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

[Lukas Sommer via llvm-commits]

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@@ -2372,6 +2372,87 @@ 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;
+
+  // This logic aims to address a specific problem: The first use of CSR for
+  // cheap-to-rematerialize live ranges because they cross calls, not because
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sommerlukas wrote:

Done in the latest commit. 

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