[BasicAA] Fix nsw handling for negated scales (PR63266)

We currently preserve the nsw flag when negating scales, which is
incorrect for INT_MIN.

However, just dropping the NSW flag in this case makes BasicAA
behavior unreliable and asymmetric, because we may or may not
drop the NSW flag depending on which side gets subtracted.

Instead, leave the Scale alone and add an additional IsNegated flag,
which indicates that the whole VarIndex should be interpreted as a
subtraction. This allows us to retain the NSW flag.

When accumulating the offset range, we need to use subtraction
instead of adding for IsNegated indices. Everything else works on
the absolute value of the scale, so the negation does not matter
there.

Fixes #63266.

Differential Revision: https://reviews.llvm.org/D153270

Author: nikic

llvm/lib/Analysis/BasicAliasAnalysis.cpp

@@ -461,6 +461,17 @@ struct VariableGEPIndex {
   /// True if all operations in this expression are NSW.
   bool IsNSW;
 
+  /// True if the index should be subtracted rather than added. We don't simply
+  /// negate the Scale, to avoid losing the NSW flag: X - INT_MIN*1 may be
+  /// non-wrapping, while X + INT_MIN*(-1) wraps.
+  bool IsNegated;
+
+  bool hasNegatedScaleOf(const VariableGEPIndex &Other) const {
+    if (IsNegated == Other.IsNegated)
+      return Scale == -Other.Scale;
+    return Scale == Other.Scale;
+  }
+
   void dump() const {
     print(dbgs());
     dbgs() << "\n";
@@ -470,7 +481,9 @@ struct VariableGEPIndex {
        << ", zextbits=" << Val.ZExtBits
        << ", sextbits=" << Val.SExtBits
        << ", truncbits=" << Val.TruncBits
-       << ", scale=" << Scale << ")";
+       << ", scale=" << Scale
+       << ", nsw=" << IsNSW
+       << ", negated=" << IsNegated << ")";
   }
 };
 }
@@ -659,7 +672,8 @@ BasicAAResult::DecomposeGEPExpression(const Value *V, const DataLayout &DL,
       Scale = adjustToIndexSize(Scale, IndexSize);
 
       if (!!Scale) {
-        VariableGEPIndex Entry = {LE.Val, Scale, CxtI, LE.IsNSW};
+        VariableGEPIndex Entry = {LE.Val, Scale, CxtI, LE.IsNSW,
+                                  /* IsNegated */ false};
         Decomposed.VarIndices.push_back(Entry);
       }
     }
@@ -1151,9 +1165,14 @@ AliasResult BasicAAResult::aliasGEP(
     assert(OffsetRange.getBitWidth() == Scale.getBitWidth() &&
            "Bit widths are normalized to MaxIndexSize");
     if (Index.IsNSW)
-      OffsetRange = OffsetRange.add(CR.smul_sat(ConstantRange(Scale)));
+      CR = CR.smul_sat(ConstantRange(Scale));
+    else
+      CR = CR.smul_fast(ConstantRange(Scale));
+
+    if (Index.IsNegated)
+      OffsetRange = OffsetRange.sub(CR);
     else
-      OffsetRange = OffsetRange.add(CR.smul_fast(ConstantRange(Scale)));
+      OffsetRange = OffsetRange.add(CR);
   }
 
   // We now have accesses at two offsets from the same base:
@@ -1220,7 +1239,7 @@ AliasResult BasicAAResult::aliasGEP(
     // 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 &&
+    if (Var0.hasNegatedScaleOf(Var1) && Var0.Val.TruncBits == 0 &&
         Var0.Val.hasSameCastsAs(Var1.Val) && !AAQI.MayBeCrossIteration &&
         isKnownNonEqual(Var0.Val.V, Var1.Val.V, DL, &AC, /* CxtI */ nullptr,
                         DT))
@@ -1701,6 +1720,13 @@ void BasicAAResult::subtractDecomposedGEPs(DecomposedGEP &DestGEP,
           !Dest.Val.hasSameCastsAs(Src.Val))
         continue;
 
+      // Normalize IsNegated if we're going to lose the NSW flag anyway.
+      if (Dest.IsNegated) {
+        Dest.Scale = -Dest.Scale;
+        Dest.IsNegated = false;
+        Dest.IsNSW = false;
+      }
+
       // If we found it, subtract off Scale V's from the entry in Dest.  If it
       // goes to zero, remove the entry.
       if (Dest.Scale != Src.Scale) {
@@ -1715,7 +1741,8 @@ void BasicAAResult::subtractDecomposedGEPs(DecomposedGEP &DestGEP,
 
     // If we didn't consume this entry, add it to the end of the Dest list.
     if (!Found) {
-      VariableGEPIndex Entry = {Src.Val, -Src.Scale, Src.CxtI, Src.IsNSW};
+      VariableGEPIndex Entry = {Src.Val, Src.Scale, Src.CxtI, Src.IsNSW,
+                                /* IsNegated */ true};
       DestGEP.VarIndices.push_back(Entry);
     }
   }
@@ -1737,7 +1764,7 @@ bool BasicAAResult::constantOffsetHeuristic(const DecomposedGEP &GEP,
   const VariableGEPIndex &Var0 = GEP.VarIndices[0], &Var1 = GEP.VarIndices[1];
 
   if (Var0.Val.TruncBits != 0 || !Var0.Val.hasSameCastsAs(Var1.Val) ||
-      Var0.Scale != -Var1.Scale ||
+      !Var0.hasNegatedScaleOf(Var1) ||
       Var0.Val.V->getType() != Var1.Val.V->getType())
     return false;
 

llvm/test/Analysis/BasicAA/range.ll

@@ -239,9 +239,8 @@ define void @benign_overflow(ptr %p, i64 %o) {
   ret void
 }
 
-; FIXME: This is a miscompile
 ; CHECK-LABEL: pr63266
-; CHECK: NoAlias:	i8* %gep2, i8* %offset16
+; CHECK: MayAlias:	i8* %gep2, i8* %offset16
 define void @pr63266(i1 %c, ptr %base) {
 entry:
   %offset16 = getelementptr inbounds i8, ptr %base, i64 16