[Loads] Handle simple cases with same base pointer with constant offsets in FindAvailableLoadedValue when AA is null.

Summary:
This will help with devirtualization (store forwarding with vtable pointers in
the presence of other stores into members in the constructor.) During inlining,
we don't have AA.

Reviewers: davidxl

Subscribers: mgorny, Prazek, hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D71307

lib/Analysis/Loads.cpp

@@ -381,6 +381,28 @@ Value *llvm::FindAvailableLoadedValue(LoadInst *Load,
       ScanFrom, MaxInstsToScan, AA, IsLoad, NumScanedInst);
 }
 
+// Check if the load and the store have the same base, constant offsets and
+// non-overlapping access ranges.
+static bool AreNonOverlapSameBaseLoadAndStore(
+    Value *LoadPtr, Type *LoadTy, Value *StorePtr, Type *StoreTy,
+    const DataLayout &DL) {
+  APInt LoadOffset(DL.getTypeSizeInBits(LoadPtr->getType()), 0);
+  APInt StoreOffset(DL.getTypeSizeInBits(StorePtr->getType()), 0);
+  Value *LoadBase = LoadPtr->stripAndAccumulateConstantOffsets(
+      DL, LoadOffset, /* AllowNonInbounds */ false);
+  Value *StoreBase = StorePtr->stripAndAccumulateConstantOffsets(
+      DL, StoreOffset, /* AllowNonInbounds */ false);
+  if (LoadBase != StoreBase)
+    return false;
+  auto LoadAccessSize = LocationSize::precise(DL.getTypeStoreSize(LoadTy));
+  auto StoreAccessSize = LocationSize::precise(DL.getTypeStoreSize(StoreTy));
+  ConstantRange LoadRange(LoadOffset,
+                          LoadOffset + LoadAccessSize.toRaw());
+  ConstantRange StoreRange(StoreOffset,
+                           StoreOffset + StoreAccessSize.toRaw());
+  return LoadRange.intersectWith(StoreRange).isEmptySet();
+}
+
 Value *llvm::FindAvailablePtrLoadStore(Value *Ptr, Type *AccessTy,
                                        bool AtLeastAtomic, BasicBlock *ScanBB,
                                        BasicBlock::iterator &ScanFrom,
@@ -459,10 +481,21 @@ Value *llvm::FindAvailablePtrLoadStore(Value *Ptr, Type *AccessTy,
           StrippedPtr != StorePtr)
         continue;
 
-      // If we have alias analysis and it says the store won't modify the loaded
-      // value, ignore the store.
-      if (AA && !isModSet(AA->getModRefInfo(SI, StrippedPtr, AccessSize)))
-        continue;
+      if (!AA) {
+        // When AA isn't available, but if the load and the store have the same
+        // base, constant offsets and non-overlapping access ranges, ignore the
+        // store. This is a simple form of alias analysis that is used by the
+        // inliner. FIXME: use BasicAA if possible.
+        if (AreNonOverlapSameBaseLoadAndStore(
+                Ptr, AccessTy, SI->getPointerOperand(),
+                SI->getValueOperand()->getType(), DL))
+          continue;
+      } else {
+        // If we have alias analysis and it says the store won't modify the
+        // loaded value, ignore the store.
+        if (!isModSet(AA->getModRefInfo(SI, StrippedPtr, AccessSize)))
+          continue;
+      }
 
       // Otherwise the store that may or may not alias the pointer, bail out.
       ++ScanFrom;

unittests/Analysis/CMakeLists.txt

@@ -20,6 +20,7 @@ add_llvm_unittest(AnalysisTests
   GlobalsModRefTest.cpp
   IVDescriptorsTest.cpp
   LazyCallGraphTest.cpp
+  LoadsTest.cpp
   LoopInfoTest.cpp
   MemoryBuiltinsTest.cpp
   MemorySSATest.cpp

unittests/Analysis/LoadsTest.cpp

@@ -0,0 +1,61 @@
+//===- LoadsTest.cpp - local load analysis unit tests ---------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "llvm/Analysis/Loads.h"
+#include "llvm/AsmParser/Parser.h"
+#include "llvm/IR/Instructions.h"
+#include "llvm/IR/LLVMContext.h"
+#include "llvm/IR/Module.h"
+#include "llvm/Support/SourceMgr.h"
+#include "gtest/gtest.h"
+
+using namespace llvm;
+
+static std::unique_ptr<Module> parseIR(LLVMContext &C, const char *IR) {
+  SMDiagnostic Err;
+  std::unique_ptr<Module> Mod = parseAssemblyString(IR, Err, C);
+  if (!Mod)
+    Err.print("AnalysisTests", errs());
+  return Mod;
+}
+
+TEST(LoadsTest, FindAvailableLoadedValueSameBasePtrConstantOffsetsNullAA) {
+  LLVMContext C;
+  std::unique_ptr<Module> M = parseIR(C,
+                                      R"IR(
+%class = type <{ i32, i32 }>
+
+define i32 @f() {
+entry:
+  %o = alloca %class
+  %f1 = getelementptr inbounds %class, %class* %o, i32 0, i32 0
+  store i32 42, i32* %f1
+  %f2 = getelementptr inbounds %class, %class* %o, i32 0, i32 1
+  store i32 43, i32* %f2
+  %v = load i32, i32* %f1
+  ret i32 %v
+}
+)IR");
+  auto *GV = M->getNamedValue("f");
+  ASSERT_TRUE(GV);
+  auto *F = dyn_cast<Function>(GV);
+  ASSERT_TRUE(F);
+  Instruction *Inst = &F->front().front();
+  auto *AI = dyn_cast<AllocaInst>(Inst);
+  ASSERT_TRUE(AI);
+  Inst = &*++F->front().rbegin();
+  auto *LI = dyn_cast<LoadInst>(Inst);
+  ASSERT_TRUE(LI);
+  BasicBlock::iterator BBI(LI);
+  Value *Loaded = FindAvailableLoadedValue(
+      LI, LI->getParent(), BBI, 0, nullptr, nullptr);
+  ASSERT_TRUE(Loaded);
+  auto *CI = dyn_cast<ConstantInt>(Loaded);
+  ASSERT_TRUE(CI);
+  ASSERT_TRUE(CI->equalsInt(42));
+}