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

[Shreyansh Chouhan via llvm-commits]

https://github.com/BK1603 updated https://github.com/llvm/llvm-project/pull/75744

>From f9eed0d56c02d4cca895f3f19bdd65e11d5272f0 Mon Sep 17 00:00:00 2001
From: Shreyansh Chouhan <chouhan.shreyansh2702 at gmail.com>
Date: Sun, 17 Dec 2023 22:48:51 +0530
Subject: [PATCH] [DeadStoreElimination] Optimize tautological assignments

If a store is immediately dominated by a condition that ensures that the value
being stored in a memory location is already present at that memory location,
consider the store a noop.

Fixes #63419
---
 .../Scalar/DeadStoreElimination.cpp           | 37 +++++++++++++++++++
 .../DeadStoreElimination/noop-stores.ll       | 24 ++++++++++++
 2 files changed, 61 insertions(+)

diff --git a/llvm/lib/Transforms/Scalar/DeadStoreElimination.cpp b/llvm/lib/Transforms/Scalar/DeadStoreElimination.cpp
index dd0a290252dae3..8ec2a47542a831 100644
--- a/llvm/lib/Transforms/Scalar/DeadStoreElimination.cpp
+++ b/llvm/lib/Transforms/Scalar/DeadStoreElimination.cpp
@@ -1926,6 +1926,43 @@ struct DSEState {
       if (InitC && InitC == StoredConstant)
         return MSSA.isLiveOnEntryDef(
             MSSA.getSkipSelfWalker()->getClobberingMemoryAccess(Def, BatchAA));
+
+      if (!Store)
+        return false;
+
+      // If there is a dominating condition, that ensures that the value
+      // being stored in a memory location is already present at the
+      // memory location, the store is a noop.
+      BasicBlock *StoreBB = DefI->getParent();
+      auto *StorePtr = Store->getOperand(1);
+
+      DomTreeNode *IDom = DT.getNode(StoreBB)->getIDom();
+      if (!IDom)
+        return false;
+
+      auto *TI = IDom->getBlock()->getTerminator();
+      ICmpInst::Predicate Pred;
+      BasicBlock *TrueBB, *FalseBB;
+
+      if (!match(TI, m_Br(m_ICmp(Pred, m_Load(m_Specific(StorePtr)),
+                                 m_Specific(StoredConstant)),
+                          TrueBB, FalseBB)))
+        return false;
+
+      MemoryAccess *LastMod =
+          MSSA.getSkipSelfWalker()->getClobberingMemoryAccess(Def, BatchAA);
+
+      DomTreeNode *CDom = DT.getNode(LastMod->getBlock());
+      if (!DT.dominates(CDom, IDom))
+        return false;
+
+      if (Pred == ICmpInst::ICMP_EQ && StoreBB == TrueBB)
+        return true;
+
+      if (Pred == ICmpInst::ICMP_NE && StoreBB == FalseBB)
+        return true;
+
+      return false;
     }
 
     if (!Store)
diff --git a/llvm/test/Transforms/DeadStoreElimination/noop-stores.ll b/llvm/test/Transforms/DeadStoreElimination/noop-stores.ll
index 3703b8d039ead0..0fce7c9ba6868c 100644
--- a/llvm/test/Transforms/DeadStoreElimination/noop-stores.ll
+++ b/llvm/test/Transforms/DeadStoreElimination/noop-stores.ll
@@ -795,3 +795,27 @@ join:
   store i8 %v, ptr %q, align 1
   ret void
 }
+
+ at x = global i32 0, align 4
+define void @remove_tautological_store() {
+; CHECK-LABEL: @remove_tautological_store(
+; CHECK-NEXT:    [[TMP1:%.*]] = load i32, ptr @x, align 4
+; CHECK-NEXT:    [[TMP2:%.*]] = icmp eq i32 [[TMP1]], 4
+; CHECK-NEXT:    br i1 [[TMP2]], label [[TMP3:%.*]], label [[TMP4:%.*]]
+; CHECK:       3:
+; CHECK-NEXT:    br label [[TMP4]]
+; CHECK:       4:
+; CHECK-NEXT:    ret void
+;
+  %1 = load i32, ptr @x, align 4
+  %2 = icmp eq i32 %1, 4
+  br i1 %2, label %3, label %4
+
+3:
+  store i32 4, ptr @x, align 4
+  br label %4
+
+4:
+  ret void
+}
+