Record copy hints for failed fragment coalescing

Interestingly, this didn't actually affect any of the test cases. Have I
done something wrong?

Author: nmraz

crates/codegen/src/regalloc/live_set.rs

@@ -8,7 +8,7 @@ use cranelift_entity::{
 use log::trace;
 
 use crate::{
-    lir::{UseOperandConstraint, VirtReg},
+    lir::{Instr, UseOperandConstraint, VirtReg},
     machine::MachineRegalloc,
 };
 
@@ -22,6 +22,7 @@ use super::{
 type VirtRegFragmentMap = SecondaryMap<VirtReg, PackedOption<LiveSetFragment>>;
 
 struct CopyCandidate {
+    instr: Instr,
     src: VirtReg,
     dest: VirtReg,
     weight: f32,
@@ -52,6 +53,7 @@ impl<M: MachineRegalloc> RegAllocContext<'_, M> {
                     if let UseOperandConstraint::TiedToDef(i) = use_op.constraint() {
                         let def = self.lir.instr_defs(instr)[i as usize];
                         candidates.push(CopyCandidate {
+                            instr,
                             src: use_op.reg(),
                             dest: def.reg(),
                             weight: get_weight_at_instr(self.lir, self.cfg_ctx, instr),
@@ -73,6 +75,7 @@ impl<M: MachineRegalloc> RegAllocContext<'_, M> {
                     // terminator for weight purposes.
                     let terminator = self.lir.block_terminator(block);
                     candidates.push(CopyCandidate {
+                        instr: terminator,
                         src: outgoing,
                         dest: incoming,
                         weight: get_weight_at_instr(self.lir, self.cfg_ctx, terminator),
@@ -97,7 +100,12 @@ impl<M: MachineRegalloc> RegAllocContext<'_, M> {
                 candidate.dest,
                 candidate.weight
             );
-            self.try_coalesce(&mut fragments_by_vreg, candidate.dest, candidate.src);
+            self.try_coalesce(
+                &mut fragments_by_vreg,
+                candidate.instr,
+                candidate.dest,
+                candidate.src,
+            );
         }
 
         // Fill in per-fragment data now that we actually know what the fragments are.
@@ -121,6 +129,7 @@ impl<M: MachineRegalloc> RegAllocContext<'_, M> {
     fn try_coalesce(
         &mut self,
         fragments_by_vreg: &mut VirtRegFragmentMap,
+        instr: Instr,
         dest: VirtReg,
         src: VirtReg,
     ) {
@@ -136,6 +145,9 @@ impl<M: MachineRegalloc> RegAllocContext<'_, M> {
             &self.live_set_fragments[dest_fragment].ranges,
             &self.live_set_fragments[src_fragment].ranges,
         ) {
+            // We couldn't coalesce these fragments now, but we might be able to later because of
+            // splitting. Record a copy hint so we remember to do so.
+            self.hint_fragment_copy(instr, src_fragment, dest_fragment);
             return;
         }