Auto merge of rust-lang#3194 - saethlin:remove-gc-heuristics, r=RalfJung

Remove Stacked Borrows GC heuristics

Removing these has no impact on our benchmarks. I think I initially added these because they have a significant impact on runtime at shorter GC intervals. But both these heuristics result in undesirable memory growth in real programs, especially `modified_since_last_gc`. I didn't realize at the time that required state becomes garbage as a result of changes to _other_ allocations.

I think this nets even primarily because we get better heap reuse. With this change I see almost all the mmap calls coming from our diagnostics infrastructure go away. Not that there were many to start with, but it's an indicator that our memory locality has improved.

Before:
```
Benchmark 1: cargo miri run  --manifest-path "/home/ben/miri/bench-cargo-miri/backtraces/Cargo.toml"
  Time (mean ± σ):      4.046 s ±  0.087 s    [User: 3.952 s, System: 0.085 s]
  Range (min … max):    3.952 s …  4.139 s    5 runs

Benchmark 1: cargo miri run  --manifest-path "/home/ben/miri/bench-cargo-miri/invalidate/Cargo.toml"
  Time (mean ± σ):      6.271 s ±  0.073 s    [User: 6.206 s, System: 0.054 s]
  Range (min … max):    6.195 s …  6.365 s    5 runs

Benchmark 1: cargo miri run  --manifest-path "/home/ben/miri/bench-cargo-miri/mse/Cargo.toml"
  Time (mean ± σ):     570.3 ms ±   6.7 ms    [User: 505.5 ms, System: 61.8 ms]
  Range (min … max):   559.6 ms … 576.0 ms    5 runs

Benchmark 1: cargo miri run  --manifest-path "/home/ben/miri/bench-cargo-miri/serde1/Cargo.toml"
  Time (mean ± σ):      2.013 s ±  0.012 s    [User: 1.938 s, System: 0.069 s]
  Range (min … max):    1.994 s …  2.024 s    5 runs

Benchmark 1: cargo miri run  --manifest-path "/home/ben/miri/bench-cargo-miri/serde2/Cargo.toml"
  Time (mean ± σ):      4.155 s ±  0.082 s    [User: 4.078 s, System: 0.067 s]
  Range (min … max):    4.011 s …  4.211 s    5 runs

Benchmark 1: cargo miri run  --manifest-path "/home/ben/miri/bench-cargo-miri/slice-get-unchecked/Cargo.toml"
  Time (mean ± σ):     541.5 ms ±   3.9 ms    [User: 477.3 ms, System: 60.0 ms]
  Range (min … max):   536.1 ms … 545.1 ms    5 runs

Benchmark 1: cargo miri run  --manifest-path "/home/ben/miri/bench-cargo-miri/unicode/Cargo.toml"
  Time (mean ± σ):      1.496 s ±  0.002 s    [User: 1.442 s, System: 0.050 s]
  Range (min … max):    1.494 s …  1.500 s    5 runs

Benchmark 1: cargo miri run  --manifest-path "/home/ben/miri/bench-cargo-miri/zip-equal/Cargo.toml"
  Time (mean ± σ):      2.190 s ±  0.043 s    [User: 2.109 s, System: 0.075 s]
  Range (min … max):    2.114 s …  2.215 s    5 runs

