[llvm] [RegisterCoalescer] Avoid redundant rematerialization when def has intervening store uses (PR #212232)

[Matt Arsenault via llvm-commits]

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@@ -1328,6 +1328,58 @@ bool RegisterCoalescer::reMaterializeDef(const CoalescerPair &CP,
   if (!TII->isReMaterializable(*DefMI))
     return false;
 
+  if (DstReg.isPhysical() && DefMI->getParent() == CopyMI->getParent()) {
+    // Check if DstReg is used by a return instruction immediately after CopyMI,
+    // with no intervening redefinition or tail call. In this case, remating a
+    // trivial def (e.g., LI 0) when SrcReg has store uses between DefMI and
+    // CopyMI would be harmful: DefMI cannot be deleted (still used by stores),
+    // so remat would introduce a redundant instruction.
+    MachineBasicBlock *MBB = CopyMI->getParent();
+    bool RedefinedAfterCopy = false;
+    bool HasMusttail = false;
+    MachineInstr *Ret = nullptr;
+    for (auto I = std::next(CopyMI->getIterator()); I != MBB->end(); ++I) {
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arsenm wrote:

This shouldn't really need a scan through the function. This should stick to computing information based on LiveIntervals 

https://github.com/llvm/llvm-project/pull/212232