optimize all ZST allocations into one single allocation

src/interpreter/terminator.rs

@@ -423,8 +423,8 @@ fn call_c_abi(
             "__rust_reallocate" => {
                 let ptr = self.memory.read_ptr(args[0])?;
                 let size = self.memory.read_usize(args[2])?;
-                self.memory.reallocate(ptr, size as usize)?;
-                self.memory.write_ptr(dest, ptr)?;
+                let new_ptr = self.memory.reallocate(ptr, size as usize)?;
+                self.memory.write_ptr(dest, new_ptr)?;
             }
 
             "memcmp" => {

src/memory.rs

@@ -39,8 +39,18 @@ pub struct Pointer {
 
 impl Pointer {
     pub fn offset(self, i: isize) -> Self {
+        // FIXME: prevent offsetting ZST ptrs in tracing mode
         Pointer { offset: (self.offset as isize + i) as usize, ..self }
     }
+    pub fn points_to_zst(&self) -> bool {
+        self.alloc_id.0 == 0
+    }
+    fn zst_ptr() -> Self {
+        Pointer {
+            alloc_id: AllocId(0),
+            offset: 0,
+        }
+    }
 }
 
 #[derive(Debug, Copy, Clone, Hash, Eq, PartialEq)]
@@ -68,13 +78,25 @@ pub struct Memory<'a, 'tcx> {
 
 impl<'a, 'tcx> Memory<'a, 'tcx> {
     pub fn new(layout: &'a TargetDataLayout) -> Self {
-        Memory {
+        let mut mem = Memory {
             alloc_map: HashMap::new(),
             functions: HashMap::new(),
             function_alloc_cache: HashMap::new(),
-            next_id: AllocId(0),
+            next_id: AllocId(1),
             layout: layout,
-        }
+        };
+        // alloc id 0 is reserved for ZSTs, this is an optimization to prevent ZST
+        // (e.g. function pointers, (), [], ...) from requiring memory
+        let alloc = Allocation {
+            bytes: Vec::new(),
+            relocations: BTreeMap::new(),
+            undef_mask: UndefMask::new(0),
+        };
+        mem.alloc_map.insert(AllocId(0), alloc);
+        // check that additional zst allocs work
+        debug_assert!(mem.allocate(0).points_to_zst());
+        debug_assert!(mem.get(AllocId(0)).is_ok());
+        mem
     }
 
     pub fn allocations<'b>(&'b self) -> ::std::collections::hash_map::Iter<'b, AllocId, Allocation> {
@@ -105,6 +127,9 @@ pub fn create_fn_ptr(&mut self, def_id: DefId, substs: &'tcx Substs<'tcx>, fn_ty
     }
 
     pub fn allocate(&mut self, size: usize) -> Pointer {
+        if size == 0 {
+            return Pointer::zst_ptr();
+        }
         let alloc = Allocation {
             bytes: vec![0; size],
             relocations: BTreeMap::new(),
@@ -121,7 +146,14 @@ pub fn allocate(&mut self, size: usize) -> Pointer {
 
     // TODO(solson): Track which allocations were returned from __rust_allocate and report an error
     // when reallocating/deallocating any others.
-    pub fn reallocate(&mut self, ptr: Pointer, new_size: usize) -> EvalResult<'tcx, ()> {
+    pub fn reallocate(&mut self, ptr: Pointer, new_size: usize) -> EvalResult<'tcx, Pointer> {
+        if ptr.points_to_zst() {
+            if new_size != 0 {
+                return Ok(self.allocate(new_size));
+            } else {
+                return Ok(ptr);
+            }
+        }
         if ptr.offset != 0 {
             // TODO(solson): Report error about non-__rust_allocate'd pointer.
             return Err(EvalError::Unimplemented(format!("bad pointer offset: {}", ptr.offset)));
@@ -141,11 +173,14 @@ pub fn reallocate(&mut self, ptr: Pointer, new_size: usize) -> EvalResult<'tcx,
             alloc.undef_mask.truncate(new_size);
         }
 
-        Ok(())
+        Ok(ptr)
     }
 
     // TODO(solson): See comment on `reallocate`.
     pub fn deallocate(&mut self, ptr: Pointer) -> EvalResult<'tcx, ()> {
+        if ptr.points_to_zst() {
+            return Ok(());
+        }
         if ptr.offset != 0 {
             // TODO(solson): Report error about non-__rust_allocate'd pointer.
             return Err(EvalError::Unimplemented(format!("bad pointer offset: {}", ptr.offset)));

tests/run-pass/zst.rs

+// the following flag prevents this test from running on the host machine
+// this should only be run on miri, because rust doesn't (yet?) optimize ZSTs of different types
+// into the same memory location
+// ignore-test
+
+
 #[derive(PartialEq, Debug)]
 struct A;
 
@@ -13,4 +19,6 @@ fn use_zst() -> A {
 fn main() {
     assert_eq!(zst_ret(), A);
     assert_eq!(use_zst(), A);
+    assert_eq!(&A as *const A as *const (), &() as *const _);
+    assert_eq!(&A as *const A, &A as *const A);
 }