```
After:
```
Benchmark 1: cargo miri run  --manifest-path "/home/ben/miri/bench-cargo-miri/backtraces/Cargo.toml"
  Time (mean ± σ):      3.954 s ±  0.057 s    [User: 3.871 s, System: 0.075 s]
  Range (min … max):    3.912 s …  4.052 s    5 runs

Benchmark 1: cargo miri run  --manifest-path "/home/ben/miri/bench-cargo-miri/invalidate/Cargo.toml"
  Time (mean ± σ):      6.200 s ±  0.108 s    [User: 6.129 s, System: 0.060 s]
  Range (min … max):    6.072 s …  6.295 s    5 runs

Benchmark 1: cargo miri run  --manifest-path "/home/ben/miri/bench-cargo-miri/mse/Cargo.toml"
  Time (mean ± σ):     575.3 ms ±   9.3 ms    [User: 511.5 ms, System: 60.2 ms]
  Range (min … max):   558.9 ms … 580.8 ms    5 runs

Benchmark 1: cargo miri run  --manifest-path "/home/ben/miri/bench-cargo-miri/serde1/Cargo.toml"
  Time (mean ± σ):      1.966 s ±  0.007 s    [User: 1.903 s, System: 0.058 s]
  Range (min … max):    1.959 s …  1.974 s    5 runs

Benchmark 1: cargo miri run  --manifest-path "/home/ben/miri/bench-cargo-miri/serde2/Cargo.toml"
  Time (mean ± σ):      4.138 s ±  0.040 s    [User: 4.057 s, System: 0.072 s]
  Range (min … max):    4.110 s …  4.207 s    5 runs

Benchmark 1: cargo miri run  --manifest-path "/home/ben/miri/bench-cargo-miri/slice-get-unchecked/Cargo.toml"
  Time (mean ± σ):     541.8 ms ±   5.6 ms    [User: 470.7 ms, System: 66.9 ms]
  Range (min … max):   535.6 ms … 549.1 ms    5 runs

Benchmark 1: cargo miri run  --manifest-path "/home/ben/miri/bench-cargo-miri/unicode/Cargo.toml"
  Time (mean ± σ):      1.490 s ±  0.021 s    [User: 1.424 s, System: 0.060 s]
  Range (min … max):    1.454 s …  1.505 s    5 runs

Benchmark 1: cargo miri run  --manifest-path "/home/ben/miri/bench-cargo-miri/zip-equal/Cargo.toml"
  Time (mean ± σ):      2.215 s ±  0.048 s    [User: 2.113 s, System: 0.072 s]
  Range (min … max):    2.183 s …  2.299 s    5 runs
```

Author: bors

src/tools/miri/src/borrow_tracker/stacked_borrows/mod.rs

@@ -37,8 +37,6 @@ pub struct Stacks {
     history: AllocHistory,
     /// The set of tags that have been exposed inside this allocation.
     exposed_tags: FxHashSet<BorTag>,
-    /// Whether this memory has been modified since the last time the tag GC ran
-    modified_since_last_gc: bool,
 }
 
 /// Indicates which permissions to grant to the retagged pointer.
@@ -450,15 +448,10 @@ impl<'tcx> Stack {
 /// Integration with the BorTag garbage collector
 impl Stacks {
     pub fn remove_unreachable_tags(&mut self, live_tags: &FxHashSet<BorTag>) {
-        if self.modified_since_last_gc {
-            for (_stack_range, stack) in self.stacks.iter_mut_all() {
-                if stack.len() > 64 {
-                    stack.retain(live_tags);
-                }
-            }
-            self.history.retain(live_tags);
-            self.modified_since_last_gc = false;
+        for (_stack_range, stack) in self.stacks.iter_mut_all() {
+            stack.retain(live_tags);
         }
+        self.history.retain(live_tags);
     }
 }
 
@@ -488,7 +481,6 @@ impl<'tcx> Stacks {
             stacks: RangeMap::new(size, stack),
             history: AllocHistory::new(id, item, machine),
             exposed_tags: FxHashSet::default(),
-            modified_since_last_gc: false,
         }
     }
 
@@ -503,7 +495,6 @@ impl<'tcx> Stacks {
             &mut FxHashSet<BorTag>,
         ) -> InterpResult<'tcx>,
     ) -> InterpResult<'tcx> {
-        self.modified_since_last_gc = true;
         for (stack_range, stack) in self.stacks.iter_mut(range.start, range.size) {
             let mut dcx = dcx_builder.build(&mut self.history, Size::from_bytes(stack_range.start));
             f(stack, &mut dcx, &mut self.exposed_tags)?;

src/tools/miri/tests/pass/stacked-borrows/stack-printing.rs

@@ -1,4 +1,7 @@
-//@compile-flags: -Zmiri-permissive-provenance
+// We disable the GC for this test because it would change what is printed. We are testing the
+// printing, not how it interacts with the GC.
+//@compile-flags: -Zmiri-permissive-provenance -Zmiri-provenance-gc=0
+
 #![feature(strict_provenance)]
 use std::{
     alloc::{self, Layout},