cranelift: Compress vcode range-lists (bytecodealliance#8506)

These lists of ranges always cover contiguous ranges of an index space,
meaning the start of one range is the same as the end of the previous
range, so we can cut storage in half by only storing one endpoint of
each range.

This in turn means we don't have to keep track of the other endpoint
while building these lists, reducing the state we need to keep while
building vcode and simplifying the various build steps.

cranelift/codegen/src/lib.rs

@@ -76,6 +76,7 @@ mod iterators;
 mod legalizer;
 mod nan_canonicalization;
 mod opts;
+mod ranges;
 mod remove_constant_phis;
 mod result;
 mod scoped_hash_map;

cranelift/codegen/src/machinst/reg.rs

@@ -288,7 +288,6 @@ pub type RegClass = regalloc2::RegClass;
 #[derive(Debug)]
 pub struct OperandCollector<'a, F: Fn(VReg) -> VReg> {
     operands: &'a mut Vec<Operand>,
-    operands_start: usize,
     clobbers: PRegSet,
 
     /// The subset of physical registers that are allocatable.
@@ -300,10 +299,8 @@ pub struct OperandCollector<'a, F: Fn(VReg) -> VReg> {
 impl<'a, F: Fn(VReg) -> VReg> OperandCollector<'a, F> {
     /// Start gathering operands into one flattened operand array.
     pub fn new(operands: &'a mut Vec<Operand>, allocatable: PRegSet, renamer: F) -> Self {
-        let operands_start = operands.len();
         Self {
             operands,
-            operands_start,
             clobbers: PRegSet::default(),
             allocatable,
             renamer,
@@ -313,10 +310,9 @@ impl<'a, F: Fn(VReg) -> VReg> OperandCollector<'a, F> {
     /// Finish the operand collection and return the tuple giving the
     /// range of indices in the flattened operand array, and the
     /// clobber set.
-    pub fn finish(self) -> ((u32, u32), PRegSet) {
-        let start = self.operands_start as u32;
-        let end = self.operands.len() as u32;
-        ((start, end), self.clobbers)
+    pub fn finish(self) -> (usize, PRegSet) {
+        let end = self.operands.len();
+        (end, self.clobbers)
     }
 }