[llvm] [GVN] MemorySSA for GVN: eliminate redundant loads via MemorySSA (PR #152859)

[Antonio Frighetto via llvm-commits]

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@@ -2146,10 +2188,530 @@ static void patchAndReplaceAllUsesWith(Instruction *I, Value *Repl) {
   I->replaceAllUsesWith(Repl);
 }
 
+/// If a load has !invariant.group, try to find the most-dominating instruction
+/// with the same metadata and equivalent pointer (modulo bitcasts and zero
+/// GEPs). If one is found that dominates the load, its value can be reused.
+static Instruction *findInvariantGroupValue(LoadInst *L, DominatorTree &DT) {
+  Value *PointerOperand = L->getPointerOperand()->stripPointerCasts();
+
+  // It's not safe to walk the use list of a global value because function
+  // passes aren't allowed to look outside their functions.
+  // FIXME: this could be fixed by filtering instructions from outside of
+  // current function.
+  if (isa<Constant>(PointerOperand))
+    return nullptr;
+
+  // Queue to process all pointers that are equivalent to load operand.
+  SmallVector<Value *, 8> PointerUsesQueue;
+  PointerUsesQueue.push_back(PointerOperand);
+
+  Instruction *MostDominatingInstruction = L;
+
+  // FIXME: This loop is potentially O(n^2) due to repeated dominates checks.
+  while (!PointerUsesQueue.empty()) {
+    Value *Ptr = PointerUsesQueue.pop_back_val();
+    assert(Ptr && !isa<GlobalValue>(Ptr) &&
+           "Null or GlobalValue should not be inserted");
+
+    for (User *U : Ptr->users()) {
+      auto *I = dyn_cast<Instruction>(U);
+      if (!I || I == L || !DT.dominates(I, MostDominatingInstruction))
+        continue;
+
+      // Add bitcasts and zero GEPs to queue.
+      // TODO: Should drop bitcast?
+      if (isa<BitCastInst>(I) ||
----------------
antoniofrighetto wrote:

Should be possible to drop the bitcast check here while looking for an equivalent dominating !invariant.group load with the same pointer.

https://github.com/llvm/llvm-project/pull/152859