[clang] [llvm] [AMDGPU] Emit a waitcnt instruction after each memory instruction (PR #79236)

[Pierre van Houtryve via cfe-commits]

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@@ -2561,6 +2567,70 @@ bool SIMemoryLegalizer::expandAtomicCmpxchgOrRmw(const SIMemOpInfo &MOI,
   return Changed;
 }
 
+bool SIMemoryLegalizer::GFX9InsertWaitcntForPreciseMem(MachineFunction &MF) {
+  const GCNSubtarget &ST = MF.getSubtarget<GCNSubtarget>();
+  const SIInstrInfo *TII = ST.getInstrInfo();
+  IsaVersion IV = getIsaVersion(ST.getCPU());
+
+  bool Changed = false;
+
+  for (auto &MBB : MF) {
+    for (auto MI = MBB.begin(); MI != MBB.end();) {
+      MachineInstr &Inst = *MI;
+      ++MI;
+      if (Inst.mayLoadOrStore() == false)
+        continue;
+
+      // Todo: if next insn is an s_waitcnt
+      AMDGPU::Waitcnt Wait;
+
+      if (!(Inst.getDesc().TSFlags & SIInstrFlags::maybeAtomic)) {
+        if (TII->isSMRD(Inst)) {          // scalar
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Pierre-vh wrote:

Can we have a shared helper, e.g. in `SIInstrInfo`  for both? It's a lot of logic to duplicate

> The counter values in SIInsertWaitcnt are precise, while in this features the counters are simply set to 0.
That could just be a boolean switch in a shared helper

https://github.com/llvm/llvm-project/pull/79236