[BasicAA] Make range check more precise

Make the range check more precise by calculating the range of
potentially accessed bytes for both accesses and checking whether
their intersection is empty. In that case there can be no overlap
between the accesses and the result is NoAlias.

This is more powerful than the previous approach, because it can
deal with sign-wrapped ranges. In the test case the original range
is [-1, INT_MAX] but becomes [0, INT_MIN] after applying the offset.
This is a wrapping range, so getSignedMin/getSignedMax will treat
it as a full range. However, the range excludes the elements
[INT_MIN+1, -1], which is enough to prove NoAlias with an access
at offset -1.

Differential Revision: https://reviews.llvm.org/D112486

Author: nikic

llvm/lib/Analysis/BasicAliasAnalysis.cpp

@@ -1252,7 +1252,7 @@ AliasResult BasicAAResult::aliasGEP(
     APInt GCD;
     bool AllNonNegative = DecompGEP1.Offset.isNonNegative();
     bool AllNonPositive = DecompGEP1.Offset.isNonPositive();
-    ConstantRange Range = ConstantRange(DecompGEP1.Offset);
+    ConstantRange OffsetRange = 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;
@@ -1277,12 +1277,12 @@ AliasResult BasicAAResult::aliasGEP(
                           (SignKnownOne && Scale.isNonNegative());
       }
 
-      assert(Range.getBitWidth() == Scale.getBitWidth() &&
+      assert(OffsetRange.getBitWidth() == Scale.getBitWidth() &&
              "Bit widths are normalized to MaxPointerSize");
-      Range = Range.add(Index.Val
+      OffsetRange = OffsetRange.add(Index.Val
                             .evaluateWith(computeConstantRange(
                                 Index.Val.V, true, &AC, Index.CxtI))
-                            .sextOrTrunc(Range.getBitWidth())
+                            .sextOrTrunc(OffsetRange.getBitWidth())
                             .smul_fast(ConstantRange(Scale)));
     }
 
@@ -1315,15 +1315,15 @@ AliasResult BasicAAResult::aliasGEP(
         (-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()))
+    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;
     }
 

llvm/test/Analysis/BasicAA/range.ll

@@ -196,13 +196,13 @@ define void @multiple(i32* %p, i32* %o1_ptr, i32* %o2_ptr) {
   ret void
 }
 
-; TODO: p.neg1 and p.o.1 don't alias, even though the addition o+1 may overflow.
+; p.neg1 and p.o.1 don't alias, even though the addition o+1 may overflow.
 ; While it makes INT_MIN a possible offset, offset -1 is not possible.
 ; CHECK-LABEL: Function: benign_overflow
 ; CHECK: MayAlias: i8* %p, i8* %p.o
 ; CHECK: MayAlias: i8* %p.neg1, i8* %p.o
 ; CHECK: MayAlias: i8* %p, i8* %p.o.1
-; CHECK: MayAlias: i8* %p.neg1, i8* %p.o.1
+; CHECK: NoAlias: i8* %p.neg1, i8* %p.o.1
 ; CHECK: NoAlias:  i8* %p.o, i8* %p.o.1
 define void @benign_overflow(i8* %p, i64 %o) {
   %c = icmp sge i64 %o, -1