[Bug c/80253] New: Optimization silences __attribute__((fallthrough)) warning

[sirl at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=80253

            Bug ID: 80253
           Summary: Optimization silences __attribute__((fallthrough))
                    warning
           Product: gcc
           Version: 7.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: sirl at gcc dot gnu.org
  Target Milestone: ---

Created attachment 41073
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=41073&action=edit
testcase

The attached testcase issues 2 warnings with -O0 and only 1 warning with -O1 or
higher. I believe 2 warnings should be issued in both cases.

# gcc-trunk -Wall -c fallthrough-1.c -O0
fallthrough-1.c: In function 'test':
fallthrough-1.c:4:21: warning: attribute 'fallthrough' not preceding a case
label or default label
 #define FALLTHROUGH __attribute__((fallthrough))
                     ^
fallthrough-1.c:25:5: note: in expansion of macro 'FALLTHROUGH'
     FALLTHROUGH;
     ^~~~~~~~~~~
fallthrough-1.c:4:21: warning: attribute 'fallthrough' not preceding a case
label or default label
 #define FALLTHROUGH __attribute__((fallthrough))
                     ^
fallthrough-1.c:32:4: note: in expansion of macro 'FALLTHROUGH'
    FALLTHROUGH;
    ^~~~~~~~~~~


# gcc-trunk -Wall -c fallthrough-1.c -O1
fallthrough-1.c: In function 'test':
fallthrough-1.c:4:21: warning: attribute 'fallthrough' not preceding a case
label or default label
 #define FALLTHROUGH __attribute__((fallthrough))
                     ^
fallthrough-1.c:25:5: note: in expansion of macro 'FALLTHROUGH'
     FALLTHROUGH;
     ^~~~~~~~~~~