[llvm] 30001af - [BasicAA] Ignore CanBeFreed in minimal extent reasoning

[Nikita Popov via llvm-commits]

Author: Nikita Popov
Date: 2021-10-04T22:08:57+02:00
New Revision: 30001af84ec5fa0c4f88da1df9c6682bdff44226

URL: https://github.com/llvm/llvm-project/commit/30001af84ec5fa0c4f88da1df9c6682bdff44226
DIFF: https://github.com/llvm/llvm-project/commit/30001af84ec5fa0c4f88da1df9c6682bdff44226.diff

LOG: [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

Added: 
    

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

Removed: 
    


################################################################################
diff  --git a/llvm/lib/Analysis/BasicAliasAnalysis.cpp b/llvm/lib/Analysis/BasicAliasAnalysis.cpp
index 705091c0b9fe1..4a54c331f11b7 100644
--- 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 c50a7d3f59c7a..78de6eeff08e3 100644
--- 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"