[InstComineLoadStoreAlloca] Optimize stores to GEP off null base

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

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@320480 91177308-0d34-0410-b5e6-96231b3b80d8

diff --git a/lib/Transforms/InstCombine/InstCombineLoadStoreAlloca.cpp b/lib/Transforms/InstCombine/InstCombineLoadStoreAlloca.cpp
index 29e0a091aa706a41d31da737e0f5e595b5b0be29..01fc1528681b79450137e92d89f3592b518762d7 100644 (file)
--- a/lib/Transforms/InstCombine/InstCombineLoadStoreAlloca.cpp
+++ b/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/test/Transforms/InstCombine/store.ll b/test/Transforms/InstCombine/store.ll
index b8730413f1b51b2da9c9bb4a1c5a2e139958ee24..5bf3683d5ea97a1294bd8e49497e3bfd261bdc4b 100644 (file)
--- a/test/Transforms/InstCombine/store.ll
+++ b/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"