[llvm] [DeadStoreElimination] Optimize tautological assignments (PR #75744)

[Nikita Popov via llvm-commits]

================
@@ -1901,6 +1901,52 @@ struct DSEState {
     return true;
   }
 
+  // Check if there is a dominating condition, that implies that the value
+  // being stored in a ptr is already present in the ptr.
+  bool dominatingConditionImpliesValue(MemoryDef *Def) {
+    StoreInst *StoreI = dyn_cast<StoreInst>(Def->getMemoryInst());
+    BasicBlock *StoreBB = StoreI->getParent();
+    Value *StorePtr = StoreI->getPointerOperand();
+    Value *StoreVal = StoreI->getValueOperand();
+
+    DomTreeNode *IDom = DT.getNode(StoreBB)->getIDom();
+    if (!IDom)
+      return false;
+
+    auto *BI = dyn_cast<BranchInst>(IDom->getBlock()->getTerminator());
+    if (!BI || !BI->isConditional())
+      return false;
+
+    // In case both blocks are the same, we cannot optimize the FalseBB when the
+    // condition is true and vice versa. Return false in this case.
+    if (BI->getSuccessor(0) == BI->getSuccessor(1))
+      return false;
+
+    Instruction *ICmpL;
+    ICmpInst::Predicate Pred;
+    if (!match(BI->getCondition(),
+               m_c_ICmp(Pred,
+                        m_CombineAnd(m_Load(m_Specific(StorePtr)),
+                                     m_Instruction(ICmpL)),
+                        m_Specific(StoreVal))) ||
+        !ICmpInst::isEquality(Pred))
+      return false;
+
+    if (Pred == ICmpInst::ICMP_EQ && StoreBB != BI->getSuccessor(0))
+      return false;
+
+    if (Pred == ICmpInst::ICMP_NE && StoreBB != BI->getSuccessor(1))
+      return false;
+
+    MemoryAccess *LoadAcc = MSSA.getMemoryAccess(ICmpL);
+    MemoryAccess *ClobAcc =
+        MSSA.getSkipSelfWalker()->getClobberingMemoryAccess(Def, BatchAA);
+    if (!MSSA.dominates(ClobAcc, LoadAcc))
+      return false;
+
+    return true;
----------------
nikic wrote:

```suggestion
    return MSSA.dominates(ClobAcc, LoadAcc);
```

https://github.com/llvm/llvm-project/pull/75744