Enable implicit fallthrough warning

eng/native/configurecompiler.cmake

@@ -323,6 +323,7 @@ if (CLR_CMAKE_HOST_UNIX)
   add_compile_options(-Wno-unused-value)
   add_compile_options(-Wno-unused-function)
   add_compile_options(-Wno-tautological-compare)
+  add_compile_options(-Wimplicit-fallthrough)
 
   #These seem to indicate real issues
   add_compile_options($<$<COMPILE_LANGUAGE:CXX>:-Wno-invalid-offsetof>)

src/coreclr/src/ToolBox/superpmi/mcs/verbildump.cpp

@@ -893,6 +893,7 @@ void DumpILToConsoleBare(unsigned char* ilCode, int len)
                         LogError("unknown ilCode 0xfe%2x at offset %d in MethodGen::PrettyPrint", ilCode[i], i);
                         break;
                 }
+                break;
             default:
                 LogError("unknown ilCode 0x%02x at offset %d in MethodGen::PrettyPrint", ilCode[i], i);
                 break;

src/coreclr/src/classlibnative/bcltype/varargsnative.cpp

@@ -575,7 +575,8 @@ VarArgsNative::GetNextArgHelper(
             }
 #endif
 
-        // fall through
+            __fallthrough;
+
         case ELEMENT_TYPE_CLASS: {
             value->type = data->SigPtr.GetTypeHandleThrowing(data->ArgCookie->pModule, &typeContext);
 

src/coreclr/src/debug/di/rstype.cpp

@@ -590,7 +590,7 @@ HRESULT CordbType::MkType(CordbAppDomain * pAppDomain,
 
       pClass->SetIsValueClass(true);
       pClass->SetIsValueClassKnown(true);
-      // drop through
+      __fallthrough;
 
     case ELEMENT_TYPE_CLASS:
         {
@@ -2009,6 +2009,7 @@ void CordbType::TypeToExpandedTypeData(DebuggerIPCE_ExpandedTypeData *data)
         }
     case ELEMENT_TYPE_END:
         _ASSERTE(!"bad element type!");
+        break;
 
     default:
         data->elementType = m_elementType;
@@ -2389,6 +2390,7 @@ HRESULT CordbType::GetTypeID(COR_TYPEID *pId)
         case ELEMENT_TYPE_BYREF:
         case ELEMENT_TYPE_FNPTR:
             IfFailThrow(CORDBG_E_UNSUPPORTED);
+            break;
         default:
             _ASSERTE(!"unexpected element type!");
             IfFailThrow(CORDBG_E_UNSUPPORTED);

src/coreclr/src/debug/ee/amd64/amd64walker.cpp

@@ -580,6 +580,7 @@ static int opSize(Amd64InstrDecode::InstrForm form, int pp, bool W, bool L, bool
     // P_M6B_or_M4B
     case Amd64InstrDecode::InstrForm::MOnly_P_M6B_or_M4B:
         opSize = P ? 6 : 4;
+        break;
     // M4B
     case Amd64InstrDecode::InstrForm::M1st_M4B:
     case Amd64InstrDecode::InstrForm::M1st_M4B_I1B:
@@ -833,7 +834,7 @@ void NativeWalker::DecodeInstructionForPatchSkip(const BYTE *address, Instructio
                 opCodeMap = Primary;
                 break;
             }
-            // Fall through
+            __fallthrough;
         case 0xc4: // Vex 3-byte
             opCodeMap = (OpcodeMap)(int(address[0]) << 8 | (address[1] & 0x1f));
             // W is the top bit of opcode2.
@@ -965,7 +966,7 @@ void NativeWalker::DecodeInstructionForPatchSkip(const BYTE *address, Instructio
                     case 2:
                     case 3:
                         pInstrAttrib->m_fIsCall = true;
-                        // fall through
+                        __fallthrough;
                     case 4:
                     case 5:
                         pInstrAttrib->m_fIsAbsBranch = true;

src/coreclr/src/debug/ee/controller.cpp

@@ -5884,6 +5884,7 @@ bool DebuggerStepper::TrapStep(ControllerStackInfo *info, bool in)
                 fIsJump = true;
 
