[BasicAA] Ignore CanBeFreed in minimal extent reasoning

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.

Differential Revision: https://reviews.llvm.org/D111028

diff --git a/llvm/lib/Analysis/BasicAliasAnalysis.cpp b/llvm/lib/Analysis/BasicAliasAnalysis.cpp
index 705091c..4a54c33 100644 (file)
--- a/llvm/lib/Analysis/BasicAliasAnalysis.cpp
+++ b/llvm/lib/Analysis/BasicAliasAnalysis.cpp
@@ -204,12 +204,12 @@ static uint64_t getMinimalExtentFrom(const Value &V,
                                      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())

diff --git a/llvm/test/Analysis/BasicAA/dereferenceable.ll b/llvm/test/Analysis/BasicAA/dereferenceable.ll
index c50a7d3..78de6ee 100644 (file)
--- a/llvm/test/Analysis/BasicAA/dereferenceable.ll
+++ b/llvm/test/Analysis/BasicAA/dereferenceable.ll
@@ -1,4 +1,5 @@
 ; 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"