BasicAA: GEPs of NoAlias'ing base ptr with equivalent indices are NoAlias

If we can show that the base pointers of two GEPs don't alias each other using
precise analysis and the indices and base offset are equal then the two GEPs
also don't alias each other.
This is primarily needed for the follow up patch that analyses NoAlias'ing PHI
nodes.

Part 1/2 of fix for PR13564.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@163317 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 65 insertions, 9 deletions

diff --git a/lib/Analysis/BasicAliasAnalysis.cpp b/lib/Analysis/BasicAliasAnalysis.cpp
index a3e6575865..d537d63317 100644
--- a/lib/Analysis/BasicAliasAnalysis.cpp
+++ b/lib/Analysis/BasicAliasAnalysis.cpp
@@ -128,6 +128,15 @@ namespace {
     const Value *V;
     ExtensionKind Extension;
     int64_t Scale;
+
+    bool operator==(const VariableGEPIndex &Other) const {
+      return V == Other.V && Extension == Other.Extension &&
+        Scale == Other.Scale;
+    }
+
+    bool operator!=(const VariableGEPIndex &Other) const {
+      return !operator==(Other);
+    }
   };
 }
 
@@ -490,6 +499,7 @@ namespace {
     // aliasGEP - Provide a bunch of ad-hoc rules to disambiguate a GEP
     // instruction against another.
     AliasResult aliasGEP(const GEPOperator *V1, uint64_t V1Size,
+                         const MDNode *V1TBAAInfo,
                          const Value *V2, uint64_t V2Size,
                          const MDNode *V2TBAAInfo,
                          const Value *UnderlyingV1, const Value *UnderlyingV2);
@@ -807,6 +817,21 @@ BasicAliasAnalysis::getModRefInfo(ImmutableCallSite CS,
   return ModRefResult(AliasAnalysis::getModRefInfo(CS, Loc) & Min);
 }
 
+static bool areVarIndicesEqual(SmallVector<VariableGEPIndex, 4> &Indices1,
+                               SmallVector<VariableGEPIndex, 4> &Indices2) {
+  unsigned Size1 = Indices1.size();
+  unsigned Size2 = Indices2.size();
+
+  if (Size1 != Size2)
+    return false;
+
+  for (unsigned I = 0; I != Size1; ++I)
+    if (Indices1[I] != Indices2[I])
+      return false;
+
+  return true;
+}
+
 /// aliasGEP - Provide a bunch of ad-hoc rules to disambiguate a GEP instruction
 /// against another pointer.  We know that V1 is a GEP, but we don't know
 /// anything about V2.  UnderlyingV1 is GetUnderlyingObject(GEP1, TD),
@@ -814,6 +839,7 @@ BasicAliasAnalysis::getModRefInfo(ImmutableCallSite CS,
 ///
 AliasAnalysis::AliasResult
 BasicAliasAnalysis::aliasGEP(const GEPOperator *GEP1, uint64_t V1Size,
+                             const MDNode *V1TBAAInfo,
                              const Value *V2, uint64_t V2Size,
                              const MDNode *V2TBAAInfo,
                              const Value *UnderlyingV1,
@@ -821,9 +847,41 @@ BasicAliasAnalysis::aliasGEP(const GEPOperator *GEP1, uint64_t V1Size,
   int64_t GEP1BaseOffset;
   SmallVector<VariableGEPIndex, 4> GEP1VariableIndices;
 