                 // fall through...
+                __fallthrough;
 
             case WALK_CALL:
                 LOG((LF_CORDB,LL_INFO10000, "DC::TS:Imm:WALK_CALL ip=%p nextip=%p skipip=%p\n", walker.GetIP(), walker.GetNextIP(), walker.GetSkipIP()));

src/coreclr/src/debug/ee/debugger.cpp

@@ -10722,6 +10722,7 @@ bool Debugger::HandleIPCEvent(DebuggerIPCEvent * pEvent)
         //
         // For regular (non-jit) attach, fall through to do an async break.
         //
+        __fallthrough;
 
     case DB_IPCE_ASYNC_BREAK:
         {
@@ -12138,7 +12139,7 @@ void Debugger::TypeHandleToExpandedTypeInfo(AreValueTypesBoxed boxed,
     case ELEMENT_TYPE_VALUETYPE:
         if (boxed == OnlyPrimitivesUnboxed || boxed == AllBoxed)
             res->elementType = ELEMENT_TYPE_CLASS;
-        // drop through
+        __fallthrough;
 
     case ELEMENT_TYPE_CLASS:
         {

src/coreclr/src/gc/unix/gcenv.unix.cpp

@@ -792,8 +792,10 @@ bool ReadMemoryValueFromFile(const char* filename, uint64_t* val)
     {
     case 'g':
     case 'G': multiplier = 1024;
+              [[fallthrough]];
     case 'm':
     case 'M': multiplier = multiplier * 1024;
+              [[fallthrough]];
     case 'k':
     case 'K': multiplier = multiplier * 1024;
     }

src/coreclr/src/ilasm/assem.cpp

@@ -1611,16 +1611,26 @@ unsigned hash(
    switch(len)              /* all the case statements fall through */
    {
        case 11: c+=((unsigned)k[10] << 24);
+                __fallthrough;
        case 10: c+=((unsigned)k[9] << 16);
+                __fallthrough;
        case 9 : c+=((unsigned)k[8] << 8);
+                __fallthrough;
           /* the first byte of c is reserved for the length */
        case 8 : b+=((unsigned)k[7] << 24);
+                __fallthrough;
        case 7 : b+=((unsigned)k[6] << 16);
+                __fallthrough;
        case 6 : b+=((unsigned)k[5] << 8);
+                __fallthrough;
        case 5 : b+=k[4];
+                __fallthrough;
        case 4 : a+=((unsigned)k[3] << 24);
+                __fallthrough;
        case 3 : a+=((unsigned)k[2] << 16);
+                __fallthrough;
        case 2 : a+=((unsigned)k[1] << 8);
+                __fallthrough;
        case 1 : a+=k[0];
      /* case 0: nothing left to add */
    }

src/coreclr/src/ilasm/assembler.cpp

@@ -1578,6 +1578,7 @@ void Assembler::EmitInstrVarByName(Instr* instr, __in __nullterminated char* lab
         case CEE_STARG:
         case CEE_STARG_S:
             nArgVarFlag++;
+            __fallthrough;
         case CEE_LDLOCA:
         case CEE_LDLOCA_S:
         case CEE_LDLOC:

src/coreclr/src/ilasm/prebuilt/asmparse.cpp

@@ -2019,6 +2019,7 @@ YYLOCAL YYNEAR YYPASCAL YYPARSER()
 
 		yyerrlab:
 			++yynerrs;
+            __fallthrough;
 
 		case 1:
 		case 2: /* incompletely recovered error ... try again */

src/coreclr/src/ildasm/dasm.cpp

@@ -2008,6 +2008,7 @@ BYTE* PrettyPrintCABlobValue(PCCOR_SIGNATURE &typePtr,
                 break;
             case ELEMENT_TYPE_CLASS        :
                 typePtr += CorSigUncompressToken(typePtr, &tk); //skip the following token
+                __fallthrough;
             case SERIALIZATION_TYPE_TYPE   :
                 appendStr(out,KEYWORD("type"));
                 appendStr(out,appendix);
@@ -2668,6 +2669,7 @@ void DumpDefaultValue(mdToken tok, __inout __nullterminated char* szString, void
                 break;
             }
             //else fall thru to default case, to report the error
+            __fallthrough;
 
         default:
             szptr+=sprintf_s(szptr,SZSTRING_REMAINING_SIZE(szptr),ERRORMSG(" /* ILLEGAL CONSTANT type:0x%02X, size:%d bytes, blob: "),MDDV.m_bType,MDDV.m_cbSize);

