_0, __arg1_0),
_ =>
::core::cmp::PartialOrd::partial_cmp(&__self_tag,
&__arg1_tag),
}
}
}