[llvm] [RegAlloc] Consider rematerialization over CSR use (PR #206756)

[Matt Arsenault via llvm-commits]

================
@@ -2372,6 +2372,86 @@ BlockFrequency RAGreedy::calcSpillCost(const LiveInterval &LI) {
   return BlockFrequency(SpillCost);
 }
 
+bool RAGreedy::shouldAvoidCSRForRemat(const LiveInterval &VirtReg,
+                                      AllocationOrder &Order) const {
+  if (!VirtReg.isSpillable())
+    return false;
+
+  // This logic is intentionally narrow: handle a single concrete value
+  // whose def can be cheaply rematerialized at every use.
+  const VNInfo *OnlyVNI = nullptr;
+  for (const VNInfo *VNI : VirtReg.vnis()) {
+    if (!VNI || VNI->isUnused())
+      continue;
+    if (VNI->isPHIDef())
+      return false;
+    if (OnlyVNI)
+      return false;
+    OnlyVNI = VNI;
+  }
+  if (!OnlyVNI)
+    return false;
+
+  MachineInstr *DefMI = LIS->getInstructionFromIndex(OnlyVNI->def);
+  if (!DefMI || DefMI->isImplicitDef() || !TII->isReMaterializable(*DefMI) ||
+      !TII->isAsCheapAsAMove(*DefMI))
+    return false;
+
+  // Only consider live ranges with register-mask interference. This limits the
+  // heuristic to avoiding a first-use CSR required by a call rather than by
+  // register pressure.
+  if (!Matrix->checkRegMaskInterference(VirtReg))
+    return false;
+
+  SmallPtrSet<MachineInstr *, 8> VisitedUses;
+  unsigned NumUses = 0;
+  for (MachineOperand &MO : MRI->use_nodbg_operands(VirtReg.reg())) {
+    if (MO.isUndef())
+      continue;
+
+    MachineInstr *UseMI = MO.getParent();
+    if (!VisitedUses.insert(UseMI).second)
----------------
arsenm wrote:

user iterator instead of use iterator?

https://github.com/llvm/llvm-project/pull/206756