Implicit switch fall through triggers warnings with GCC 7

[akopytov]

ck_ht_hash.h contains a few switch() statements with implicit fall through logic. Which triggers GCC warnings with the -Wimplicit-fallthrough option introduced in GCC 7.

Code:

  switch(len & 3)
  {
  case 3: k1 ^= tail[2] << 16;
  case 2: k1 ^= tail[1] << 8;
  case 1: k1 ^= tail[0];
          k1 *= c1; k1 = ROTL32(k1,15); k1 *= c2; h1 ^= k1;
  };
...
  switch(len & 7)
  {
  case 7: h ^= (uint64_t)(data2[6]) << 48;
  case 6: h ^= (uint64_t)(data2[5]) << 40;
  case 5: h ^= (uint64_t)(data2[4]) << 32;
  case 4: h ^= (uint64_t)(data2[3]) << 24;
  case 3: h ^= (uint64_t)(data2[2]) << 16;
  case 2: h ^= (uint64_t)(data2[1]) << 8;
  case 1: h ^= (uint64_t)(data2[0]);
          h *= m;
  };
...
  switch(len)
  {
  case 3: h2 ^= ((const unsigned char*)data)[2] << 16;
  case 2: h2 ^= ((const unsigned char*)data)[1] << 8;
  case 1: h2 ^= ((const unsigned char*)data)[0];
      h2 *= m;
  };

Compiler warnings:

In file included from /build/sysbench-1.0.10.13/third_party/concurrency_kit/tmp/ck/src/ck_ht.c:44:0:

/build/sysbench-1.0.10.13/third_party/concurrency_kit/tmp/ck/src/ck_ht_hash.h: In function ‘MurmurHash64A’:

/build/sysbench-1.0.10.13/third_party/concurrency_kit/tmp/ck/src/ck_ht_hash.h:198:13: warning: this statement may fall through [-Wimplicit-fallthrough=]

   case 7: h ^= (uint64_t)(data2[6]) << 48;

           ~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~

/build/sysbench-1.0.10.13/third_party/concurrency_kit/tmp/ck/src/ck_ht_hash.h:199:3: note: here

   case 6: h ^= (uint64_t)(data2[5]) << 40;

   ^~~~

/build/sysbench-1.0.10.13/third_party/concurrency_kit/tmp/ck/src/ck_ht_hash.h:199:13: warning: this statement may fall through [-Wimplicit-fallthrough=]

   case 6: h ^= (uint64_t)(data2[5]) << 40;

           ~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~

/build/sysbench-1.0.10.13/third_party/concurrency_kit/tmp/ck/src/ck_ht_hash.h:200:3: note: here

   case 5: h ^= (uint64_t)(data2[4]) << 32;

   ^~~~

/build/sysbench-1.0.10.13/third_party/concurrency_kit/tmp/ck/src/ck_ht_hash.h:200:13: warning: this statement may fall through [-Wimplicit-fallthrough=]

   case 5: h ^= (uint64_t)(data2[4]) << 32;

           ~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~

/build/sysbench-1.0.10.13/third_party/concurrency_kit/tmp/ck/src/ck_ht_hash.h:201:3: note: here

   case 4: h ^= (uint64_t)(data2[3]) << 24;

   ^~~~

/build/sysbench-1.0.10.13/third_party/concurrency_kit/tmp/ck/src/ck_ht_hash.h:201:13: warning: this statement may fall through [-Wimplicit-fallthrough=]

   case 4: h ^= (uint64_t)(data2[3]) << 24;

           ~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~

/build/sysbench-1.0.10.13/third_party/concurrency_kit/tmp/ck/src/ck_ht_hash.h:202:3: note: here

   case 3: h ^= (uint64_t)(data2[2]) << 16;

   ^~~~

/build/sysbench-1.0.10.13/third_party/concurrency_kit/tmp/ck/src/ck_ht_hash.h:202:13: warning: this statement may fall through [-Wimplicit-fallthrough=]

   case 3: h ^= (uint64_t)(data2[2]) << 16;

           ~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~

/build/sysbench-1.0.10.13/third_party/concurrency_kit/tmp/ck/src/ck_ht_hash.h:203:3: note: here

   case 2: h ^= (uint64_t)(data2[1]) << 8;

   ^~~~

/build/sysbench-1.0.10.13/third_party/concurrency_kit/tmp/ck/src/ck_ht_hash.h:203:13: warning: this statement may fall through [-Wimplicit-fallthrough=]

   case 2: h ^= (uint64_t)(data2[1]) << 8;

           ~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~

/build/sysbench-1.0.10.13/third_party/concurrency_kit/tmp/ck/src/ck_ht_hash.h:204:3: note: here

   case 1: h ^= (uint64_t)(data2[0]);

   ^~~~

The manual lists a number of options to silence the warnings, which I guess is the right thing to do here. The most portable way seems to be adding /* fall through */ comments, which are also supported by Clang.

[vlm]

+1 /* Fall through */ is indeed the most portable and have been around for ages.