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

[Antonio Frighetto via llvm-commits]

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@@ -1545,12 +1559,24 @@ LoadInst *GVNPass::findLoadToHoistIntoPred(BasicBlock *Pred, BasicBlock *LoadBB,
     if (!Inst.isIdenticalTo(Load))
       continue;
 
-    MemDepResult Dep = MD->getDependency(&Inst);
+    bool HasLocalDep = true;
+    if (MD && !MSSAU) {
+      MemDepResult Dep = MD->getDependency(&Inst);
+      HasLocalDep = !Dep.isNonLocal();
+    } else {
+      auto *MSSA = MSSAU->getMemorySSA();
+      // Do not hoist if the identical load has ordering constraint.
+      if (auto *MA = MSSA->getMemoryAccess(&Inst); MA && isa<MemoryUse>(MA)) {
+        auto *Clobber = MSSA->getWalker()->getClobberingMemoryAccess(MA);
+        HasLocalDep = isa<MemoryDef>(Clobber) && Clobber->getBlock() == SuccBB;
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antoniofrighetto wrote:

This is a recent addition. If not missing anything, it should suffice to check whether the identical load has a local dominating clobber, in order not to be hoisted.

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