src/coreclr/src/ildasm/dasm_sz.cpp

@@ -211,6 +211,7 @@ unsigned SizeOfField(PCCOR_SIGNATURE *ppSig, ULONG cSig, IMDInternalImport* pImp
                     case ELEMENT_TYPE_CMOD_OPT  :
                     case ELEMENT_TYPE_CMOD_REQD :
                         *ppSig += CorSigUncompressToken(*ppSig, &tk);
+                         __fallthrough; // TODO: is this correct?
                     case ELEMENT_TYPE_PINNED    :
                     case ELEMENT_TYPE_SZARRAY    : // uElementNumber doesn't change
                         if(*ppSig < pSigEnd) Reiterate = TRUE;

src/coreclr/src/jit/codegenlinear.cpp

@@ -731,6 +731,7 @@ void CodeGen::genCodeForBBlist()
 
             case BBJ_EHCATCHRET:
                 noway_assert(!"Unexpected BBJ_EHCATCHRET"); // not used on x86
+                break;
 
             case BBJ_EHFINALLYRET:
             case BBJ_EHFILTERRET:

src/coreclr/src/jit/emitarm.cpp

@@ -2133,7 +2133,6 @@ void emitter::emitIns_R_R(
                 assert(isFloatReg(reg1));
                 assert(isFloatReg(reg2));
             }
-            __fallthrough;
 
         VCVT_COMMON:
             fmt = IF_T2_VFP2;
@@ -2847,7 +2846,6 @@ void emitter::emitIns_R_R_I(instruction ins,
             //
             // If we did not find a thumb-1 encoding above
             //
-            __fallthrough;
 
         COMMON_THUMB2_LDST:
             assert(fmt == IF_NONE);
@@ -3070,9 +3068,9 @@ void emitter::emitIns_R_R_R(instruction ins,
                     assert(!"Instruction cannot be encoded");
                 }
             }
-            __fallthrough;
 
 #if !defined(USE_HELPERS_FOR_INT_DIV)
+            __fallthrough;
         case INS_sdiv:
         case INS_udiv:
 #endif // !USE_HELPERS_FOR_INT_DIV

src/coreclr/src/jit/emitarm64.cpp

@@ -13920,6 +13920,7 @@ emitter::insExecutionCharacteristics emitter::getInsExecutionCharacteristics(ins
                     perfScoreUnhandledInstruction(id, &result);
                     break;
             }
+            break;
 
         // ALU, basic immediate
         case IF_DI_1A: // cmp, cmn

src/coreclr/src/jit/flowgraph.cpp

@@ -4968,6 +4968,7 @@ void Compiler::fgFindJumpTargets(const BYTE* codeAddr, IL_OFFSET codeSize, Fixed
                 break;
             case CEE_RET:
                 retBlocks++;
+                break;
 
             default:
                 break;
@@ -5299,7 +5300,8 @@ void Compiler::fgLinkBasicBlocks()
                     BADCODE("Fall thru the end of a method");
                 }
 
-            // Fall through, the next block is also reachable
+                // Fall through, the next block is also reachable
+                __fallthrough;
 
             case BBJ_NONE:
                 curBBdesc->bbNext->bbRefs++;
@@ -5661,7 +5663,7 @@ unsigned Compiler::fgMakeBasicBlocks(const BYTE* codeAddr, IL_OFFSET codeSize, F
                the target of a jump.
              */
 
-            // fall-through
+            __fallthrough;
 
             case CEE_JMP:
             /* These are equivalent to a return from the current method
@@ -16628,6 +16630,7 @@ void Compiler::fgDetermineFirstColdBlock()
             {
                 default:
                     noway_assert(!"Unhandled jumpkind in fgDetermineFirstColdBlock()");
+                    break;
 
                 case BBJ_CALLFINALLY:
                     // A BBJ_CALLFINALLY that falls through is always followed
@@ -21507,6 +21510,7 @@ void Compiler::fgDebugCheckFlags(GenTree* tree)
                 break;
             case GT_ADDR:
                 assert(!op1->CanCSE());
+                break;
 
             case GT_IND:
                 // Do we have a constant integer address as op1?
@@ -21578,6 +21582,7 @@ void Compiler::fgDebugCheckFlags(GenTree* tree)
                         //                 +--------------+----------------+
                     }
                 }
+                break;
 
             default:
                 break;

src/coreclr/src/jit/gentree.cpp

@@ -4183,7 +4183,8 @@ unsigned Compiler::gtSetEvalOrder(GenTree* tree)
                         break;
                     }
 
