[llvm] Limit Alloca->LDS promotion based on speculations as to eventual register pressure (PR #152814)

[Tim Gymnich via llvm-commits]

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@@ -1471,9 +1485,83 @@ bool AMDGPUPromoteAllocaImpl::hasSufficientLocalMem(const Function &F) {
   return true;
 }
 
+size_t AMDGPUPromoteAllocaImpl::getSGPRPressureEstimate(AllocaInst &I) {
+  Function &F = *I.getParent()->getParent();
+  size_t MaxLive = 0;
+  for (BasicBlock *BB : post_order(&F)) {
+    if (SGPRLiveIns.count(BB))
+      continue;
+
+    std::unordered_set<Instruction *> CurrentlyLive;
+    for (BasicBlock *SuccBB : successors(BB))
+      if (SGPRLiveIns.count(SuccBB))
+        for (const auto &R : SGPRLiveIns[SuccBB])
+          CurrentlyLive.insert(R);
+
+    for (auto RIt = BB->rbegin(); RIt != BB->rend(); RIt++) {
----------------
tgymnich wrote:

```suggestion
    for (auto Inst : reverse(BB)) {
```

https://github.com/llvm/llvm-project/pull/152814