[llvm] [AMDGPU] Eliminate redundant SGPR restores (PR #222680)

[Matt Arsenault via llvm-commits]

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@@ -121,6 +129,59 @@ INITIALIZE_PASS_END(SILowerSGPRSpillsLegacy, DEBUG_TYPE,
 
 char &llvm::SILowerSGPRSpillsLegacyID = SILowerSGPRSpillsLegacy::ID;
 
+/// Remove a restore which immediately reads back the value just saved from the
+/// same SGPR. Keep the save, since a later restore may still need it.
+bool SILowerSGPRSpills::eliminateRedundantRestores(
+    MachineFunction &MF, ArrayRef<int> CalleeSavedFIs) {
+  bool Changed = false;
+
+  for (MachineBasicBlock &MBB : MF) {
+    for (MachineInstr &Restore : llvm::make_early_inc_range(MBB)) {
+      if (!TII->isSGPRSpill(Restore) || !Restore.mayLoad() ||
+          Restore.isBundled() || Restore.getFlag(MachineInstr::FrameSetup) ||
+          Restore.getFlag(MachineInstr::FrameDestroy))
+        continue;
+
+      auto RestoreIt = Restore.getIterator();
+      if (RestoreIt == MBB.begin())
+        continue;
+      MachineInstr &Save = *prev_nodbg(RestoreIt, MBB.instr_begin(),
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arsenm wrote:

std::prev?

https://github.com/llvm/llvm-project/pull/222680