Add a shim that lets us manually run the GC

Author: saethlin

src/machine.rs

@@ -1358,7 +1358,7 @@ impl<'mir, 'tcx> Machine<'mir, 'tcx> for MiriMachine<'mir, 'tcx> {
         // where it mistakenly removes an important tag become visible.
         if ecx.machine.gc_interval > 0 && ecx.machine.since_gc >= ecx.machine.gc_interval {
             ecx.machine.since_gc = 0;
-            ecx.garbage_collect_tags()?;
+            ecx.run_provenance_gc();
         }
 
         // These are our preemption points.

src/provenance_gc.rs

@@ -149,12 +149,8 @@ impl VisitTags for crate::MiriInterpCx<'_, '_> {
 
 impl<'mir, 'tcx: 'mir> EvalContextExt<'mir, 'tcx> for crate::MiriInterpCx<'mir, 'tcx> {}
 pub trait EvalContextExt<'mir, 'tcx: 'mir>: MiriInterpCxExt<'mir, 'tcx> {
-    fn garbage_collect_tags(&mut self) -> InterpResult<'tcx> {
+    fn run_provenance_gc(&mut self) {
         let this = self.eval_context_mut();
-        // No reason to do anything at all if stacked borrows is off.
-        if this.machine.borrow_tracker.is_none() {
-            return Ok(());
-        }
 
         let mut tags = FxHashSet::default();
         let mut alloc_ids = FxHashSet::default();
@@ -168,8 +164,6 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: MiriInterpCxExt<'mir, 'tcx> {
         });
         self.remove_unreachable_tags(tags);
         self.remove_unreachable_allocs(alloc_ids);
-
-        Ok(())
     }
 
     fn remove_unreachable_tags(&mut self, tags: FxHashSet<BorTag>) {

src/shims/foreign_items.rs

@@ -459,6 +459,10 @@ trait EvalContextExtPriv<'mir, 'tcx: 'mir>: crate::MiriInterpCxExt<'mir, 'tcx> {
         // shim, add it to the corresponding submodule.
         match link_name.as_str() {
             // Miri-specific extern functions
+            "miri_run_provenance_gc" => {
+                let [] = this.check_shim(abi, Abi::Rust, link_name, args)?;
+                this.run_provenance_gc();
+            }
             "miri_get_alloc_id" => {
                 let [ptr] = this.check_shim(abi, Abi::Rust, link_name, args)?;
                 let ptr = this.read_pointer(ptr)?;

tests/pass/ptr_int_casts.rs

@@ -4,6 +4,10 @@
 use std::mem;
 use std::ptr;
 
+extern "Rust" {
+    fn miri_run_provenance_gc();
+}
+
 fn eq_ref<T>(x: &T, y: &T) -> bool {
     x as *const _ == y as *const _
 }
@@ -47,25 +51,19 @@ fn ptr_int_casts() {
     // involving types other than usize
     assert_eq!((-1i32) as usize as *const i32 as usize, (-1i32) as usize);
 
-    // Stress-test that the GC doesn't delete context that would prevent us from casting from int
+    // Check that the GC doesn't delete context that would prevent us from casting from int
     // to pointer correctly after the allocation is dead.
-    let mut ptrs: Vec<*const u8> = Vec::new();
-    let mut ints: Vec<usize> = Vec::new();
-
-    for _ in 0..100 {
+    let (ptr, int) = {
         let local = 0u8;
         let ptr = &local as *const u8;
-        ptrs.push(ptr);
-        ints.push(ptr as usize);
-
-        let b = Box::new(0u8);
-        let ptr = &*b as *const u8;
-        ptrs.push(ptr);
-        ints.push(ptr as usize);
-    }
-
-    let later_ints: Vec<usize> = ptrs.iter().map(|p| *p as usize).collect();
-    assert_eq!(ints, later_ints);
+        (ptr, ptr as usize)
+    };
+    // Manually run the GC, instead of just hoping that it runs at the right time.
+    unsafe { miri_run_provenance_gc() }
+    let later_int = ptr as usize;
+    assert_eq!(int, later_int);
+    let later_ptr = int as *const u8;
+    assert_eq!(ptr, later_ptr);
 }
 
 fn ptr_int_ops() {