[compiler-rt] [flang] [lldb] [libcxx] [lld] [clang] [clang-tools-extra] [libc] [llvm] [AMDGPU] Use alias info to relax waitcounts for LDS DMA (PR #74537)

[Matt Arsenault via llvm-commits]

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@@ -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
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arsenm wrote:

PseudoSourceValue::mayAlias is supposed to report aliasing to possible IR values. It looks like it's layered weirdly, and expects you to go through MachineInstr::mayAlias. MachineInstr::mayAlias ought to be using the AA tags, it shouldn't be a fundamental limitation 

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