[PATCH] D124540: [clang][dataflow] Perform structural comparison of indirection values in `join`.

Yitzhak Mandelbaum via Phabricator via cfe-commits cfe-commits at lists.llvm.org
Wed Apr 27 10:35:32 PDT 2022

ymandel created this revision.
ymandel added reviewers: xazax.hun, sgatev.
Herald added subscribers: tschuett, steakhal, rnkovacs.
Herald added a project: All.
ymandel requested review of this revision.
Herald added a project: clang.

This patch changes `Environment::join`, in the case that two values at the same
location are not (pointer) equal, to structurally compare indirection values
(pointers and references) for equivalence (that is, equivalent pointees) before
resorting to merging.

This change makes join consistent with equivalence, which also performs
structural comparison. It also fixes a bug where the values are `ReferenceValue`
but the merge creates a non-reference value. This case arises when the
`ReferenceValue`s were created to represent an lvalue, so the "reference-ness"
is not reflected in the type. In this case, the pointees will always be
equivalent, because lvalues at the same code location point to the location of a
fixed declaration, whose location is itself stable across blocks.

We were unable to reproduce a unit test for this latter bug, but have verified
the fix in the context of a larger piece of code that triggers the bug.

  rG LLVM Github Monorepo



Index: clang/lib/Analysis/FlowSensitive/DataflowEnvironment.cpp
--- clang/lib/Analysis/FlowSensitive/DataflowEnvironment.cpp
+++ clang/lib/Analysis/FlowSensitive/DataflowEnvironment.cpp
@@ -21,6 +21,7 @@
 #include "clang/Analysis/FlowSensitive/Value.h"
 #include "llvm/ADT/DenseMap.h"
 #include "llvm/ADT/DenseSet.h"
+#include "llvm/Support/Casting.h"
 #include "llvm/Support/ErrorHandling.h"
 #include <cassert>
 #include <memory>
@@ -353,6 +354,16 @@
+    // FIXME: add unit tests that cover this statement.
+    if (auto *IndVal1 = dyn_cast<IndirectionValue>(Val)) {
+      auto *IndVal2 = cast<IndirectionValue>(It->second);
+      assert(IndVal1->getKind() == IndVal2->getKind());
+      if (&IndVal1->getPointeeLoc() == &IndVal2->getPointeeLoc()) {
+        JoinedEnv.LocToVal.insert({Loc, Val});
+        continue;
+      }
+    }
     if (Value *MergedVal = mergeDistinctValues(
             Loc->getType(), Val, *this, It->second, Other, JoinedEnv, Model))
       JoinedEnv.LocToVal.insert({Loc, MergedVal});

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