[BasicAA] Always strip single-argument phi nodes

We can always look through single-argument (LCSSA) phi nodes when
performing alias analysis. getUnderlyingObject() already does this,
but stripPointerCastsAndInvariantGroups() does not. We still look
through these phi nodes with the usual aliasPhi() logic, but
sometimes get sub-optimal results due to the restrictions on value
equivalence when looking through arbitrary phi nodes. I think it's
generally beneficial to keep the underlying object logic and the
pointer cast stripping logic in sync, insofar as it is possible.

With this patch we get marginally better results:

  aa.NumMayAlias | 5010069 | 5009861
  aa.NumMustAlias | 347518 | 347674
  aa.NumNoAlias | 27201336 | 27201528
  ...
  licm.NumPromoted | 1293 | 1296

I've renamed the relevant strip method to stripPointerCastsForAliasAnalysis(),
as we're past the point where we can explicitly spell out everything
that's getting stripped.

Differential Revision: https://reviews.llvm.org/D96668

diff --git a/llvm/include/llvm/IR/Value.h b/llvm/include/llvm/IR/Value.h
index 01dfdf3..5a7e90a 100644 (file)
--- a/llvm/include/llvm/IR/Value.h
+++ b/llvm/include/llvm/IR/Value.h
@@ -654,15 +654,16 @@ public:
                                    ->stripPointerCastsSameRepresentation());
   }
 
-  /// Strip off pointer casts, all-zero GEPs and invariant group info.
+  /// Strip off pointer casts, all-zero GEPs, single-argument phi nodes and
+  /// invariant group info.
   ///
   /// Returns the original uncasted value.  If this is called on a non-pointer
   /// value, it returns 'this'. This function should be used only in
   /// Alias analysis.
-  const Value *stripPointerCastsAndInvariantGroups() const;
-  Value *stripPointerCastsAndInvariantGroups() {
+  const Value *stripPointerCastsForAliasAnalysis() const;
+  Value *stripPointerCastsForAliasAnalysis() {
     return const_cast<Value *>(static_cast<const Value *>(this)
-                                   ->stripPointerCastsAndInvariantGroups());
+                                   ->stripPointerCastsForAliasAnalysis());
   }
 
   /// Strip off pointer casts and all-constant inbounds GEPs.

diff --git a/llvm/lib/Analysis/BasicAliasAnalysis.cpp b/llvm/lib/Analysis/BasicAliasAnalysis.cpp
index ac3a327..81996f3 100644 (file)
--- a/llvm/lib/Analysis/BasicAliasAnalysis.cpp
+++ b/llvm/lib/Analysis/BasicAliasAnalysis.cpp
@@ -1470,8 +1470,8 @@ AliasResult BasicAAResult::aliasCheck(const Value *V1, LocationSize V1Size,
     return NoAlias;
 
   // Strip off any casts if they exist.
-  V1 = V1->stripPointerCastsAndInvariantGroups();
-  V2 = V2->stripPointerCastsAndInvariantGroups();
+  V1 = V1->stripPointerCastsForAliasAnalysis();
+  V2 = V2->stripPointerCastsForAliasAnalysis();
 
   // If V1 or V2 is undef, the result is NoAlias because we can always pick a
   // value for undef that aliases nothing in the program.

diff --git a/llvm/lib/Analysis/GlobalsModRef.cpp b/llvm/lib/Analysis/GlobalsModRef.cpp
index 145baf8..aafc19a 100644 (file)
--- a/llvm/lib/Analysis/GlobalsModRef.cpp
+++ b/llvm/lib/Analysis/GlobalsModRef.cpp
@@ -828,9 +828,9 @@ AliasResult GlobalsAAResult::alias(const MemoryLocation &LocA,
                                    AAQueryInfo &AAQI) {
   // Get the base object these pointers point to.
   const Value *UV1 =
-      getUnderlyingObject(LocA.Ptr->stripPointerCastsAndInvariantGroups());
+      getUnderlyingObject(LocA.Ptr->stripPointerCastsForAliasAnalysis());
   const Value *UV2 =
-      getUnderlyingObject(LocB.Ptr->stripPointerCastsAndInvariantGroups());
+      getUnderlyingObject(LocB.Ptr->stripPointerCastsForAliasAnalysis());
 
   // If either of the underlying values is a global, they may be non-addr-taken
   // globals, which we can answer queries about.

