[MSSA] Add special handling for invariant/constant loads.

This is a follow-up to r277637. It teaches MemorySSA that invariant
loads (and loads of provably constant memory) are always liveOnEntry.

llvm-svn: 277640

lib/Transforms/Utils/MemorySSA.cpp

@@ -170,6 +170,7 @@ template <> struct DenseMapInfo<MemoryLocOrCall> {
   }
 };
 }
+
 namespace {
 struct UpwardsMemoryQuery {
   // True if our original query started off as a call
@@ -251,6 +252,17 @@ static Reorderability getLoadReorderability(const LoadInst *Use,
   return Result;
 }
 
+static bool isUseTriviallyOptimizableToLiveOnEntry(AliasAnalysis &AA,
+                                                   const Instruction *I) {
+  // If the memory can't be changed, then loads of the memory can't be
+  // clobbered.
+  //
+  // FIXME: We should handle invariant groups, as well. It's a bit harder,
+  // because we need to pay close attention to invariant group barriers.
+  return isa<LoadInst>(I) && (I->getMetadata(LLVMContext::MD_invariant_load) ||
+                              AA.pointsToConstantMemory(I));
+}
+
 static bool instructionClobbersQuery(MemoryDef *MD,
                                      const MemoryLocation &UseLoc,
                                      const Instruction *UseInst,
@@ -1332,6 +1344,11 @@ void MemorySSA::OptimizeUses::optimizeUsesInBlock(
       continue;
     }
 
+    if (isUseTriviallyOptimizableToLiveOnEntry(*AA, MU->getMemoryInst())) {
+      MU->setDefiningAccess(MSSA->getLiveOnEntryDef());
+      continue;
+    }
+
     MemoryLocOrCall UseMLOC(MU);
     auto &LocInfo = LocStackInfo[UseMLOC];
     // If the pop epoch changed, it means we've removed stuff from top of
@@ -2246,6 +2263,12 @@ MemorySSA::CachingWalker::getClobberingMemoryAccess(MemoryAccess *MA) {
   if (auto *CacheResult = Cache.lookup(StartingAccess, Q.StartingLoc, Q.IsCall))
     return CacheResult;
 
+  if (isUseTriviallyOptimizableToLiveOnEntry(*MSSA->AA, I)) {
+    MemoryAccess *LiveOnEntry = MSSA->getLiveOnEntryDef();
+    Cache.insert(StartingAccess, LiveOnEntry, Q.StartingLoc, Q.IsCall);
+    return LiveOnEntry;
+  }
+
   // Start with the thing we already think clobbers this location
   MemoryAccess *DefiningAccess = StartingAccess->getDefiningAccess();
 

test/Transforms/Util/MemorySSA/load-invariant.ll

@@ -1,4 +1,3 @@
-; XFAIL: *
 ; RUN: opt -basicaa -print-memoryssa -verify-memoryssa -analyze < %s 2>&1 | FileCheck %s
 ; RUN: opt -aa-pipeline=basic-aa -passes='print<memoryssa>' -verify-memoryssa -disable-output < %s 2>&1 | FileCheck %s
 ;
@@ -12,6 +11,7 @@
 
 declare void @clobberAllTheThings()
 
+; CHECK-LABEL: define i32 @foo
 define i32 @foo() {
 ; CHECK: 1 = MemoryDef(liveOnEntry)
 ; CHECK-NEXT: call void @clobberAllTheThings()
@@ -22,4 +22,20 @@ define i32 @foo() {
   ret i32 %1
 }
 
+; CHECK-LABEL: define i32 @bar
+define i32 @bar(i32* %a) {
+; CHECK: 1 = MemoryDef(liveOnEntry)
+; CHECK-NEXT: call void @clobberAllTheThings()
+  call void @clobberAllTheThings()
+
+; CHECK: 2 = MemoryDef(1)
+; CHECK-NEXT: %1 = load atomic i32
+  %1 = load atomic i32, i32* %a acquire, align 4, !invariant.load !0
+
+; CHECK: MemoryUse(2)
+; CHECK-NEXT: %2 = load i32
+  %2 = load i32, i32* %a, align 4
+  ret i32 %2
+}
+
 !0 = !{}

unittests/Transforms/Utils/MemorySSA.cpp

@@ -418,3 +418,36 @@ TEST_F(MemorySSATest, PartialWalkerCacheWithPhis) {
   MemoryAccess *UseClobber = Walker->getClobberingMemoryAccess(ALoad);
   EXPECT_EQ(UseClobber, MSSA.getMemoryAccess(FirstStore));
 }
+
+// Test that our walker properly handles loads with the invariant group
+// attribute. It's a bit hacky, since we add the invariant attribute *after*
+// building MSSA. Otherwise, the use optimizer will optimize it for us, which
+// isn't what we want.
+// FIXME: It may be easier/cleaner to just add an 'optimize uses?' flag to MSSA.
+TEST_F(MemorySSATest, WalkerInvariantLoadOpt) {
+  F = Function::Create(FunctionType::get(B.getVoidTy(), {}, false),
+                       GlobalValue::ExternalLinkage, "F", &M);
+  B.SetInsertPoint(BasicBlock::Create(C, "", F));
+  Type *Int8 = Type::getInt8Ty(C);
+  Constant *One = ConstantInt::get(Int8, 1);
+  Value *AllocA = B.CreateAlloca(Int8, One, "");
+
+  Instruction *Store = B.CreateStore(One, AllocA);
+  Instruction *Load = B.CreateLoad(AllocA);
+
+  setupAnalyses();
+  MemorySSA &MSSA = Analyses->MSSA;
+  MemorySSAWalker *Walker = Analyses->Walker;
+
+  auto *LoadMA = cast<MemoryUse>(MSSA.getMemoryAccess(Load));
+  auto *StoreMA = cast<MemoryDef>(MSSA.getMemoryAccess(Store));
+  EXPECT_EQ(LoadMA->getDefiningAccess(), StoreMA);
+
+  // ...At the time of writing, no cache should exist for LoadMA. Be a bit
+  // flexible to future changes.
+  Walker->invalidateInfo(LoadMA);
+  Load->setMetadata(LLVMContext::MD_invariant_load, MDNode::get(C, {}));
+
+  MemoryAccess *LoadClobber = Walker->getClobberingMemoryAccess(LoadMA);
+  EXPECT_EQ(LoadClobber, MSSA.getLiveOnEntryDef());
+}