[PATCH] D111028: [BasicAA] Ignore CanBeFreed in minimal extent reasoning

[Nikita Popov via Phabricator via llvm-commits]

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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 <https://reviews.llvm.org/D110745>.


https://reviews.llvm.org/D111028

Files:
  llvm/lib/Analysis/BasicAliasAnalysis.cpp
  llvm/test/Analysis/BasicAA/dereferenceable.ll


Index: llvm/test/Analysis/BasicAA/dereferenceable.ll
===================================================================
--- llvm/test/Analysis/BasicAA/dereferenceable.ll
+++ 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"
 
Index: llvm/lib/Analysis/BasicAliasAnalysis.cpp
===================================================================
--- llvm/lib/Analysis/BasicAliasAnalysis.cpp
+++ llvm/lib/Analysis/BasicAliasAnalysis.cpp
@@ -204,12 +204,12 @@
                                      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())


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