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

[Stanislav Mekhanoshin via libc-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
----------------
rampitec 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

This is all PSV::mayAlias() does:
```
bool PseudoSourceValue::mayAlias(const MachineFrameInfo *) const {
  return !(isGOT() || isConstantPool() || isJumpTable());
}
```
No very useful. Then even to get to the AA tags check MI:mayAlias() shall go through all IR values' checks first.

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