Make Alloc use only 30 bits.

This is useful in the in-progress Cranelift glue in particular, where we
wrap `Operand` and `Allocation` together in one 32-bit union, using the
MSB to disambiguate. In the `Allocation` case, we still want to carry
def/use info so we can tell reads and writes apart for
machine-code-level analyses. So stealing an extra bit from `Allocation`
is necessary; this just reduces our stack-slot limit to 2^28 slots,
which should be no problem.

src/lib.rs

@@ -204,7 +204,7 @@ pub struct Operand {
     /// Bit-pack into 31 bits. This allows a `Reg` to encode an
     /// `Operand` or an `Allocation` in 32 bits.
     ///
-    /// op-or-alloc:1 pos:2 kind:1 policy:2 class:1 preg:5 vreg:20
+    /// free:1 pos:2 kind:1 policy:2 class:1 preg:5 vreg:20
     bits: u32,
 }
 
@@ -422,9 +422,9 @@ pub enum OperandPos {
 /// Operand.
 #[derive(Clone, Copy, PartialEq, Eq, PartialOrd, Ord, Hash)]
 pub struct Allocation {
-    /// Bit-pack in 31 bits:
+    /// Bit-pack in 30 bits:
     ///
-    /// op-or-alloc:1 kind:2  index:29
+    /// free:2 kind:2 index:28
     bits: u32,
 }
 
@@ -452,8 +452,9 @@ impl std::fmt::Display for Allocation {
 impl Allocation {
     #[inline(always)]
     pub(crate) fn new(kind: AllocationKind, index: usize) -> Self {
+        assert!(index < (1 << 28));
         Self {
-            bits: ((kind as u8 as u32) << 29) | (index as u32),
+            bits: ((kind as u8 as u32) << 28) | (index as u32),
         }
     }
 
@@ -474,7 +475,7 @@ impl Allocation {
 
     #[inline(always)]
     pub fn kind(self) -> AllocationKind {
-        match (self.bits >> 29) & 3 {
+        match (self.bits >> 28) & 3 {
             0 => AllocationKind::None,
             1 => AllocationKind::Reg,
             2 => AllocationKind::Stack,
@@ -484,7 +485,7 @@ impl Allocation {
 
     #[inline(always)]
     pub fn index(self) -> usize {
-        (self.bits & ((1 << 29) - 1)) as usize
+        (self.bits & ((1 << 28) - 1)) as usize
     }
 
     #[inline(always)]