[InstComineLoadStoreAlloca] Optimize stores to GEP off null base

annamthomas · annamthomas · commit 2dd9835f351b · 2017-12-12T14:12:33.000Z
Summary: Currently, in InstCombineLoadStoreAlloca, we have simplification rules for the following cases: 1. load off a null 2. load off a GEP with null base 3. store to a null This patch adds support for the fourth case which is store into a GEP with null base. Since this is UB as well (and directly analogous to the load off a GEP with null base), we can substitute the stored val with undef in instcombine, so that SimplifyCFG can optimize this code into unreachable code. Note: Right now, simplifyCFG hasn't been taught about optimizing this to unreachable and adding an llvm.trap (this is already done for the above 3 cases). Reviewers: majnemer, hfinkel, sanjoy, davide Reviewed by: sanjoy, davide Subscribers: llvm-commits Differential Revision: https://reviews.llvm.org/D41026 llvm-svn: 320480
diff --git a/llvm/lib/Transforms/InstCombine/InstCombineLoadStoreAlloca.cpp b/llvm/lib/Transforms/InstCombine/InstCombineLoadStoreAlloca.cpp
@@ -959,6 +959,16 @@ static Instruction *replaceGEPIdxWithZero(InstCombiner &IC, Value *Ptr,
   return nullptr;
 }
 
+static bool canSimplifyNullStoreOrGEP(StoreInst &SI) {
+  if (SI.getPointerAddressSpace() != 0)
+    return false;
+
+  auto *Ptr = SI.getPointerOperand();
+  if (GetElementPtrInst *GEPI = dyn_cast<GetElementPtrInst>(Ptr))
+    Ptr = GEPI->getOperand(0);
+  return isa<ConstantPointerNull>(Ptr);
+}
+
 static bool canSimplifyNullLoadOrGEP(LoadInst &LI, Value *Op) {
   if (GetElementPtrInst *GEPI = dyn_cast<GetElementPtrInst>(Op)) {
     const Value *GEPI0 = GEPI->getOperand(0);
@@ -1447,7 +1457,8 @@ Instruction *InstCombiner::visitStoreInst(StoreInst &SI) {
   }
 
   // store X, null    -> turns into 'unreachable' in SimplifyCFG
-  if (isa<ConstantPointerNull>(Ptr) && SI.getPointerAddressSpace() == 0) {
+  // store X, GEP(null, Y) -> turns into 'unreachable' in SimplifyCFG
+  if (canSimplifyNullStoreOrGEP(SI)) {
     if (!isa<UndefValue>(Val)) {
       SI.setOperand(0, UndefValue::get(Val->getType()));
       if (Instruction *U = dyn_cast<Instruction>(Val))
diff --git a/llvm/test/Transforms/InstCombine/store.ll b/llvm/test/Transforms/InstCombine/store.ll
@@ -20,6 +20,14 @@ define void @test2(i32* %P) {
 ; CHECK-NEXT: ret void
 }
 
+define void @store_at_gep_off_null(i64 %offset) {
+; CHECK-LABEL: @store_at_gep_off_null
+; CHECK: store i32 undef, i32* %ptr
+   %ptr = getelementptr i32, i32 *null, i64 %offset
+   store i32 24, i32* %ptr
+   ret void
+}
+
 ;; Simple sinking tests
 
 ; "if then else"
