Tweak comment and guarantee correct alignment.

ykrt/src/mt.rs

@@ -1,6 +1,7 @@
 #[cfg(test)]
 use std::time::Duration;
 use std::{
+    alloc::{alloc, dealloc, Layout},
     io, mem,
     panic::{catch_unwind, resume_unwind, UnwindSafe},
     rc::Rc,
@@ -311,9 +312,10 @@ impl MTThread {
 /// The innards of a meta-tracer thread.
 struct MTThreadInner {
     mt: MT,
-    /// A value that uniquely identifies a thread. For simplicities sake this is a pointer to a
-    /// malloc'd chunk of memory since that pointer's alignment means that we won't conflict with
-    /// the bits set in PHASE_TAG.
+    /// A value that uniquely identifies a thread. Since this ID needs to be ORable with PHASE_TAG,
+    /// we use a pointer to a malloc'd chunk of memory. We guarantee a) that chunk is aligned to a
+    /// machine word (or greater) b) that it is a non-zero chunk of memory (and thus guaranteed to
+    /// be a unique pointer).
     tid: *mut u8,
     hot_threshold: HotThreshold,
     #[allow(dead_code)]
@@ -328,9 +330,14 @@ impl MTThreadInner {
     fn init(mt: MT) -> MTThread {
         let hot_threshold = mt.hot_threshold();
         let tracing_kind = mt.tracing_kind();
+        let tid = {
+            let layout =
+                Layout::from_size_align(mem::size_of::<usize>(), mem::size_of::<usize>()).unwrap();
+            unsafe { alloc(layout) }
+        };
         let inner = MTThreadInner {
             mt,
-            tid: Box::into_raw(Box::new(0u8)),
+            tid,
             hot_threshold,
             tracing_kind,
             tracer: None,
@@ -343,7 +350,9 @@ impl MTThreadInner {
 
 impl Drop for MTThreadInner {
     fn drop(&mut self) {
-        unsafe { Box::from_raw(self.tid) };
+        let layout =
+            Layout::from_size_align(mem::size_of::<usize>(), mem::size_of::<usize>()).unwrap();
+        unsafe { dealloc(self.tid, layout) };
     }
 }