-                // fall through and set GTF_REVERSE_OPS
+                    // fall through and set GTF_REVERSE_OPS
+                    __fallthrough;
 
                 case GT_LCL_VAR:
                 case GT_LCL_FLD:
@@ -5689,9 +5690,9 @@ bool GenTree::OperMayThrow(Compiler* comp)
                 //
                 return true;
             }
+            break;
         }
 #endif // FEATURE_HW_INTRINSICS
-
         default:
             break;
     }
@@ -11320,6 +11321,7 @@ void Compiler::gtDispLeaf(GenTree* tree, IndentStack* indentStack)
         case GT_JCMP:
             printf(" cond=%s%s", (tree->gtFlags & GTF_JCMP_TST) ? "TEST_" : "",
                    (tree->gtFlags & GTF_JCMP_EQ) ? "EQ" : "NE");
+            break;
 
         default:
             assert(!"don't know how to display tree leaf node");
@@ -14581,6 +14583,7 @@ GenTree* Compiler::gtFoldExprConst(GenTree* tree)
 #endif
                         goto DONE;
                     }
+                    break;
 
                 default:
                     break;
@@ -17456,8 +17459,8 @@ GenTree* Compiler::gtGetSIMDZero(var_types simdType, var_types baseType, CORINFO
                         break;
                     case TYP_UINT:
                         assert(simdHandle == m_simdHandleCache->Vector64UIntHandle);
-                        break;
 #endif // defined(TARGET_ARM64) && defined(FEATURE_HW_INTRINSICS)
+                        break;
                     default:
                         break;
                 }

src/coreclr/src/jit/importer.cpp

@@ -5630,6 +5630,7 @@ void Compiler::verVerifyCall(OPCODE                  opcode,
             }
         }
         // fall thru to default checks
+        __fallthrough;
         default:
             VerifyOrReturn(!(mflags & CORINFO_FLG_ABSTRACT), "method abstract");
     }
@@ -13139,6 +13140,7 @@ void Compiler::impImportBlockCode(BasicBlock* block)
                         case CEE_CONV_OVF_U_UN:
                         case CEE_CONV_U:
                             isNative = true;
+                            break;
                         default:
                             // leave 'isNative' = false;
                             break;
@@ -14108,7 +14110,7 @@ void Compiler::impImportBlockCode(BasicBlock* block)
                     }
                 }
 
-            // fall through
+                __fallthrough;
 
             case CEE_CALLVIRT:
             case CEE_CALL:
@@ -16346,6 +16348,7 @@ void Compiler::impImportBlockCode(BasicBlock* block)
 
             case 0xCC:
                 OutputDebugStringA("CLR: Invalid x86 breakpoint in IL stream\n");
+                __fallthrough;
 
             case CEE_ILLEGAL:
             case CEE_MACRO_END:

src/coreclr/src/jit/inlinepolicy.cpp

@@ -1536,6 +1536,7 @@ void DiscretionaryPolicy::ComputeOpcodeBin(OPCODE opcode)
 
         case CEE_RET:
             m_ReturnCount++;
+            break;
 
         default:
             break;

src/coreclr/src/jit/instr.cpp

@@ -2231,8 +2231,10 @@ instruction CodeGen::ins_FloatConv(var_types to, var_types from)
             {
                 case TYP_FLOAT:
                     NYI("long to float");
+                    break;
                 case TYP_DOUBLE:
                     NYI("long to double");
+                    break;
                 default:
                     unreached();
             }
@@ -2246,6 +2248,7 @@ instruction CodeGen::ins_FloatConv(var_types to, var_types from)
                     return INS_vcvt_f2u;
                 case TYP_LONG:
                     NYI("float to long");
+                    break;
                 case TYP_DOUBLE:
                     return INS_vcvt_f2d;
                 case TYP_FLOAT:
@@ -2263,6 +2266,7 @@ instruction CodeGen::ins_FloatConv(var_types to, var_types from)
                     return INS_vcvt_d2u;
                 case TYP_LONG:
                     NYI("double to long");
+                    break;
                 case TYP_FLOAT:
                     return INS_vcvt_d2f;
                 case TYP_DOUBLE:
@@ -2274,6 +2278,7 @@ instruction CodeGen::ins_FloatConv(var_types to, var_types from)
         default:
             unreached();
     }
+    unreached();
 }
 
 #elif defined(TARGET_ARM64)