[InstCombine] Fold PHIs with equal incoming pointers

In case when all incoming values of a PHI are equal pointers, this
transformation inserts a definition of such a pointer right after
definition of the base pointer and replaces with this value both PHI and
all it's incoming pointers. Primary goal of this transformation is
canonicalization of this pattern in order to enable optimizations that
can't handle PHIs. Non-inbounds pointers aren't currently supported.

Reviewers: spatel, RKSimon, lebedev.ri, apilipenko

Reviewed By: apilipenko

Tags: #llvm

Subscribers: hiraditya, llvm-commits

Differential Revision: https://reviews.llvm.org/D68128

llvm/lib/Transforms/InstCombine/InstCombineInternal.h

@@ -895,6 +895,11 @@ class LLVM_LIBRARY_VISIBILITY InstCombiner
   /// insert a new pointer typed PHI and replace the original one.
   Instruction *FoldIntegerTypedPHI(PHINode &PN);
 
+  /// If all incoming values of a pointer typed PHI are pointers with the same
+  /// base and offset, replace the PHI and all incoming values with one
+  /// definition of such pointer.
+  Instruction *FoldPHIWithEqualPointers(PHINode &PN);
+
   /// Helper function for FoldPHIArgXIntoPHI() to set debug location for the
   /// folded operation.
   void PHIArgMergedDebugLoc(Instruction *Inst, PHINode &PN);

llvm/lib/Transforms/InstCombine/InstCombinePHI.cpp

@@ -1122,6 +1122,63 @@ Instruction *InstCombiner::SliceUpIllegalIntegerPHI(PHINode &FirstPhi) {
   return replaceInstUsesWith(FirstPhi, Undef);
 }
 
+Instruction *InstCombiner::FoldPHIWithEqualPointers(PHINode &PN) {
+  auto *PhiTy = dyn_cast<PointerType>(PN.getType());
+  if (!PhiTy)
+    return nullptr;
+
+  // Make sure all incoming pointers have the same base pointers and offsets.
+  // Also, make sure no addrspacecasts involved.
+  // Note: only inbounds GEPs are supported!
+  const DataLayout &DL = PN.getModule()->getDataLayout();
+  Value *FirstValue = PN.getIncomingValue(0);
+  int64_t Offset;
+  Value *Base = GetPointerBaseWithConstantOffset(
+      FirstValue, Offset, DL, /* AllowNonInbounds */ false);
+
+  auto *BaseTy = cast<PointerType>(Base->getType());
+  if (BaseTy->getAddressSpace() != PhiTy->getAddressSpace())
+    return nullptr;
+
+  for (Use &Incoming : PN.incoming_values()) {
+    if (!isa<Instruction>(Incoming))
+      return nullptr;
+    int64_t CurrentOffset;
+    Value *CurrentBase = GetPointerBaseWithConstantOffset(
+        Incoming, CurrentOffset, DL, /* AllowNonInbounds */ false);
+    if (CurrentBase != Base || CurrentOffset != Offset)
+      return nullptr;
+  }
+
+  Instruction *InsertPt = nullptr;
+  if (auto *BaseInst = dyn_cast<Instruction>(Base)) {
+    if (isa<PHINode>(BaseInst)) {
+      BasicBlock *InsertBB = BaseInst->getParent();
+      BasicBlock::iterator InsertPtIter = InsertBB->getFirstInsertionPt();
+      // Make sure the insertion point exists. At the moment the only reason why
+      // insertion point may not exist is EHPad being a terminator. This check
+      // is a bit more future-proof than just `if (!TI->isEHPad())`.
+      if (InsertPtIter != InsertBB->end())
+        InsertPt = &*InsertPtIter;
+    } else
+      InsertPt = BaseInst->getNextNode();
+  } else
+    InsertPt = &*PN.getFunction()->getEntryBlock().getFirstInsertionPt();
+
+  if (!InsertPt)
+    return nullptr;
+
+  Builder.SetInsertPoint(InsertPt);
+  Type *I8PtrTy = Builder.getInt8PtrTy(PhiTy->getAddressSpace());
+  Value *BaseI8Ptr = Builder.CreateBitCast(Base, I8PtrTy);
+  Value *GEP = Builder.CreateConstInBoundsGEP1_64(BaseI8Ptr, Offset);
+  Value *GEPTyped = Builder.CreateBitCast(GEP, PhiTy);
+
+  for (Use &Incoming : PN.incoming_values())
+    replaceInstUsesWith(*cast<Instruction>(Incoming), GEPTyped);
+  return replaceInstUsesWith(PN, GEPTyped);
+}
+
 // PHINode simplification
 //
 Instruction *InstCombiner::visitPHINode(PHINode &PN) {
@@ -1143,6 +1200,9 @@ Instruction *InstCombiner::visitPHINode(PHINode &PN) {
     if (Instruction *Result = FoldPHIArgOpIntoPHI(PN))
       return Result;
 
+  if (Instruction *Result = FoldPHIWithEqualPointers(PN))
+    return Result;
+
   // If this is a trivial cycle in the PHI node graph, remove it.  Basically, if
   // this PHI only has a single use (a PHI), and if that PHI only has one use (a
   // PHI)... break the cycle.