diff --git a/llvm/lib/IR/Value.cpp b/llvm/lib/IR/Value.cpp
index 572f37a..92ffae1 100644 (file)
--- a/llvm/lib/IR/Value.cpp
+++ b/llvm/lib/IR/Value.cpp
@@ -551,7 +551,7 @@ enum PointerStripKind {
   PSK_ZeroIndices,
   PSK_ZeroIndicesAndAliases,
   PSK_ZeroIndicesSameRepresentation,
-  PSK_ZeroIndicesAndInvariantGroups,
+  PSK_ForAliasAnalysis,
   PSK_InBoundsConstantIndices,
   PSK_InBounds
 };
@@ -577,7 +577,7 @@ static const Value *stripPointerCastsAndOffsets(
       case PSK_ZeroIndices:
       case PSK_ZeroIndicesAndAliases:
       case PSK_ZeroIndicesSameRepresentation:
-      case PSK_ZeroIndicesAndInvariantGroups:
+      case PSK_ForAliasAnalysis:
         if (!GEP->hasAllZeroIndices())
           return V;
         break;
@@ -602,6 +602,9 @@ static const Value *stripPointerCastsAndOffsets(
       V = cast<Operator>(V)->getOperand(0);
     } else if (StripKind == PSK_ZeroIndicesAndAliases && isa<GlobalAlias>(V)) {
       V = cast<GlobalAlias>(V)->getAliasee();
+    } else if (StripKind == PSK_ForAliasAnalysis && isa<PHINode>(V) &&
+               cast<PHINode>(V)->getNumIncomingValues() == 1) {
+      V = cast<PHINode>(V)->getIncomingValue(0);
     } else {
       if (const auto *Call = dyn_cast<CallBase>(V)) {
         if (const Value *RV = Call->getReturnedArgOperand()) {
@@ -611,7 +614,7 @@ static const Value *stripPointerCastsAndOffsets(
         // The result of launder.invariant.group must alias it's argument,
         // but it can't be marked with returned attribute, that's why it needs
         // special case.
-        if (StripKind == PSK_ZeroIndicesAndInvariantGroups &&
+        if (StripKind == PSK_ForAliasAnalysis &&
             (Call->getIntrinsicID() == Intrinsic::launder_invariant_group ||
              Call->getIntrinsicID() == Intrinsic::strip_invariant_group)) {
           V = Call->getArgOperand(0);
@@ -643,8 +646,8 @@ const Value *Value::stripInBoundsConstantOffsets() const {
   return stripPointerCastsAndOffsets<PSK_InBoundsConstantIndices>(this);
 }
 
-const Value *Value::stripPointerCastsAndInvariantGroups() const {
-  return stripPointerCastsAndOffsets<PSK_ZeroIndicesAndInvariantGroups>(this);
+const Value *Value::stripPointerCastsForAliasAnalysis() const {
+  return stripPointerCastsAndOffsets<PSK_ForAliasAnalysis>(this);
 }
 
 const Value *Value::stripAndAccumulateConstantOffsets(

diff --git a/llvm/lib/Target/AMDGPU/AMDGPUAliasAnalysis.cpp b/llvm/lib/Target/AMDGPU/AMDGPUAliasAnalysis.cpp
index 0ed89e9..31e1767 100644 (file)
--- a/llvm/lib/Target/AMDGPU/AMDGPUAliasAnalysis.cpp
+++ b/llvm/lib/Target/AMDGPU/AMDGPUAliasAnalysis.cpp
@@ -87,7 +87,7 @@ AliasResult AMDGPUAAResult::alias(const MemoryLocation &LocA,
   if (asA == AMDGPUAS::FLAT_ADDRESS &&
       (asB == AMDGPUAS::LOCAL_ADDRESS || asB == AMDGPUAS::PRIVATE_ADDRESS)) {
     const auto *ObjA =
-        getUnderlyingObject(A.Ptr->stripPointerCastsAndInvariantGroups());
+        getUnderlyingObject(A.Ptr->stripPointerCastsForAliasAnalysis());
     if (const LoadInst *LI = dyn_cast<LoadInst>(ObjA)) {
       // If a generic pointer is loaded from the constant address space, it
       // could only be a GLOBAL or CONSTANT one as that address space is soley

diff --git a/llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp b/llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp
index 99524f5..df8fade 100644 (file)
--- a/llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp
@@ -390,7 +390,13 @@ static Instruction *simplifyInvariantGroupIntrinsic(IntrinsicInst &II,
                                                     InstCombinerImpl &IC) {
   auto *Arg = II.getArgOperand(0);
   auto *StrippedArg = Arg->stripPointerCasts();
-  auto *StrippedInvariantGroupsArg = Arg->stripPointerCastsAndInvariantGroups();
+  auto *StrippedInvariantGroupsArg = StrippedArg;
+  while (auto *Intr = dyn_cast<IntrinsicInst>(StrippedInvariantGroupsArg)) {
+    if (Intr->getIntrinsicID() != Intrinsic::launder_invariant_group &&
+        Intr->getIntrinsicID() != Intrinsic::strip_invariant_group)
+      break;
+    StrippedInvariantGroupsArg = Intr->getArgOperand(0)->stripPointerCasts();
+  }
   if (StrippedArg == StrippedInvariantGroupsArg)
     return nullptr; // No launders/strips to remove.
 

diff --git a/llvm/test/Analysis/BasicAA/phi-aa.ll b/llvm/test/Analysis/BasicAA/phi-aa.ll
index 39a3180..357fd49 100644 (file)
--- a/llvm/test/Analysis/BasicAA/phi-aa.ll
+++ b/llvm/test/Analysis/BasicAA/phi-aa.ll
@@ -199,9 +199,9 @@ for.body:                                         ; preds = %for.body, %entry
 } 
 
 ; CHECK-LABEL: single_arg_phi
-; CHECK: MayAlias: i32* %ptr, i32* %ptr.next
+; CHECK: NoAlias: i32* %ptr, i32* %ptr.next
+; CHECK: MustAlias: i32* %ptr, i32* %ptr.phi
 ; CHECK: MustAlias: i32* %ptr.next, i32* %ptr.next.phi
-; TODO: The first one could be MustAlias as well.
 define void @single_arg_phi(i32* %ptr.base) {
 entry:
   br label %loop