[llvm] [GreedyRA] Improve RA for nested loop induction variables (PR #72093)

[via llvm-commits]

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@@ -767,10 +767,27 @@ bool RAGreedy::growRegion(GlobalSplitCandidate &Cand) {
     if (Cand.PhysReg) {
       if (!addThroughConstraints(Cand.Intf, NewBlocks))
         return false;
-    } else
-      // Provide a strong negative bias on through blocks to prevent unwanted
-      // liveness on loop backedges.
-      SpillPlacer->addPrefSpill(NewBlocks, /* Strong= */ true);
+    } else {
+      // Providing that the variable being spilled does not look like a loop
+      // induction variable, which is expensive to spill around and better
+      // pushed into a condition inside the loop if possible, provide a strong
+      // negative bias on through blocks to prevent unwanted liveness on loop
+      // backedges.
+      bool PrefSpill = true;
+      if (SA->looksLikeLoopIV() && NewBlocks.size() == 3) {
+        // NewBlocks==3 means we are (possible) adding a Header + start+end of
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qcolombet wrote:

Nit: (possible) => (possibly)

https://github.com/llvm/llvm-project/pull/72093