[analyzer] RegionStore: Enable loading default bindings from variables.

[NoQ]

Summary

D44934 added modeling for memset() that, for the case of setting memory to 0, boils down to default-binding a concrete 0 to the memory region.

Surprisingly, in some cases RegionStore fails to load default bindings from a region. That is, it assumes that the region either has a direct binding (i.e., initialized via assignment), or it is uninitialized. In particular, this applies to local variables regions of integer/pointer/float types.

It seems that a lot of code (eg., invalidateRegions()) carefully works around this problem by adding either a direct binding or a default binding, depending on what RegionStore expects. This essentially duplicates RegionStore's already-convoluted logic on this subject on the caller side.

Our options are:

1. Duplicate this logic again in CStringChecker.
2. Let RegionStore automatically do the direct binding for such plain integers.
3. Add support for loading default bindings from variables.

Option 1 is a failure to improve checker API in an aspect in which it's already very bad, so i didn't go for it. In options 2 and 3 the difference is entirely within RegionStore. I believe that option 2 is slightly superior because it produces a more "normalized" data structure and, additionally, because stores are more rare than loads (unless they are "dead stores"^^), so it should be slightly more performant. But it is also harder to implement, as it requires gathering the logic of when exactly do we need a direct binding in one place, so i went for option 3 in order to address the regression and simplify that logic. I did not yet try to see how other callers of bindDefault***() can be simplified.

[NoQ]

Fix test file name in the top comment.

[NoQ]

Whoops, copy-paste error.

So, like, the reason why i added a unittest was that i wanted to test separation of concerns: that it's specifically our Store that works correctly, not only the system as a whole.

[dcoughlin]

LGTM.

clang/unittests/StaticAnalyzer/StoreTest.cpp

21

It is awesome to see unit tests for this!

[a_sidorin]

I like the test even more than the change itself!

clang/unittests/StaticAnalyzer/StoreTest.cpp

50

This can fit one line.

83

Do we need this dtor declaration?

94

= default? (Or it seems like we can remove it at all)

[NoQ]

clang/unittests/StaticAnalyzer/StoreTest.cpp

50

Just wanted to make line breaks similar in all tests, for the sake of OCD-friendliness.

[NoQ]

Fxd constructors and destructors.

clang/lib/StaticAnalyzer/Core/RegionStore.cpp

SVal RegionStoreManager::getBindingForObjCIvar(RegionBindingsConstRef B,
 
   return getBindingForLazySymbol(R);
 }
 
 SVal RegionStoreManager::getBindingForVar(RegionBindingsConstRef B,
                                           const VarRegion *R) {
 
   // Check if the region has a binding.
-  if (const Optional<SVal> &V = B.getDirectBinding(R))
+  if (Optional<SVal> V = B.getDirectBinding(R))
+    return *V;
+
+  if (Optional<SVal> V = B.getDefaultBinding(R))
     return *V;
 
   // Lazily derive a value for the VarRegion.
   const VarDecl *VD = R->getDecl();
   const MemSpaceRegion *MS = R->getMemorySpace();
 
   // Arguments are always symbolic.
   if (isa<StackArgumentsSpaceRegion>(MS))

clang/test/Analysis/string.c

 }
 
 void memset28() {
   short x;
   memset(&x, 1, sizeof(short));
   // This should be true.
   clang_analyzer_eval(x == 0x101); // expected-warning{{UNKNOWN}}
 }
+
+void memset29_plain_int_zero() {
+  short x;
+  memset(&x, 0, sizeof(short));
+  clang_analyzer_eval(x == 0); // expected-warning{{TRUE}}
+}

clang/unittests/StaticAnalyzer/CMakeLists.txt

 set(LLVM_LINK_COMPONENTS
   Support
   )
 
 add_clang_unittest(StaticAnalysisTests
   AnalyzerOptionsTest.cpp
+  StoreTest.cpp
   RegisterCustomCheckersTest.cpp
   SymbolReaperTest.cpp
   )
 
 target_link_libraries(StaticAnalysisTests
   PRIVATE
   clangBasic
   clangAnalysis

