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Auto merge of #77257 - ecstatic-morse:optimize-int-range-from-pat, r=Mark-Simulacrum

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Optimize `IntRange::from_pat`, then shrink `ParamEnv`

Resolves #77058.

r? `@Mark-Simulacrum`
cc `@vandenheuvel`

Looking at the output of `perf report` for #76244, the hot instructions seemed to be around the call to `pat_constructor` in `IntRange::from_pat`. I carried out an obvious optimization, but it actually made the instruction count higher (see #77075). However, it seems to have mitigated whatever was causing the pipeline stalls, so when combined with #76244, it's a net win.

As you can see below, the regression in #76244 seems to have originated from something measured by `stalled-cycles-backend`. I'll try to collect some finer-grained stats to see if I can isolate it. I wish I had a better idea of what was going on here. I'd like to prevent the regression from reappearing in the future due to small changes in unrelated code.

<details>
<summary>Current `master`:</summary>

```
 Performance counter stats for 'cargo +baseline-stage1 check':

          2,275.67 msec task-clock:u              #    0.998 CPUs utilized
                 0      context-switches:u        #    0.000 K/sec
                 0      cpu-migrations:u          #    0.000 K/sec
            49,826      page-faults:u             #    0.022 M/sec
     5,117,221,678      cycles:u                  #    2.249 GHz
       299,655,943      stalled-cycles-frontend:u #    5.86% frontend cycles idle
     2,284,213,395      stalled-cycles-backend:u  #   44.64% backend cycles idle
     8,051,871,959      instructions:u            #    1.57  insn per cycle
                                                  #    0.28  stalled cycles per insn
     1,359,589,402      branches:u                #  597.447 M/sec
         7,359,347      branch-misses:u           #    0.54% of all branches

       2.281030026 seconds time elapsed

       2.108197000 seconds user
       0.164183000 seconds sys
```
</details>

<details>
<summary>Shrink `ParamEnv` without changing `IntRange::from_pat`:</summary>

```
 Performance counter stats for 'cargo +perf-stage1 check':

          2,751.79 msec task-clock:u              #    0.996 CPUs utilized
                 0      context-switches:u        #    0.000 K/sec
                 0      cpu-migrations:u          #    0.000 K/sec
            50,103      page-faults:u             #    0.018 M/sec
     6,260,590,019      cycles:u                  #    2.275 GHz
       317,355,920      stalled-cycles-frontend:u #    5.07% frontend cycles idle
     3,397,743,582      stalled-cycles-backend:u  #   54.27% backend cycles idle
     8,276,224,367      instructions:u            #    1.32  insn per cycle
                                                  #    0.41  stalled cycles per insn
     1,370,453,386      branches:u                #  498.023 M/sec
         7,281,031      branch-misses:u           #    0.53% of all branches

       2.763265838 seconds time elapsed

       2.544578000 seconds user
       0.204548000 seconds sys
```
</details>

<details>
<summary>Shrink `ParamEnv` and change `IntRange::from_pat`: </summary>

```
 Performance counter stats for 'cargo +perf-stage1 check':

          2,295.57 msec task-clock:u              #    0.996 CPUs utilized
                 0      context-switches:u        #    0.000 K/sec
                 0      cpu-migrations:u          #    0.000 K/sec
            49,959      page-faults:u             #    0.022 M/sec
     5,151,407,066      cycles:u                  #    2.244 GHz
       324,517,829      stalled-cycles-frontend:u #    6.30% frontend cycles idle
     2,301,671,001      stalled-cycles-backend:u  #   44.68% backend cycles idle
     8,130,868,329      instructions:u            #    1.58  insn per cycle
                                                  #    0.28  stalled cycles per insn
     1,356,618,512      branches:u                #  590.972 M/sec
         7,323,800      branch-misses:u           #    0.54% of all branches

       2.304509653 seconds time elapsed

       2.128090000 seconds user
       0.163909000 seconds sys
```
</details>
This commit is contained in:
bors 2020-09-29 02:29:13 +00:00
commit 48cab67447
4 changed files with 30 additions and 9 deletions
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5
compiler/rustc_middle/src/ty/mod.rs
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@ -1751,9 +1751,6 @@ 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 {
@ -1834,7 +1831,7 @@ 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), unused_field: None }
ty::ParamEnv { packed: CopyTaggedPtr::new(caller_bounds, reveal) }
}
pub fn with_user_facing(mut self) -> Self {