[clang] detect switch fallthrough marked by a comment (PR43465)

The regex can be extended if needed, but this should probably handle
most of the cases.

Differential Revision: https://reviews.llvm.org/D73852

clang/lib/Sema/AnalysisBasedWarnings.cpp

@@ -1148,6 +1148,11 @@ namespace {
           continue;
         }
 
+        if (isFollowedByFallThroughComment(LastStmt)) {
+          ++AnnotatedCnt;
+          continue; // Fallthrough comment, good.
+        }
+
         ++UnannotatedCnt;
       }
       return !!UnannotatedCnt;
@@ -1208,10 +1213,41 @@ namespace {
       return nullptr;
     }
 
+    bool isFollowedByFallThroughComment(const Stmt *Statement) {
+      // Try to detect whether the fallthough is marked by a comment like
+      // /*FALLTHOUGH*/.
+      bool Invalid;
+      const char *SourceData = S.getSourceManager().getCharacterData(
+          Statement->getEndLoc(), &Invalid);
+      if (Invalid)
+        return false;
+      const char *LineStart = SourceData;
+      for (;;) {
+        LineStart = strchr(LineStart, '\n');
+        if (LineStart == nullptr)
+          return false;
+        ++LineStart; // Start of next line.
+        const char *LineEnd = strchr(LineStart, '\n');
+        StringRef Line(LineStart,
+                       LineEnd ? LineEnd - LineStart : strlen(LineStart));
+        if (LineStart == LineEnd ||
+            Line.find_first_not_of(" \t\r") == StringRef::npos)
+          continue; // Whitespace-only line.
+        if (!FallthroughRegex.isValid())
+          FallthroughRegex =
+              llvm::Regex("(/\\*[ \\t]*fall(s | |-)?thr(ough|u)\\.?[ \\t]*\\*/)"
+                          "|(//[ \\t]*fall(s | |-)?thr(ough|u)\\.?[ \\t]*)",
+                          llvm::Regex::IgnoreCase);
+        assert(FallthroughRegex.isValid());
+        return FallthroughRegex.match(Line);
+      }
+    }
+
     bool FoundSwitchStatements;
     AttrStmts FallthroughStmts;
     Sema &S;
     llvm::SmallPtrSet<const CFGBlock *, 16> ReachableBlocks;
+    llvm::Regex FallthroughRegex;
   };
 } // anonymous namespace
 

clang/test/Sema/fallthrough-comment.c

@@ -0,0 +1,20 @@
+// RUN: %clang_cc1 -fsyntax-only -std=c11 -verify -Wimplicit-fallthrough %s
+
+int fallthrough_comment(int n) {
+  switch (n) {
+  case 0:
+    n++;
+    // FALLTHROUGH
+  case 1:
+    n++;
+
+    /*fall-through.*/
+
+  case 2:
+    n++;
+  case 3: // expected-warning{{unannotated fall-through between switch labels}} expected-note{{insert '__attribute__((fallthrough));' to silence this warning}} expected-note{{insert 'break;' to avoid fall-through}}
+    n++;
+    break;
+  }
+  return n;
+}