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Auto merge of #76913 - vandenheuvel:performance_debug, r=lcnr

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Fixing the performance regression of #76244

Issue https://github.com/rust-lang/rust/issues/74865 suggested that removing the `def_id` field from `ParamEnv` would improve performance. PR https://github.com/rust-lang/rust/pull/76244 implemented this change.

Generally, [results](https://perf.rust-lang.org/compare.html?start=80fc9b0ecb29050d45b17c64af004200afd3cfc2&end=5ef250dd2ad618ee339f165e9b711a1b4746887d) were as expected: an instruction count decrease of about a percent. The instruction count for the unicode crates increased by about 3%, which `@nnethercote` speculated to be caused by a quirk of inlining or codegen. As the results were generally positive, and for chalk integration, this was also a step in the right direction, the PR was r+'d regardless.

However, [wall-time performance results](https://perf.rust-lang.org/compare.html?start=a055c5a1bd95e029e9b31891db63b6dc8258b472&end=7402a394471a6738a40fea7d4f1891666e5a80c5&stat=task-clock) show a much larger performance degradation: 25%, as [mentioned](https://github.com/rust-lang/rust/pull/76244#issuecomment-694459840) by `@Mark-Simulacrum.`

This PR, for now, reverts #76244 and attempts to find out, which change caused the regression.
This commit is contained in:
bors 2020-09-22 00:22:24 +00:00
commit 45198456be
3 changed files with 6 additions and 3 deletions
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2
compiler/rustc_infer/src/traits/mod.rs
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@ -57,7 +57,7 @@ pub type TraitObligation<'tcx> = Obligation<'tcx, ty::PolyTraitPredicate<'tcx>>;
// `PredicateObligation` is used a lot. Make sure it doesn't unintentionally get bigger.
#[cfg(target_arch = "x86_64")]
static_assert_size!(PredicateObligation<'_>, 32);
static_assert_size!(PredicateObligation<'_>, 40);
pub type Obligations<'tcx, O> = Vec<Obligation<'tcx, O>>;
pub type PredicateObligations<'tcx> = Vec<PredicateObligation<'tcx>>;

5
compiler/rustc_middle/src/ty/mod.rs
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@ -1745,6 +1745,9 @@ pub struct ParamEnv<'tcx> {
///
/// Note: This is packed, use the reveal() method to access it.
packed: CopyTaggedPtr<&'tcx List<Predicate<'tcx>>, traits::Reveal, true>,
/// FIXME: This field is not used, but removing it causes a performance degradation. See #76913.
unused_field: Option<DefId>,
}
unsafe impl rustc_data_structures::tagged_ptr::Tag for traits::Reveal {
@ -1825,7 +1828,7 @@ impl<'tcx> ParamEnv<'tcx> {
/// Construct a trait environment with the given set of predicates.
#[inline]
pub fn new(caller_bounds: &'tcx List<Predicate<'tcx>>, reveal: Reveal) -> Self {
ty::ParamEnv { packed: CopyTaggedPtr::new(caller_bounds, reveal) }
ty::ParamEnv { packed: CopyTaggedPtr::new(caller_bounds, reveal), unused_field: None }
}
pub fn with_user_facing(mut self) -> Self {

2
compiler/rustc_trait_selection/src/traits/fulfill.rs
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@ -87,7 +87,7 @@ pub struct PendingPredicateObligation<'tcx> {
// `PendingPredicateObligation` is used a lot. Make sure it doesn't unintentionally get bigger.
#[cfg(target_arch = "x86_64")]
static_assert_size!(PendingPredicateObligation<'_>, 56);
static_assert_size!(PendingPredicateObligation<'_>, 64);
impl<'a, 'tcx> FulfillmentContext<'tcx> {
/// Creates a new fulfillment context.