[BasicAA] Check known access sizes earlier (NFC)

All heuristics for variable accesses require both access sizes to
be known, so check this once at the start, rather than for each
particular heuristic.

Author: nikic

llvm/lib/Analysis/BasicAliasAnalysis.cpp

@@ -1229,6 +1229,10 @@ AliasResult BasicAAResult::aliasGEP(
     return AliasResult::NoAlias;
   }
 
+  // We need to know both acess sizes for all the following heuristics.
+  if (!V1Size.hasValue() || !V2Size.hasValue())
+    return AliasResult::MayAlias;
+
   APInt GCD;
   ConstantRange OffsetRange = ConstantRange(DecompGEP1.Offset);
   for (unsigned i = 0, e = DecompGEP1.VarIndices.size(); i != e; ++i) {
@@ -1270,58 +1274,53 @@ AliasResult BasicAAResult::aliasGEP(
   APInt ModOffset = DecompGEP1.Offset.srem(GCD);
   if (ModOffset.isNegative())
     ModOffset += GCD; // We want mod, not rem.
-  if (V1Size.hasValue() && V2Size.hasValue() &&
-      ModOffset.uge(V2Size.getValue()) &&
+  if (ModOffset.uge(V2Size.getValue()) &&
       (GCD - ModOffset).uge(V1Size.getValue()))
     return AliasResult::NoAlias;
 
-  if (V1Size.hasValue() && V2Size.hasValue()) {
-    // Compute ranges of potentially accessed bytes for both accesses. If the
-    // interseciton is empty, there can be no overlap.
-    unsigned BW = OffsetRange.getBitWidth();
-    ConstantRange Range1 = OffsetRange.add(
-        ConstantRange(APInt(BW, 0), APInt(BW, V1Size.getValue())));
-    ConstantRange Range2 =
-        ConstantRange(APInt(BW, 0), APInt(BW, V2Size.getValue()));
-    if (Range1.intersectWith(Range2).isEmptySet())
-      return AliasResult::NoAlias;
-  }
+  // Compute ranges of potentially accessed bytes for both accesses. If the
+  // interseciton is empty, there can be no overlap.
+  unsigned BW = OffsetRange.getBitWidth();
+  ConstantRange Range1 = OffsetRange.add(
+      ConstantRange(APInt(BW, 0), APInt(BW, V1Size.getValue())));
+  ConstantRange Range2 =
+      ConstantRange(APInt(BW, 0), APInt(BW, V2Size.getValue()));
+  if (Range1.intersectWith(Range2).isEmptySet())
+    return AliasResult::NoAlias;
 
-  if (V1Size.hasValue() && V2Size.hasValue()) {
-    // Try to determine the range of values for VarIndex such that
-    // VarIndex <= -MinAbsVarIndex || MinAbsVarIndex <= VarIndex.
-    Optional<APInt> MinAbsVarIndex;
-    if (DecompGEP1.VarIndices.size() == 1) {
-      // VarIndex = Scale*V.
-      const VariableGEPIndex &Var = DecompGEP1.VarIndices[0];
-      if (Var.Val.TruncBits == 0 &&
-          isKnownNonZero(Var.Val.V, DL, 0, &AC, Var.CxtI, DT)) {
-        // If V != 0 then abs(VarIndex) >= abs(Scale).
-        MinAbsVarIndex = Var.Scale.abs();
-      }
-    } else if (DecompGEP1.VarIndices.size() == 2) {
-      // VarIndex = Scale*V0 + (-Scale)*V1.
-      // If V0 != V1 then abs(VarIndex) >= abs(Scale).
-      // Check that VisitedPhiBBs is empty, to avoid reasoning about
-      // inequality of values across loop iterations.
-      const VariableGEPIndex &Var0 = DecompGEP1.VarIndices[0];
-      const VariableGEPIndex &Var1 = DecompGEP1.VarIndices[1];
-      if (Var0.Scale == -Var1.Scale && Var0.Val.TruncBits == 0 &&
-          Var0.Val.hasSameCastsAs(Var1.Val) && VisitedPhiBBs.empty() &&
-          isKnownNonEqual(Var0.Val.V, Var1.Val.V, DL, &AC, /* CxtI */ nullptr,
-                          DT))
-        MinAbsVarIndex = Var0.Scale.abs();
+  // Try to determine the range of values for VarIndex such that
+  // VarIndex <= -MinAbsVarIndex || MinAbsVarIndex <= VarIndex.
+  Optional<APInt> MinAbsVarIndex;
+  if (DecompGEP1.VarIndices.size() == 1) {
+    // VarIndex = Scale*V.
+    const VariableGEPIndex &Var = DecompGEP1.VarIndices[0];
+    if (Var.Val.TruncBits == 0 &&
+        isKnownNonZero(Var.Val.V, DL, 0, &AC, Var.CxtI, DT)) {
+      // If V != 0 then abs(VarIndex) >= abs(Scale).
+      MinAbsVarIndex = Var.Scale.abs();
     }
+  } else if (DecompGEP1.VarIndices.size() == 2) {
+    // VarIndex = Scale*V0 + (-Scale)*V1.
+    // If V0 != V1 then abs(VarIndex) >= abs(Scale).
+    // Check that VisitedPhiBBs is empty, to avoid reasoning about
+    // inequality of values across loop iterations.
+    const VariableGEPIndex &Var0 = DecompGEP1.VarIndices[0];
+    const VariableGEPIndex &Var1 = DecompGEP1.VarIndices[1];
+    if (Var0.Scale == -Var1.Scale && Var0.Val.TruncBits == 0 &&
+        Var0.Val.hasSameCastsAs(Var1.Val) && VisitedPhiBBs.empty() &&
+        isKnownNonEqual(Var0.Val.V, Var1.Val.V, DL, &AC, /* CxtI */ nullptr,
+                        DT))
+      MinAbsVarIndex = Var0.Scale.abs();
+  }
 
-    if (MinAbsVarIndex) {
-      // The constant offset will have added at least +/-MinAbsVarIndex to it.
-      APInt OffsetLo = DecompGEP1.Offset - *MinAbsVarIndex;
-      APInt OffsetHi = DecompGEP1.Offset + *MinAbsVarIndex;
-      // We know that Offset <= OffsetLo || Offset >= OffsetHi
-      if (OffsetLo.isNegative() && (-OffsetLo).uge(V1Size.getValue()) &&
-          OffsetHi.isNonNegative() && OffsetHi.uge(V2Size.getValue()))
-        return AliasResult::NoAlias;
-    }
+  if (MinAbsVarIndex) {
+    // The constant offset will have added at least +/-MinAbsVarIndex to it.
+    APInt OffsetLo = DecompGEP1.Offset - *MinAbsVarIndex;
+    APInt OffsetHi = DecompGEP1.Offset + *MinAbsVarIndex;
+    // We know that Offset <= OffsetLo || Offset >= OffsetHi
+    if (OffsetLo.isNegative() && (-OffsetLo).uge(V1Size.getValue()) &&
+        OffsetHi.isNonNegative() && OffsetHi.uge(V2Size.getValue()))
+      return AliasResult::NoAlias;
   }
 
   if (constantOffsetHeuristic(DecompGEP1, V1Size, V2Size, &AC, DT))