[llvm] r320480 - [InstComineLoadStoreAlloca] Optimize stores to GEP off null base

[Anna Thomas via llvm-commits]

Author: annat
Date: Tue Dec 12 06:12:33 2017
New Revision: 320480

URL: http://llvm.org/viewvc/llvm-project?rev=320480&view=rev
Log:
[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

Modified:
    llvm/trunk/lib/Transforms/InstCombine/InstCombineLoadStoreAlloca.cpp
    llvm/trunk/test/Transforms/InstCombine/store.ll

Modified: llvm/trunk/lib/Transforms/InstCombine/InstCombineLoadStoreAlloca.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Transforms/InstCombine/InstCombineLoadStoreAlloca.cpp?rev=320480&r1=320479&r2=320480&view=diff
==============================================================================
--- llvm/trunk/lib/Transforms/InstCombine/InstCombineLoadStoreAlloca.cpp (original)
+++ llvm/trunk/lib/Transforms/InstCombine/InstCombineLoadStoreAlloca.cpp Tue Dec 12 06:12:33 2017
@@ -959,6 +959,16 @@ static Instruction *replaceGEPIdxWithZer
   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::visitStoreIns
   }
 
   // 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))

Modified: llvm/trunk/test/Transforms/InstCombine/store.ll
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Transforms/InstCombine/store.ll?rev=320480&r1=320479&r2=320480&view=diff
==============================================================================
--- llvm/trunk/test/Transforms/InstCombine/store.ll (original)
+++ llvm/trunk/test/Transforms/InstCombine/store.ll Tue Dec 12 06:12:33 2017
@@ -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"