Revert "[AA] Teach BasicAA to recognize basic GEP range information."

We have found a miscompile with this change, reverting while working on a
reproducer.

This reverts commit 455b60ccfbfdbb5d2b652666050544c31e6673b1.

1 files changed, 5 insertions, 31 deletions

diff --git a/llvm/lib/Analysis/BasicAliasAnalysis.cpp b/llvm/lib/Analysis/BasicAliasAnalysis.cpp
index 4a54c33..6be3778 100644
--- a/llvm/lib/Analysis/BasicAliasAnalysis.cpp
+++ b/llvm/lib/Analysis/BasicAliasAnalysis.cpp
@@ -31,7 +31,6 @@
 #include "llvm/IR/Argument.h"
 #include "llvm/IR/Attributes.h"
 #include "llvm/IR/Constant.h"
-#include "llvm/IR/ConstantRange.h"
 #include "llvm/IR/Constants.h"
 #include "llvm/IR/DataLayout.h"
 #include "llvm/IR/DerivedTypes.h"
@@ -1317,24 +1316,13 @@ AliasResult BasicAAResult::aliasGEP(
       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 whether abs(VarIndex) > 0.
       Optional<APInt> MinAbsVarIndex;
-      Optional<ConstantRange> VarIndexRange;
       if (DecompGEP1.VarIndices.size() == 1) {
-        // VarIndex = Scale*V.
+        // VarIndex = Scale*V. If V != 0 then abs(VarIndex) >= abs(Scale).
         const VariableGEPIndex &Var = DecompGEP1.VarIndices[0];
-        if (isKnownNonZero(Var.Val.V, DL, 0, &AC, Var.CxtI, DT)) {
-          // If V != 0 then abs(VarIndex) >= abs(Scale).
+        if (isKnownNonZero(Var.Val.V, DL, 0, &AC, Var.CxtI, DT))
           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())
-                              .multiply(ConstantRange(Var.Scale));
       } else if (DecompGEP1.VarIndices.size() == 2) {
         // VarIndex = Scale*V0 + (-Scale)*V1.
         // If V0 != V1 then abs(VarIndex) >= abs(Scale).
@@ -1353,26 +1341,12 @@ AliasResult BasicAAResult::aliasGEP(
         // 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
+        // Check that an access at OffsetLo or lower, and an access at OffsetHi
+        // or higher both do not alias.
         if (OffsetLo.isNegative() && (-OffsetLo).uge(V1Size.getValue()) &&
             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))