Index: clang/include/clang/Analysis/Analyses/ReachableCode.h
===================================================================
--- clang/include/clang/Analysis/Analyses/ReachableCode.h
+++ clang/include/clang/Analysis/Analyses/ReachableCode.h
@@ -61,8 +61,8 @@
                                 llvm::BitVector &Reachable);
 
 void FindUnreachableCode(AnalysisDeclContext &AC, Preprocessor &PP,
-                         Callback &CB);
-
+                         Callback &CB,
+                         const bool UnreachableFallThroughDiagIsEnabled);
 }} // end namespace clang::reachable_code
 
 #endif
Index: clang/lib/Analysis/ReachableCode.cpp
===================================================================
--- clang/lib/Analysis/ReachableCode.cpp
+++ clang/lib/Analysis/ReachableCode.cpp
@@ -12,6 +12,7 @@
 //===----------------------------------------------------------------------===//
 
 #include "clang/Analysis/Analyses/ReachableCode.h"
+#include "clang/AST/Attr.h"
 #include "clang/AST/Expr.h"
 #include "clang/AST/ExprCXX.h"
 #include "clang/AST/ExprObjC.h"
@@ -409,15 +410,16 @@
 
     void enqueue(const CFGBlock *block);
     unsigned scanBackwards(const CFGBlock *Start,
-    clang::reachable_code::Callback &CB);
+                           clang::reachable_code::Callback &CB,
+                           const bool UnreachableFallThroughDiagIsEnabled);
 
     bool isDeadCodeRoot(const CFGBlock *Block);
 
     const Stmt *findDeadCode(const CFGBlock *Block);
 
-    void reportDeadCode(const CFGBlock *B,
-                        const Stmt *S,
-                        clang::reachable_code::Callback &CB);
+    void reportDeadCode(const CFGBlock *B, const Stmt *S,
+                        clang::reachable_code::Callback &CB,
+                        const bool UnreachableFallThroughDiagIsEnabled);
   };
 }
 
@@ -487,8 +489,10 @@
   return 0;
 }
 
-unsigned DeadCodeScan::scanBackwards(const clang::CFGBlock *Start,
-                                     clang::reachable_code::Callback &CB) {
+unsigned
+DeadCodeScan::scanBackwards(const clang::CFGBlock *Start,
+                            clang::reachable_code::Callback &CB,
+                            const bool UnreachableFallThroughDiagIsEnabled) {
 
   unsigned count = 0;
   enqueue(Start);
@@ -521,7 +525,7 @@
     }
 
     if (isDeadCodeRoot(Block)) {
-      reportDeadCode(Block, S, CB);
+      reportDeadCode(Block, S, CB, UnreachableFallThroughDiagIsEnabled);
       count += scanMaybeReachableFromBlock(Block, PP, Reachable);
     }
     else {
@@ -540,7 +544,7 @@
       const CFGBlock *Block = I.first;
       if (Reachable[Block->getBlockID()])
         continue;
-      reportDeadCode(Block, I.second, CB);
+      reportDeadCode(Block, I.second, CB, UnreachableFallThroughDiagIsEnabled);
       count += scanMaybeReachableFromBlock(Block, PP, Reachable);
     }
   }
@@ -613,9 +617,9 @@
   return S->getBeginLoc();
 }
 
-void DeadCodeScan::reportDeadCode(const CFGBlock *B,
-                                  const Stmt *S,
-                                  clang::reachable_code::Callback &CB) {
+void DeadCodeScan::reportDeadCode(
+    const CFGBlock *B, const Stmt *S, clang::reachable_code::Callback &CB,
+    const bool UnreachableFallThroughDiagIsEnabled) {
   // Classify the unreachable code found, or suppress it in some cases.
   reachable_code::UnreachableKind UK = reachable_code::UK_Other;
 
@@ -663,6 +667,14 @@
     }
   }
 
+  // If S is `[[fallthrough]];` and `-Wunreachable-code-fallthrough` is enabled,
+  // suppress `code will never be executed` warning to avoid generating
+  // diagnostic twice
+  const AttributedStmt *AS = dyn_cast<AttributedStmt>(S);
+  if (AS && hasSpecificAttr<FallThroughAttr>(AS->getAttrs()) &&
+      UnreachableFallThroughDiagIsEnabled)
+    return;
+
   SourceRange R1, R2;
   SourceLocation Loc = GetUnreachableLoc(S, R1, R2);
   CB.HandleUnreachable(UK, Loc, SilenceableCondVal, R1, R2);
@@ -682,7 +694,8 @@
 }
 
 void FindUnreachableCode(AnalysisDeclContext &AC, Preprocessor &PP,
-                         Callback &CB) {
+                         Callback &CB,
+                         const bool UnreachableFallThroughDiagIsEnabled) {
 
   CFG *cfg = AC.getCFG();
   if (!cfg)
@@ -713,11 +726,11 @@
       continue;
 
     DeadCodeScan DS(reachable, PP, AC.getASTContext());
-    numReachable += DS.scanBackwards(block, CB);
+    numReachable +=
+        DS.scanBackwards(block, CB, UnreachableFallThroughDiagIsEnabled);
 
     if (numReachable == cfg->getNumBlockIDs())
       return;
   }
 }
-
 }} // end namespace clang::reachable_code
Index: clang/lib/Sema/AnalysisBasedWarnings.cpp
===================================================================
--- clang/lib/Sema/AnalysisBasedWarnings.cpp
+++ clang/lib/Sema/AnalysisBasedWarnings.cpp
@@ -123,7 +123,10 @@
     return;
 
   UnreachableCodeHandler UC(S);
-  reachable_code::FindUnreachableCode(AC, S.getPreprocessor(), UC);
+  reachable_code::FindUnreachableCode(
+      AC, S.getPreprocessor(), UC,
+      !S.getDiagnostics().isIgnored(diag::warn_unreachable_fallthrough_attr,
+                                    SourceLocation()));
 }
 
 namespace {
Index: clang/test/Sema/warn-unreachable-fallthrough.c
===================================================================
--- /dev/null
+++ clang/test/Sema/warn-unreachable-fallthrough.c
@@ -0,0 +1,13 @@
+// RUN: %clang_cc1 -fsyntax-only -verify -std=c2x -Wunreachable-code-fallthrough %s
+// RUN: %clang_cc1 -fsyntax-only -verify -std=c2x -Wunreachable-code %s
+// RUN: %clang_cc1 -fsyntax-only -verify=code -std=c2x -Wunreachable-code -Wno-unreachable-code-fallthrough %s
+// RUN: %clang_cc1 -fsyntax-only -verify -std=c2x -Wno-unreachable-code -Wunreachable-code-fallthrough %s
+
+int n;
+void f(void){
+     switch (n){
+         [[fallthrough]]; // expected-warning{{fallthrough annotation in unreachable code}}
+                          // code-warning@-1{{never be executed}}
+         case 1:;
+     }
+}