[BasicAA] Remove checks for GEP decomposition limit reached

The GEP aliasing code currently checks for the GEP decomposition
limit being reached (i.e., we did not reach the "final" underlying
object). As far as I can see, these checks are not necessary. It is
perfectly fine to work with a GEP whose base can still be further
decomposed.

Looking back through the commit history, these checks were originally
introduced in 1a44448. However, I
believe that the problem this was intended to address was later
properly fixed with 1726fc6, and
the checks are no longer necessary since then (and were not the
right fix in the first place).

Differential Revision: https://reviews.llvm.org/D91010

llvm/lib/Analysis/BasicAliasAnalysis.cpp

@@ -1259,10 +1259,8 @@ AliasResult BasicAAResult::aliasGEP(
   DecompGEP1.HasCompileTimeConstantScale =
       DecompGEP2.HasCompileTimeConstantScale = true;
 
-  bool GEP1MaxLookupReached =
-    DecomposeGEPExpression(GEP1, DecompGEP1, DL, &AC, DT);
-  bool GEP2MaxLookupReached =
-    DecomposeGEPExpression(V2, DecompGEP2, DL, &AC, DT);
+  DecomposeGEPExpression(GEP1, DecompGEP1, DL, &AC, DT);
+  DecomposeGEPExpression(V2, DecompGEP2, DL, &AC, DT);
 
   // Don't attempt to analyze the decomposed GEP if index scale is not a
   // compile-time constant.
@@ -1280,17 +1278,15 @@ AliasResult BasicAAResult::aliasGEP(
   // If the GEP's offset relative to its base is such that the base would
   // fall below the start of the object underlying V2, then the GEP and V2
   // cannot alias.
-  if (!GEP1MaxLookupReached && !GEP2MaxLookupReached &&
-      isGEPBaseAtNegativeOffset(GEP1, DecompGEP1, DecompGEP2, V2Size))
+  if (isGEPBaseAtNegativeOffset(GEP1, DecompGEP1, DecompGEP2, V2Size))
     return NoAlias;
   // If we have two gep instructions with must-alias or not-alias'ing base
   // pointers, figure out if the indexes to the GEP tell us anything about the
   // derived pointer.
   if (const GEPOperator *GEP2 = dyn_cast<GEPOperator>(V2)) {
     // Check for the GEP base being at a negative offset, this time in the other
     // direction.
-    if (!GEP1MaxLookupReached && !GEP2MaxLookupReached &&
-        isGEPBaseAtNegativeOffset(GEP2, DecompGEP2, DecompGEP1, V1Size))
+    if (isGEPBaseAtNegativeOffset(GEP2, DecompGEP2, DecompGEP1, V1Size))
       return NoAlias;
     // Do the base pointers alias?
     AliasResult BaseAlias =
@@ -1301,8 +1297,7 @@ AliasResult BasicAAResult::aliasGEP(
     // and AAInfo when performing the alias check on the underlying objects.
     if (BaseAlias == MayAlias && V1Size == V2Size &&
         GEP1BaseOffset == GEP2BaseOffset &&
-        DecompGEP1.VarIndices == DecompGEP2.VarIndices &&
-        !GEP1MaxLookupReached && !GEP2MaxLookupReached) {
+        DecompGEP1.VarIndices == DecompGEP2.VarIndices) {
       AliasResult PreciseBaseAlias = aliasCheck(
           UnderlyingV1, V1Size, V1AAInfo, UnderlyingV2, V2Size, V2AAInfo, AAQI);
       if (PreciseBaseAlias == NoAlias)
@@ -1331,10 +1326,6 @@ AliasResult BasicAAResult::aliasGEP(
         return R;
     }
 
-    // If the max search depth is reached, the result is undefined
-    if (GEP2MaxLookupReached || GEP1MaxLookupReached)
-      return MayAlias;
-
     // Subtract the GEP2 pointer from the GEP1 pointer to find out their
     // symbolic difference.
     GEP1BaseOffset -= GEP2BaseOffset;
@@ -1361,10 +1352,6 @@ AliasResult BasicAAResult::aliasGEP(
       assert(R == NoAlias || R == MayAlias);
       return R;
     }
-
-    // If the max search depth is reached the result is undefined
-    if (GEP1MaxLookupReached)
-      return MayAlias;
   }
 
   // In the two GEP Case, if there is no difference in the offsets of the

llvm/test/Analysis/BasicAA/gep-decomposition-limit.ll

@@ -6,13 +6,13 @@
 ; CHECK-DAG: NoAlias: i8* %gep.inc3, i8* %gep.inc5
 ; CHECK-DAG: NoAlias: i8* %gep.inc4, i8* %gep.inc5
 ;; At limit:
-; CHECK-DAG: MayAlias: i8* %gep.add6, i8* %gep.inc6
-; CHECK-DAG: MayAlias: i8* %gep.inc4, i8* %gep.inc6
-; CHECK-DAG: MayAlias: i8* %gep.inc5, i8* %gep.inc6
+; CHECK-DAG: MustAlias: i8* %gep.add6, i8* %gep.inc6
+; CHECK-DAG: NoAlias: i8* %gep.inc4, i8* %gep.inc6
+; CHECK-DAG: NoAlias: i8* %gep.inc5, i8* %gep.inc6
 ;; After limit:
 ; CHECK-DAG: MayAlias: i8* %gep.add7, i8* %gep.inc7
 ; CHECK-DAG: MayAlias: i8* %gep.inc5, i8* %gep.inc7
-; CHECK-DAG: MayAlias: i8* %gep.inc6, i8* %gep.inc7
+; CHECK-DAG: NoAlias: i8* %gep.inc6, i8* %gep.inc7
 
 define void @test(i8* %base) {
   %gep.add5 = getelementptr i8, i8* %base, i64 5