[PATCH] -Wimplicit-fallthrough: fixed two cases where "fallthrough annotation in unreachable code" was issued incorrectly.

[Alexander Kornienko]

Hi rsmith,

1. In actual unreachable code, but not immediately on a fall-through execution
path "fallthrough annotation does not directly precede switch label" is better;
2. After default: in a switch with covered enum cases. Actually, these shouldn't
be treated as unreachable code for our purpose.

http://llvm-reviews.chandlerc.com/D374

Files:
  lib/Sema/AnalysisBasedWarnings.cpp
  test/SemaCXX/switch-implicit-fallthrough.cpp

Index: lib/Sema/AnalysisBasedWarnings.cpp
===================================================================
--- lib/Sema/AnalysisBasedWarnings.cpp
+++ lib/Sema/AnalysisBasedWarnings.cpp
@@ -708,6 +708,16 @@
 
       ReachableBlocks.insert(&Cfg->getEntry());
       BlockQueue.push_back(&Cfg->getEntry());
+      // Mark all default: blocks as reachable, even when isAllEnumCasesCovered.
+      // These blocks can contain fall-through annotations, and we don't want to
+      // issue a warn_fallthrough_attr_unreachable for them.
+      for (CFG::iterator I = Cfg->begin(), E = Cfg->end(); I != E; ++I) {
+        const CFGBlock *B = *I;
+        const Stmt *L = B->getLabel();
+        if (L && isa<DefaultStmt>(L) && ReachableBlocks.insert(B))
+          BlockQueue.push_back(B);
+      }
+
       while (!BlockQueue.empty()) {
         const CFGBlock *P = BlockQueue.front();
         BlockQueue.pop_front();
@@ -747,14 +757,16 @@
           continue; // Case label is preceded with a normal label, good.
 
         if (!ReachableBlocks.count(P)) {
-          for (CFGBlock::const_iterator ElIt = P->begin(), ElEnd = P->end();
-               ElIt != ElEnd; ++ElIt) {
-            if (const CFGStmt *CS = ElIt->getAs<CFGStmt>()){
+          for (CFGBlock::const_reverse_iterator ElemIt = P->rbegin(),
+                                                ElemEnd = P->rend();
+               ElemIt != ElemEnd; ++ElemIt) {
+            if (const CFGStmt *CS = ElemIt->getAs<CFGStmt>()) {
               if (const AttributedStmt *AS = asFallThroughAttr(CS->getStmt())) {
                 S.Diag(AS->getLocStart(),
                        diag::warn_fallthrough_attr_unreachable);
                 markFallthroughVisited(AS);
                 ++AnnotatedCnt;
+                break;
               }
               // Don't care about other unreachable statements.
             }
Index: test/SemaCXX/switch-implicit-fallthrough.cpp
===================================================================
--- test/SemaCXX/switch-implicit-fallthrough.cpp
+++ test/SemaCXX/switch-implicit-fallthrough.cpp
@@ -179,13 +179,15 @@
 
 int fallthrough_position(int n) {
   switch (n) {
+      [[clang::fallthrough]];  // expected-warning{{fallthrough annotation does not directly precede switch label}}
+      n += 300;
       [[clang::fallthrough]];  // expected-warning{{fallthrough annotation in unreachable code}}
     case 221:
-      [[clang::fallthrough]]; // expected-warning{{fallthrough annotation does not directly precede switch label}}
+      [[clang::fallthrough]];  // expected-warning{{fallthrough annotation does not directly precede switch label}}
       return 1;
       [[clang::fallthrough]];  // expected-warning{{fallthrough annotation in unreachable code}}
     case 222:
-      [[clang::fallthrough]]; // expected-warning{{fallthrough annotation does not directly precede switch label}}
+      [[clang::fallthrough]];  // expected-warning{{fallthrough annotation does not directly precede switch label}}
       n += 400;
     case 223:          // expected-warning{{unannotated fall-through between switch labels}} expected-note{{insert '[[clang::fallthrough]];' to silence this warning}} expected-note{{insert 'break;' to avoid fall-through}}
       [[clang::fallthrough]]; // expected-warning{{fallthrough annotation does not directly precede switch label}}
@@ -211,6 +213,26 @@
   return n;
 }
 
+enum Enum {
+  Value1, Value2
+};
+
+int fallthrough_covered_enums(Enum e) {
+  int n = 0;
+  switch (e) {
+    default:
+      n += 17;
+      [[clang::fallthrough]];  // no warning here, this shouldn't be treated as unreachable code
+    case Value1:
+      n += 19;
+      break;
+    case Value2:
+      n += 21;
+      break;
+  }
+  return n;
+}
+
 int fallthrough_targets(int n) {
   [[clang::fallthrough]]; // expected-error{{fallthrough annotation is outside switch statement}}
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