[llvm] [AMDGPU] Use alias info to relax waitcounts for LDS DMA (PR #74537)

[Matt Arsenault via llvm-commits]

================
@@ -703,8 +713,37 @@ void WaitcntBrackets::updateByEvent(const SIInstrInfo *TII,
         setRegScore(RegNo, T, CurrScore);
       }
     }
-    if (Inst.mayStore() && (TII->isDS(Inst) || mayWriteLDSThroughDMA(Inst))) {
-      setRegScore(SQ_MAX_PGM_VGPRS + EXTRA_VGPR_LDS, T, CurrScore);
+    if (Inst.mayStore() &&
+        (TII->isDS(Inst) || TII->mayWriteLDSThroughDMA(Inst))) {
+      // MUBUF and FLAT LDS DMA operations need a wait on vmcnt before LDS
+      // written can be accessed. A load from LDS to VMEM does not need a wait.
+      unsigned Slot = 0;
+      for (const auto *MemOp : Inst.memoperands()) {
+        if (!MemOp->isStore() ||
+            MemOp->getAddrSpace() != AMDGPUAS::LOCAL_ADDRESS)
+          continue;
+        // Comparing just AA info does not guarantee memoperands are equal
+        // in general, but this is so for LDS DMA on practice.
+        auto AAI = MemOp->getAAInfo();
+        if (!AAI)
+          break;
+        for (unsigned I = 0, E = LDSDMAStores.size(); I != E && !Slot; ++I) {
+          for (const auto *MemOp : LDSDMAStores[I]->memoperands()) {
+            if (MemOp->isStore() && AAI == MemOp->getAAInfo()) {
----------------
arsenm wrote:

I'm not sure I understand the equality check here; you perform an actual mayAlias query later? What is the point of this filter? Don't you need to consider any possibly aliasing write as an event?

https://github.com/llvm/llvm-project/pull/74537