[PATCH] D66371: [CaptureTracker] Let subclasses provide dereferenceability information

[Johannes Doerfert via Phabricator via llvm-commits]

jdoerfert created this revision.
jdoerfert added reviewers: sanjoy, hfinkel, aykevl, sstefan1, uenoku, xbolva00.
Herald added subscribers: bollu, hiraditya.
Herald added a project: LLVM.

CaptureTracker subclasses might have better dereferenceability
information which allows null pointer checks to be no-capturing.
The first user will be D59922 <https://reviews.llvm.org/D59922>.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D66371

Files:
  llvm/include/llvm/Analysis/CaptureTracking.h
  llvm/lib/Analysis/CaptureTracking.cpp


Index: llvm/lib/Analysis/CaptureTracking.cpp
===================================================================
--- llvm/lib/Analysis/CaptureTracking.cpp
+++ llvm/lib/Analysis/CaptureTracking.cpp
@@ -33,6 +33,20 @@
 
 bool CaptureTracker::shouldExplore(const Use *U) { return true; }
 
+bool CaptureTracker::isDereferenceableOrNull(Value *O, const DataLayout &DL) {
+  // An inbounds GEP can either be a valid pointer (pointing into
+  // or to the end of an allocation), or be null in the default
+  // address space. So for an inbounds GEPs there is no way to let
+  // the pointer escape using clever GEP hacking because doing so
+  // would make the pointer point outside of the allocated object
+  // and thus make the GEP result a poison value.
+  if (auto *GEP = dyn_cast<GetElementPtrInst>(O))
+    if (GEP->isInBounds())
+      return true;
+  bool CanBeNull;
+  return O->getPointerDereferenceableBytes(DL, CanBeNull);
+}
+
 namespace {
   struct SimpleCaptureTracker : public CaptureTracker {
     explicit SimpleCaptureTracker(bool ReturnCaptures)
@@ -342,21 +356,10 @@
             break;
         if (!I->getFunction()->nullPointerIsDefined()) {
           auto *O = I->getOperand(Idx)->stripPointerCastsSameRepresentation();
-          // An inbounds GEP can either be a valid pointer (pointing into
-          // or to the end of an allocation), or be null in the default
-          // address space. So for an inbounds GEPs there is no way to let
-          // the pointer escape using clever GEP hacking because doing so
-          // would make the pointer point outside of the allocated object
-          // and thus make the GEP result a poison value.
-          if (auto *GEP = dyn_cast<GetElementPtrInst>(O))
-            if (GEP->isInBounds())
-              break;
-          // Comparing a dereferenceable_or_null argument against null
-          // cannot lead to pointer escapes, because if it is not null it
-          // must be a valid (in-bounds) pointer.
-          bool CanBeNull;
-          if (O->getPointerDereferenceableBytes(I->getModule()->getDataLayout(),
-                                                CanBeNull))
+          // Comparing a dereferenceable_or_null pointer against null cannot
+          // lead to pointer escapes, because if it is not null it must be a
+          // valid (in-bounds) pointer.
+          if (Tracker->isDereferenceableOrNull(O, I->getModule()->getDataLayout()))
             break;
         }
       }
Index: llvm/include/llvm/Analysis/CaptureTracking.h
===================================================================
--- llvm/include/llvm/Analysis/CaptureTracking.h
+++ llvm/include/llvm/Analysis/CaptureTracking.h
@@ -17,6 +17,7 @@
 
   class Value;
   class Use;
+  class DataLayout;
   class Instruction;
   class DominatorTree;
   class OrderedBasicBlock;
@@ -83,6 +84,11 @@
     /// use U. Return true to stop the traversal or false to continue looking
     /// for more capturing instructions.
     virtual bool captured(const Use *U) = 0;
+
+    /// isDereferenceableOrNull - Overload to allow clients with additional
+    /// knowledge about pointer dereferenceability to provide it and thereby
+    /// avoid conservative responses when a pointer is compared to null.
+    virtual bool isDereferenceableOrNull(Value *O, const DataLayout &DL);
   };
 
   /// PointerMayBeCaptured - Visit the value and the values derived from it and


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