[BasicAA] Ignore CanBeFreed in minimal extent reasoning

[nikic]

Summary

When determining NoAlias based on object size and dereferenceability information, we can ignore frees for the same reason we can ignore possible null pointers (if null is not a valid pointer): Actually accessing the null pointer / freed pointer would be immediate UB, and AA results are only valid under the assumption of an access.

This addresses a minor regression from D110745.

[nlopes]

good catch!
This function is only used to heck if one of the accesses is larger than the 2nd object. It doesn't matter whether it is live or not.

llvm/lib/Analysis/BasicAliasAnalysis.cpp

 /// assuming the result is used in an aliasing query. E.g., we do use the query
 /// location size and the fact that null pointers cannot alias here.
 static uint64_t getMinimalExtentFrom(const Value &V,
                                      const LocationSize &LocSize,
                                      const DataLayout &DL,
                                      bool NullIsValidLoc) {
   // If we have dereferenceability information we know a lower bound for the
   // extent as accesses for a lower offset would be valid. We need to exclude
-  // the "or null" part if null is a valid pointer.
+  // the "or null" part if null is a valid pointer. We can ignore frees, as an
+  // access after free would be undefined behavior.
   bool CanBeNull, CanBeFreed;
   uint64_t DerefBytes =
     V.getPointerDereferenceableBytes(DL, CanBeNull, CanBeFreed);
   DerefBytes = (CanBeNull && NullIsValidLoc) ? 0 : DerefBytes;
-  DerefBytes = CanBeFreed ? 0 : DerefBytes;
   // If queried with a precise location size, we assume that location size to be
   // accessed, thus valid.
   if (LocSize.isPrecise())
     DerefBytes = std::max(DerefBytes, LocSize.getValue());
   return DerefBytes;
 }
 
 /// Returns true if we can prove that the object specified by V has size Size.

llvm/test/Analysis/BasicAA/dereferenceable.ll

 ; RUN: opt -basic-aa -print-all-alias-modref-info -aa-eval < %s 2>&1 | FileCheck %s
+; RUN: opt -basic-aa -print-all-alias-modref-info -aa-eval -use-dereferenceable-at-point-semantics=1 < %s 2>&1 | FileCheck %s
 
 target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
 
 @G = global i32 0, align 4
 
 define i64 @global_and_deref_arg_1(i64* dereferenceable(8) %arg) nofree nosync {
 ; CHECK:     Function: global_and_deref_arg_1: 2 pointers, 0 call sites
 ; CHECK-NEXT:  NoAlias: i32* @G, i64* %arg