[analyzer; new edges] add simplifySimpleBranches() to reduce edges for branches.

In many cases, the edge from the "if" to the condition, followed by an edge from the branch condition to the target code, is uninteresting.

In such cases, we should fold the two edges into one from the "if" to the target.

This also applies to loops.

Implements <rdar://problem/14034763>.

llvm-svn: 183018

clang/lib/StaticAnalyzer/Core/BugReporter.cpp

@@ -1582,6 +1582,9 @@ static const Stmt *getTerminatorCondition(const CFGBlock *B) {
   return S;
 }
 
+static const char *StrEnteringLoop = "Entering loop body";
+static const char *StrLoopBodyZero = "Loop body executed 0 times";
+
 static bool
 GenerateAlternateExtensivePathDiagnostic(PathDiagnostic& PD,
                                          PathDiagnosticBuilder &PDB,
@@ -1775,11 +1778,11 @@ GenerateAlternateExtensivePathDiagnostic(PathDiagnostic& PD,
 
             if (isJumpToFalseBranch(&*BE)) {
               if (!IsInLoopBody) {
-                str = "Loop body executed 0 times";
+                str = StrLoopBodyZero;
               }
             }
             else {
-              str = "Entering loop body";
+              str = StrEnteringLoop;
             }
 
             if (str) {
@@ -2338,6 +2341,92 @@ static void splitBranchConditionEdges(PathPieces &pieces,
   }
 }
 
+/// \brief Move edges from a branch condition to a branch target
+///        when the condition is simple.
+///
+/// This is the dual of splitBranchConditionEdges.  That function creates
+/// edges this may destroy, but they work together to create a more
+/// aesthetically set of edges around branches.  After the call to
+/// splitBranchConditionEdges, we may have (1) an edge to the branch,
+/// (2) an edge from the branch to the branch condition, and (3) an edge from
+/// the branch condition to the branch target.  We keep (1), but may wish
+/// to remove (2) and move the source of (3) to the branch if the branch
+/// condition is simple.
+///
+static void simplifySimpleBranches(PathPieces &pieces) {
+
+
+  for (PathPieces::iterator I = pieces.begin(), E = pieces.end(); I != E; ++I) {
+    // Adjust edges in subpaths.
+    if (PathDiagnosticCallPiece *Call = dyn_cast<PathDiagnosticCallPiece>(*I)) {
+      simplifySimpleBranches(Call->path);
+      continue;
+    }
+
+    PathDiagnosticControlFlowPiece *PieceI =
+      dyn_cast<PathDiagnosticControlFlowPiece>(*I);
+
+    if (!PieceI)
+      continue;
+
+    const Stmt *s1Start = getLocStmt(PieceI->getStartLocation());
+    const Stmt *s1End   = getLocStmt(PieceI->getEndLocation());
+
+    if (!s1Start || !s1End)
+      continue;
+
+    PathPieces::iterator NextI = I; ++NextI;
+    if (NextI == E)
+      break;
+
+    PathDiagnosticControlFlowPiece *PieceNextI = 0;
+
+    while (true) {
+      if (NextI == E)
+        break;
+
+      PathDiagnosticEventPiece *EV = dyn_cast<PathDiagnosticEventPiece>(*NextI);
+      if (EV) {
+        StringRef S = EV->getString();
+        if (S == StrEnteringLoop || S == StrLoopBodyZero) {
+          ++NextI;
+          continue;
+        }
+        break;
+      }
+
+      PieceNextI = dyn_cast<PathDiagnosticControlFlowPiece>(*NextI);
+      break;
+    }
+
+    if (!PieceNextI)
+      continue;
+
+    const Stmt *s2Start = getLocStmt(PieceNextI->getStartLocation());
+    const Stmt *s2End   = getLocStmt(PieceNextI->getEndLocation());
+
+    if (!s2Start || !s2End || s1End != s2Start)
+      continue;
+
+    // We only perform this transformation for specific branch kinds.
+    // We do want to do this for do..while, for example.
+    if (!(isa<ForStmt>(s1Start) || isa<WhileStmt>(s1Start) ||
+          isa<IfStmt>(s1Start) || isa<ObjCForCollectionStmt>(s1Start)))
+      continue;
+
+    // Is s1End the branch condition?
+    if (!isConditionForTerminator(s1Start, s1End))
+      continue;
+
+    // Perform the hoisting by eliminating (2) and changing the start
+    // location of (3).
+    PathDiagnosticLocation L = PieceI->getStartLocation();
+    pieces.erase(I);
+    I = NextI;
+    PieceNextI->setStartLocation(L);
+  }
+}
+
 //===----------------------------------------------------------------------===//
 // Methods for BugType and subclasses.
 //===----------------------------------------------------------------------===//
@@ -3033,6 +3122,10 @@ bool GRBugReporter::generatePathDiagnostic(PathDiagnostic& PD,
         // Adjust edges into loop conditions to make them more uniform
         // and aesthetically pleasing.
         splitBranchConditionEdges(PD.getMutablePieces(), LCM, SM);
+
+        // Hoist edges originating from branch conditions to branches
+        // for simple branches.
+        simplifySimpleBranches(PD.getMutablePieces());
       }
     }