From 00c9b6461f65380deed20676147083a6e8c66c3e Mon Sep 17 00:00:00 2001
From: Philip Reames <listmail@philipreames.com>
Date: Tue, 5 Aug 2014 17:48:20 +0000
Subject: [PATCH] Remove dead zero store to calloc initialized memory

Optimize the following IR:

%1 = tail call noalias i8* @calloc(i64 1, i64 4)
%2 = bitcast i8* %1 to i32*
; This store is dead and should be removed
store i32 0, i32* %2, align 4

Memory returned by calloc is guaranteed to be zero initialized. If the value being stored is the constant zero (and the store is not otherwise observable across threads), we can delete the store.  If the store is to an out of bounds address, it is undefined and thus also removable.

Reviewed By: nicholas

Differential Revision: http://reviews.llvm.org/D3942

llvm-svn: 214897
---
 .../lib/Transforms/Scalar/DeadStoreElimination.cpp | 50 +++++++++++++++-------
 .../DeadStoreElimination/calloc-store.ll           | 17 ++++++++
 2 files changed, 52 insertions(+), 15 deletions(-)
 create mode 100644 llvm/test/Transforms/DeadStoreElimination/calloc-store.ll

diff --git a/llvm/lib/Transforms/Scalar/DeadStoreElimination.cpp b/llvm/lib/Transforms/Scalar/DeadStoreElimination.cpp
index 3af8ee7..b8cd38b 100644
--- a/llvm/lib/Transforms/Scalar/DeadStoreElimination.cpp
+++ b/llvm/lib/Transforms/Scalar/DeadStoreElimination.cpp
@@ -514,30 +514,50 @@ bool DSE::runOnBasicBlock(BasicBlock &BB) {
     if (!InstDep.isDef() && !InstDep.isClobber())
       continue;
 
-    // If we're storing the same value back to a pointer that we just
-    // loaded from, then the store can be removed.
+    // Check for cases where the store is redundant.
     if (StoreInst *SI = dyn_cast<StoreInst>(Inst)) {
+      bool DeleteStore = false;
+      // If we're storing the same value back to a pointer that we just
+      // loaded from, then the store can be removed.
       if (LoadInst *DepLoad = dyn_cast<LoadInst>(InstDep.getInst())) {
         if (SI->getPointerOperand() == DepLoad->getPointerOperand() &&
             SI->getOperand(0) == DepLoad && isRemovable(SI)) {
           DEBUG(dbgs() << "DSE: Remove Store Of Load from same pointer:\n  "
                        << "LOAD: " << *DepLoad << "\n  STORE: " << *SI << '\n');
+          DeleteStore = true;
+        }
+      }
 
-          // DeleteDeadInstruction can delete the current instruction.  Save BBI
-          // in case we need it.
-          WeakVH NextInst(BBI);
-
-          DeleteDeadInstruction(SI, *MD, TLI);
-
-          if (!NextInst)  // Next instruction deleted.
-            BBI = BB.begin();
-          else if (BBI != BB.begin())  // Revisit this instruction if possible.
-            --BBI;
-          ++NumFastStores;
-          MadeChange = true;
-          continue;
+      // If we find a store to memory which was defined by calloc
+      // we can remove the store if the value being stored is a
+      // constant zero (since calloc initialized the memory to
+      // that same value) or the store is undefined (if out of
+      // bounds).  
+      if (isCallocLikeFn(InstDep.getInst(), TLI) && isRemovable(SI)) {
+        Value *V = SI->getValueOperand();
+        if (isa<Constant>(V) && cast<Constant>(V)->isNullValue()) {
+          DEBUG(dbgs() << "DSE: Remove Store Of Zero to Calloc:\n  "
+                << "CALLOC: " << *InstDep.getInst() << "\n"
+                << "STORE: " << *SI << '\n');
+          DeleteStore = true;
         }
       }
+
+      if (DeleteStore) {
+        // DeleteDeadInstruction can delete the current instruction.  Save BBI
+        // in case we need it.
+        WeakVH NextInst(BBI);
+        
+        DeleteDeadInstruction(SI, *MD, TLI);
+        
+        if (!NextInst)  // Next instruction deleted.
+          BBI = BB.begin();
+        else if (BBI != BB.begin())  // Revisit this instruction if possible.
+          --BBI;
+        ++NumFastStores;
+        MadeChange = true;
+        continue;
+      }
     }
 
     // Figure out what location is being stored to.
diff --git a/llvm/test/Transforms/DeadStoreElimination/calloc-store.ll b/llvm/test/Transforms/DeadStoreElimination/calloc-store.ll
new file mode 100644
index 0000000..32b62a0
--- /dev/null
+++ b/llvm/test/Transforms/DeadStoreElimination/calloc-store.ll
@@ -0,0 +1,17 @@
+; RUN: opt < %s -basicaa -dse -S | FileCheck %s
+
+target datalayout = "e-p:64:64:64-i1:8:8-i8:8:8-i16:16:16-i32:32:32-i64:64:64-f32:32:32-f64:64:64-v64:64:64-v128:128:128-a0:0:64-s0:64:64-f80:128:128-n8:16:32:64-S128"
+
+; Function Attrs: nounwind
+declare noalias i8* @calloc(i64, i64)
+
+; Function Attrs: nounwind uwtable
+define noalias i32* @test_store() {
+; CHECK-LABEL: test_store
+  %1 = tail call noalias i8* @calloc(i64 1, i64 4)
+  %2 = bitcast i8* %1 to i32*
+  ; This store is dead and should be removed
+  store i32 0, i32* %2, align 4
+; CHECK-NOT: store i32 0, i32* %2, align 4
+  ret i32* %2
+}
-- 
2.7.4