-  // If we have two gep instructions with must-alias'ing base pointers, figure
-  // out if the indexes to the GEP tell us anything about the derived pointer.
+  // If we have two gep instructions with must-alias or not-alias'ing base
+  // pointers, figure out if the indexes to the GEP tell us anything about the
+  // derived pointer.
   if (const GEPOperator *GEP2 = dyn_cast<GEPOperator>(V2)) {
+    // Check for geps of non-aliasing underlying pointers where the offsets are
+    // identical.
+    if (V1Size == V2Size) {
+      // Do the base pointers alias assuming type and size.
+      AliasResult PreciseBaseAlias = aliasCheck(UnderlyingV1, V1Size,
+                                                V1TBAAInfo, UnderlyingV2,
+                                                V2Size, V2TBAAInfo);
+      if (PreciseBaseAlias == NoAlias) {
+        // See if the computed offset from the common pointer tells us about the
+        // relation of the resulting pointer.
+        int64_t GEP2BaseOffset;
+        SmallVector<VariableGEPIndex, 4> GEP2VariableIndices;
+        const Value *GEP2BasePtr =
+          DecomposeGEPExpression(GEP2, GEP2BaseOffset, GEP2VariableIndices, TD);
+        const Value *GEP1BasePtr =
+          DecomposeGEPExpression(GEP1, GEP1BaseOffset, GEP1VariableIndices, TD);
+        // DecomposeGEPExpression and GetUnderlyingObject should return the
+        // same result except when DecomposeGEPExpression has no TargetData.
+        if (GEP1BasePtr != UnderlyingV1 || GEP2BasePtr != UnderlyingV2) {
+          assert(TD == 0 &&
+             "DecomposeGEPExpression and GetUnderlyingObject disagree!");
+          return MayAlias;
+        }
+        // Same offsets.
+        if (GEP1BaseOffset == GEP2BaseOffset &&
+            areVarIndicesEqual(GEP1VariableIndices, GEP2VariableIndices))
+          return NoAlias;
+        GEP1VariableIndices.clear();
+      }
+    }
+
     // Do the base pointers alias?
     AliasResult BaseAlias = aliasCheck(UnderlyingV1, UnknownSize, 0,
                                        UnderlyingV2, UnknownSize, 0);
@@ -843,9 +901,8 @@ BasicAliasAnalysis::aliasGEP(const GEPOperator *GEP1, uint64_t V1Size,
     const Value *GEP2BasePtr =
       DecomposeGEPExpression(GEP2, GEP2BaseOffset, GEP2VariableIndices, TD);
     
-    // If DecomposeGEPExpression isn't able to look all the way through the
-    // addressing operation, we must not have TD and this is too complex for us
-    // to handle without it.
+    // DecomposeGEPExpression and GetUnderlyingObject should return the
+    // same result except when DecomposeGEPExpression has no TargetData.
     if (GEP1BasePtr != UnderlyingV1 || GEP2BasePtr != UnderlyingV2) {
       assert(TD == 0 &&
              "DecomposeGEPExpression and GetUnderlyingObject disagree!");
@@ -879,9 +936,8 @@ BasicAliasAnalysis::aliasGEP(const GEPOperator *GEP1, uint64_t V1Size,
     const Value *GEP1BasePtr =
       DecomposeGEPExpression(GEP1, GEP1BaseOffset, GEP1VariableIndices, TD);
     
-    // If DecomposeGEPExpression isn't able to look all the way through the
-    // addressing operation, we must not have TD and this is too complex for us
-    // to handle without it.
+    // DecomposeGEPExpression and GetUnderlyingObject should return the
+    // same result except when DecomposeGEPExpression has no TargetData.
     if (GEP1BasePtr != UnderlyingV1) {
       assert(TD == 0 &&
              "DecomposeGEPExpression and GetUnderlyingObject disagree!");
@@ -1156,7 +1212,7 @@ BasicAliasAnalysis::aliasCheck(const Value *V1, uint64_t V1Size,
     std::swap(O1, O2);
   }
   if (const GEPOperator *GV1 = dyn_cast<GEPOperator>(V1)) {
-    AliasResult Result = aliasGEP(GV1, V1Size, V2, V2Size, V2TBAAInfo, O1, O2);
+    AliasResult Result = aliasGEP(GV1, V1Size, V1TBAAInfo, V2, V2Size, V2TBAAInfo, O1, O2);
     if (Result != MayAlias) return AliasCache[Locs] = Result;
   }