Make -Zmiri-tag-gc=0 disable the GC, document the flag

Author: saethlin

README.md

@@ -323,6 +323,10 @@ environment variable. We first document the most relevant and most commonly used
   ensure alignment.  (The standard library `align_to` method works fine in both modes; under
   symbolic alignment it only fills the middle slice when the allocation guarantees sufficient
   alignment.)
+* `-Zmiri-tag-gc=<blocks>` configures how often the pointer tag garbage collector runs. The default
+  is to search for and remove unreachable tags once every `10,000` basic blocks. Setting this to
+  `0` disables the garbage collector, which causes some programs to have explosive memory usage
+  and/or super-linear runtime.
 
 The remaining flags are for advanced use only, and more likely to change or be removed.
 Some of these are **unsound**, which means they can lead

src/machine.rs

@@ -971,7 +971,9 @@ impl<'mir, 'tcx> Machine<'mir, 'tcx> for Evaluator<'mir, 'tcx> {
         // stacks.
         // When debug assertions are enabled, run the GC as often as possible so that any cases
         // where it mistakenly removes an important tag become visible.
-        if ecx.machine.since_gc >= ecx.machine.gc_interval || cfg!(debug_assertions) {
+        if cfg!(debug_assertions)
+            || (ecx.machine.gc_interval > 0 && ecx.machine.since_gc >= ecx.machine.gc_interval)
+        {
             ecx.machine.since_gc = 0;
             ecx.garbage_collect_tags()?;
         } else {