[BasicAA] Use dereferenceability to reason about aliasing
Summary:
We already use the fact that an object with known size X does not alias
another objection of size Y > X before. With this commit, we use
dereferenceability information to determine a lower bound for Y and not
only rely on the user provided query size.
The result for @global_and_deref_arg_2() and @local_and_deref_ret_2()
in test/Analysis/BasicAA/dereferenceable.ll improved with this patch.
Reviewers: asbirlea, chandlerc, hfinkel, sanjoy
Subscribers: hiraditya, bollu, llvm-commits
Tags: #llvm
Differential Revision: https://reviews.llvm.org/D66157
llvm-svn: 369786
diff --git a/llvm/lib/Analysis/BasicAliasAnalysis.cpp b/llvm/lib/Analysis/BasicAliasAnalysis.cpp
index 16bdded..2702566 100644
--- a/llvm/lib/Analysis/BasicAliasAnalysis.cpp
+++ b/llvm/lib/Analysis/BasicAliasAnalysis.cpp
@@ -233,6 +233,26 @@
return ObjectSize != MemoryLocation::UnknownSize && ObjectSize < Size;
}
+/// Return the minimal extent from \p V to the end of the underlying object,
+/// 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.
+ bool CanBeNull;
+ uint64_t DerefBytes = V.getPointerDereferenceableBytes(DL, CanBeNull);
+ DerefBytes = (CanBeNull && NullIsValidLoc) ? 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.
static bool isObjectSize(const Value *V, uint64_t Size, const DataLayout &DL,
const TargetLibraryInfo &TLI, bool NullIsValidLoc) {
@@ -1792,10 +1812,12 @@
// If the size of one access is larger than the entire object on the other
// side, then we know such behavior is undefined and can assume no alias.
bool NullIsValidLocation = NullPointerIsDefined(&F);
- if ((V1Size.isPrecise() && isObjectSmallerThan(O2, V1Size.getValue(), DL, TLI,
- NullIsValidLocation)) ||
- (V2Size.isPrecise() && isObjectSmallerThan(O1, V2Size.getValue(), DL, TLI,
- NullIsValidLocation)))
+ if ((isObjectSmallerThan(
+ O2, getMinimalExtentFrom(*V1, V1Size, DL, NullIsValidLocation), DL,
+ TLI, NullIsValidLocation)) ||
+ (isObjectSmallerThan(
+ O1, getMinimalExtentFrom(*V2, V2Size, DL, NullIsValidLocation), DL,
+ TLI, NullIsValidLocation)))
return NoAlias;
// Check the cache before climbing up use-def chains. This also terminates