Changes from review comments.

Author: cfallin

src/lib.rs

@@ -355,6 +355,7 @@ impl Operand {
 
     #[inline(always)]
     pub fn from_bits(bits: u32) -> Self {
+        debug_assert!(bits >> 29 <= 4);
         Operand { bits }
     }
 }
@@ -429,9 +430,9 @@ pub struct Allocation {
     /// `policy` field in `Operand`, and we are careful to use
     /// disjoint ranges of values in this field for each type. We also
     /// leave the def-or-use bit (`kind` for `Operand`) unused here so
-    /// that the client may use it to mark `Allocation`s on
-    /// instructions as read or write when it edits instructions
-    /// (which is sometimes useful for post-allocation analyses).
+    /// that we can use it below in `OperandOrAllocation` to record
+    /// whether `Allocation`s are defs or uses (which is often useful
+    /// to know).
     ///
     /// kind:3 unused:1 index:28
     bits: u32,
@@ -532,6 +533,7 @@ impl Allocation {
 
     #[inline(always)]
     pub fn from_bits(bits: u32) -> Self {
+        debug_assert!(bits >> 29 >= 5);
         Self { bits }
     }
 }
@@ -566,11 +568,13 @@ pub struct OperandOrAllocation {
 
 impl OperandOrAllocation {
     pub fn from_operand(operand: Operand) -> Self {
+        debug_assert!(operand.bits() >> 29 <= 4);
         Self {
             bits: operand.bits(),
         }
     }
     pub fn from_alloc(alloc: Allocation) -> Self {
+        debug_assert!(alloc.bits() >> 29 >= 5);
         Self { bits: alloc.bits() }
     }
     pub fn is_operand(&self) -> bool {
@@ -588,6 +592,10 @@ impl OperandOrAllocation {
     }
     pub fn as_allocation(&self) -> Option<Allocation> {
         if self.is_allocation() {
+            // Remove the def/use bit -- the canonical `Allocation`
+            // does not have this, and we want allocs to continue to
+            // be comparable whether they are used for reads or
+            // writes.
             Some(Allocation::from_bits(self.bits & !(1 << 28)))
         } else {
             None