[PATCH] D112378: [BasicAA] Use ranges for more than one index

[Nikita Popov via Phabricator via llvm-commits]

nikic created this revision.
nikic added reviewers: courbet, fhahn, jdoerfert, reames.
Herald added subscribers: jeroen.dobbelaere, arphaman, hiraditya.
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D109746 <https://reviews.llvm.org/D109746> made BasicAA use range information to determine the minimum/maximum GEP offset. However, it was limited to the case of a single variable index. This patch extends support to multiple indices by adding all the ranges together.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D112378

Files:
  llvm/lib/Analysis/BasicAliasAnalysis.cpp
  llvm/test/Analysis/BasicAA/range.ll


Index: llvm/test/Analysis/BasicAA/range.ll
===================================================================
--- llvm/test/Analysis/BasicAA/range.ll
+++ llvm/test/Analysis/BasicAA/range.ll
@@ -185,8 +185,7 @@
 ; CHECK: NoAlias:  i32* %p.01, i32* %p.2
 ; CHECK: MayAlias: i32* %p.02, i32* %p.2
 ; CHECK: NoAlias:  i32* %p.01, i32* %p.3
-; TODO: This can be NoAlias.
-; CHECK: MayAlias: i32* %p.02, i32* %p.3
+; CHECK: NoAlias:  i32* %p.02, i32* %p.3
 define void @multiple(i32* %p, i32* %o1_ptr, i32* %o2_ptr) {
   %o1 = load i32, i32* %o1_ptr, !range !0
   %o2 = load i32, i32* %o2_ptr, !range !0
Index: llvm/lib/Analysis/BasicAliasAnalysis.cpp
===================================================================
--- llvm/lib/Analysis/BasicAliasAnalysis.cpp
+++ llvm/lib/Analysis/BasicAliasAnalysis.cpp
@@ -1252,6 +1252,7 @@
     APInt GCD;
     bool AllNonNegative = DecompGEP1.Offset.isNonNegative();
     bool AllNonPositive = DecompGEP1.Offset.isNonPositive();
+    ConstantRange Range = ConstantRange(DecompGEP1.Offset);
     for (unsigned i = 0, e = DecompGEP1.VarIndices.size(); i != e; ++i) {
       const VariableGEPIndex &Index = DecompGEP1.VarIndices[i];
       const APInt &Scale = Index.Scale;
@@ -1275,6 +1276,11 @@
         AllNonPositive &= (SignKnownZero && Scale.isNonPositive()) ||
                           (SignKnownOne && Scale.isNonNegative());
       }
+
+      Range = Range.add(
+          Index.Val.evaluateWith(computeConstantRange(Index.Val.V, true, &AC,
+                                                      Index.CxtI))
+          .sextOrTrunc(Range.getBitWidth()).smul_fast(ConstantRange(Scale)));
     }
 
     // We now have accesses at two offsets from the same base:
@@ -1306,13 +1312,22 @@
         (-DecompGEP1.Offset).uge(V1Size.getValue()))
       return AliasResult::NoAlias;
 
+    if (!Range.isEmptySet()) {
+      // We know that Offset >= MinOffset.
+      // (MinOffset >= V2Size) => (Offset >= V2Size) => NoAlias.
+      if (V2Size.hasValue() && Range.getSignedMin().sge(V2Size.getValue()))
+        return AliasResult::NoAlias;
+
+      // We know that Offset <= MaxOffset.
+      // (MaxOffset <= -V1Size) => (Offset <= -V1Size) => NoAlias.
+      if (V1Size.hasValue() && Range.getSignedMax().sle(-V1Size.getValue()))
+        return AliasResult::NoAlias;
+    }
+
     if (V1Size.hasValue() && V2Size.hasValue()) {
-      // Try to determine the range of values for VarIndex.
-      // VarIndexRange is such that:
-      //    (VarIndex <= -MinAbsVarIndex || MinAbsVarIndex <= VarIndex) &&
-      //    VarIndexRange.contains(VarIndex)
+      // Try to determine the range of values for VarIndex such that
+      // VarIndex <= -MinAbsVarIndex || MinAbsVarIndex <= VarIndex.
       Optional<APInt> MinAbsVarIndex;
-      Optional<ConstantRange> VarIndexRange;
       if (DecompGEP1.VarIndices.size() == 1) {
         // VarIndex = Scale*V.
         const VariableGEPIndex &Var = DecompGEP1.VarIndices[0];
@@ -1321,11 +1336,6 @@
           // If V != 0 then abs(VarIndex) >= abs(Scale).
           MinAbsVarIndex = Var.Scale.abs();
         }
-        ConstantRange R = Var.Val.evaluateWith(
-            computeConstantRange(Var.Val.V, true, &AC, Var.CxtI));
-        if (!R.isFullSet() && !R.isEmptySet())
-          VarIndexRange = R.sextOrTrunc(Var.Scale.getBitWidth())
-                              .smul_fast(ConstantRange(Var.Scale));
       } else if (DecompGEP1.VarIndices.size() == 2) {
         // VarIndex = Scale*V0 + (-Scale)*V1.
         // If V0 != V1 then abs(VarIndex) >= abs(Scale).
@@ -1349,21 +1359,6 @@
             OffsetHi.isNonNegative() && OffsetHi.uge(V2Size.getValue()))
           return AliasResult::NoAlias;
       }
-
-      if (VarIndexRange) {
-        ConstantRange OffsetRange =
-            VarIndexRange->add(ConstantRange(DecompGEP1.Offset));
-
-        // We know that Offset >= MinOffset.
-        // (MinOffset >= V2Size) => (Offset >= V2Size) => NoAlias.
-        if (OffsetRange.getSignedMin().sge(V2Size.getValue()))
-          return AliasResult::NoAlias;
-
-        // We know that Offset <= MaxOffset.
-        // (MaxOffset <= -V1Size) => (Offset <= -V1Size) => NoAlias.
-        if (OffsetRange.getSignedMax().sle(-V1Size.getValue()))
-          return AliasResult::NoAlias;
-      }
     }
 
     if (constantOffsetHeuristic(DecompGEP1, V1Size, V2Size, &AC, DT))


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