[analyzer] Fix some expressions staying live too long. Add a debug checker.

StaticAnalyzer uses the CFG-based RelaxedLiveVariables analysis in order to,
in particular, figure out values of which expressions are still needed.
When the expression becomes "dead", it is garbage-collected during
the dead binding scan.

Expressions that constitute branches/bodies of control flow statements,
eg. `E1' in `if (C1) E1;' but not `E2' in `if (C2) { E2; }', were kept alive
for too long. This caused false positives in MoveChecker because it relies
on cleaning up loop-local variables when they go out of scope, but some of those
live-for-too-long expressions were keeping a reference to those variables.

Fix liveness analysis to correctly mark these expressions as dead.

Add a debug checker, debug.DumpLiveStmts, in order to test expressions liveness.

Differential Revision: https://reviews.llvm.org/D55566

llvm-svn: 349320

1 files changed, 48 insertions, 0 deletions

diff --git a/clang/lib/Analysis/LiveVariables.cpp b/clang/lib/Analysis/LiveVariables.cpp
index 4f22ccc55a7..afe2d264907 100644
--- a/clang/lib/Analysis/LiveVariables.cpp
+++ b/clang/lib/Analysis/LiveVariables.cpp
@@ -93,6 +93,7 @@ public:
              LiveVariables::Observer *obs = nullptr);
 
   void dumpBlockLiveness(const SourceManager& M);
+  void dumpStmtLiveness(const SourceManager& M);
 
   LiveVariablesImpl(AnalysisDeclContext &ac, bool KillAtAssign)
     : analysisContext(ac),
@@ -327,6 +328,35 @@ void TransferFunctions::Visit(Stmt *S) {
       // No need to unconditionally visit subexpressions.
       return;
     }
+    case Stmt::IfStmtClass: {
+      // If one of the branches is an expression rather than a compound
+      // statement, it will be bad if we mark it as live at the terminator
+      // of the if-statement (i.e., immediately after the condition expression).
+      AddLiveStmt(val.liveStmts, LV.SSetFact, cast<IfStmt>(S)->getCond());
+      return;
+    }
+    case Stmt::WhileStmtClass: {
+      // If the loop body is an expression rather than a compound statement,
+      // it will be bad if we mark it as live at the terminator of the loop
+      // (i.e., immediately after the condition expression).
+      AddLiveStmt(val.liveStmts, LV.SSetFact, cast<WhileStmt>(S)->getCond());
+      return;
+    }
+    case Stmt::DoStmtClass: {
+      // If the loop body is an expression rather than a compound statement,
+      // it will be bad if we mark it as live at the terminator of the loop
+      // (i.e., immediately after the condition expression).
+      AddLiveStmt(val.liveStmts, LV.SSetFact, cast<DoStmt>(S)->getCond());
+      return;
+    }
+    case Stmt::ForStmtClass: {
+      // If the loop body is an expression rather than a compound statement,
+      // it will be bad if we mark it as live at the terminator of the loop
+      // (i.e., immediately after the condition expression).
+      AddLiveStmt(val.liveStmts, LV.SSetFact, cast<ForStmt>(S)->getCond());
+      return;
+    }
+
   }
 
   for (Stmt *Child : S->children()) {
@@ -632,5 +662,23 @@ void LiveVariablesImpl::dumpBlockLiveness(const SourceManager &M) {
   llvm::errs() << "\n";
 }
 
+void LiveVariables::dumpStmtLiveness(const SourceManager &M) {
+  getImpl(impl).dumpStmtLiveness(M);
+}
+
+void LiveVariablesImpl::dumpStmtLiveness(const SourceManager &M) {
+  // Don't iterate over blockEndsToLiveness directly because it's not sorted.
+  for (auto I : *analysisContext.getCFG()) {
+
+    llvm::errs() << "\n[ B" << I->getBlockID()
+                 << " (live statements at block exit) ]\n";
+    for (auto S : blocksEndToLiveness[I].liveStmts) {
+      llvm::errs() << "\n";
+      S->dump();
+    }
+    llvm::errs() << "\n";
+  }
+}
+
 const void *LiveVariables::getTag() { static int x; return &x; }
 const void *RelaxedLiveVariables::getTag() { static int x; return &x; }