clang/unittests/StaticAnalyzer/StoreTest.cpp

+//===- unittests/StaticAnalyzer/StoreTest.cpp -----------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "Reusables.h"
+
+#include "clang/Tooling/Tooling.h"
+#include "gtest/gtest.h"
+
+namespace clang {
+namespace ento {
+namespace {
+
+// Test that we can put a value into an int-type variable and load it
+// back from that variable. Test what happens if default bindings are used.
+class VariableBindConsumer : public ExprEngineConsumer {
+  void performTest(const Decl *D) {
+    StoreManager &StMgr = Eng.getStoreManager();
+    SValBuilder &SVB = Eng.getSValBuilder();
+    MemRegionManager &MRMgr = StMgr.getRegionManager();
+    const ASTContext &ACtx = Eng.getContext();
+
+    const auto *VDX0 = findDeclByName<VarDecl>(D, "x0");
+    const auto *VDY0 = findDeclByName<VarDecl>(D, "y0");
+    const auto *VDZ0 = findDeclByName<VarDecl>(D, "z0");
+    const auto *VDX1 = findDeclByName<VarDecl>(D, "x1");
+    const auto *VDY1 = findDeclByName<VarDecl>(D, "y1");
+    assert(VDX0 && VDY0 && VDZ0 && VDX1 && VDY1);
+
+    const StackFrameContext *SFC =
+        Eng.getAnalysisDeclContextManager().getStackFrame(D);
+
+    Loc LX0 = loc::MemRegionVal(MRMgr.getVarRegion(VDX0, SFC));
+    Loc LY0 = loc::MemRegionVal(MRMgr.getVarRegion(VDY0, SFC));
+    Loc LZ0 = loc::MemRegionVal(MRMgr.getVarRegion(VDZ0, SFC));
+    Loc LX1 = loc::MemRegionVal(MRMgr.getVarRegion(VDX1, SFC));
+    Loc LY1 = loc::MemRegionVal(MRMgr.getVarRegion(VDY1, SFC));
+
+    Store StInit = StMgr.getInitialStore(SFC).getStore();
+    SVal Zero = SVB.makeZeroVal(ACtx.IntTy);
+    SVal One = SVB.makeIntVal(1, ACtx.IntTy);
+    SVal NarrowZero = SVB.makeZeroVal(ACtx.CharTy);
+
+    // Bind(Zero)
+    Store StX0 =
+        StMgr.Bind(StInit, LX0, Zero).getStore();
+    ASSERT_EQ(Zero, StMgr.getBinding(StX0, LX0, ACtx.IntTy));
+
+    // BindDefaultInitial(Zero)
+    Store StY0 =
+        StMgr.BindDefaultInitial(StInit, LY0.getAsRegion(), Zero).getStore();
+    ASSERT_EQ(Zero, StMgr.getBinding(StY0, LY0, ACtx.IntTy));
+    ASSERT_EQ(Zero, *StMgr.getDefaultBinding(StY0, LY0.getAsRegion()));
+
+    // BindDefaultZero()
+    Store StZ0 =
+        StMgr.BindDefaultZero(StInit, LZ0.getAsRegion()).getStore();
+    // BindDefaultZero wipes the region with '0 S8b', not with out Zero.
+    // Direct load, however, does give us back the object of the type
+    // that we specify for loading.
+    ASSERT_EQ(Zero, StMgr.getBinding(StZ0, LZ0, ACtx.IntTy));
+    ASSERT_EQ(NarrowZero, *StMgr.getDefaultBinding(StZ0, LZ0.getAsRegion()));
+
+    // Bind(One)
+    Store StX1 =
+        StMgr.Bind(StInit, LX1, One).getStore();
+    ASSERT_EQ(One, StMgr.getBinding(StX1, LX1, ACtx.IntTy));
+
+    // BindDefaultInitial(One)
+    Store StY1 =
+        StMgr.BindDefaultInitial(StInit, LY1.getAsRegion(), One).getStore();
+    ASSERT_EQ(One, StMgr.getBinding(StY1, LY1, ACtx.IntTy));
+    ASSERT_EQ(One, *StMgr.getDefaultBinding(StY1, LY1.getAsRegion()));
+  }
+
+public:
+  VariableBindConsumer(CompilerInstance &C) : ExprEngineConsumer(C) {}
+
+  bool HandleTopLevelDecl(DeclGroupRef DG) override {
+    for (const auto *D : DG)
+      performTest(D);
+    return true;
+  }
+};
+
+class VariableBindAction : public ASTFrontendAction {
+public:
+  std::unique_ptr<ASTConsumer> CreateASTConsumer(CompilerInstance &Compiler,
+                                                 StringRef File) override {
+    return llvm::make_unique<VariableBindConsumer>(Compiler);
+  }
+};
+
+TEST(Store, VariableBind) {
+  EXPECT_TRUE(tooling::runToolOnCode(
+      new VariableBindAction, "void foo() { int x0, y0, z0, x1, y1; }"));
+}
+
+} // namespace
+} // namespace ento
+} // namespace clang