[Bug c/96956] New: When gcc does not see a label used in a goto it gives the wrong label address &&label

["thomas.lynch at reasoningtechnology dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96956

            Bug ID: 96956
           Summary: When gcc does not see a label used in a goto it gives
                    the wrong label  address &&label
           Product: gcc
           Version: 10.2.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: thomas.lynch at reasoningtechnology dot com
  Target Milestone: ---

Created attachment 49193
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=49193&action=edit
.c .i .s gcc version system info

The attached is an example distilled from much larger code.  It makes use of
the gcc extension  of  taking the address of a label. All optimizations are
turned off `-O0`.

The && operator returns the address for a different label than the one
requested.  Specificaly in this exmaple `&&nominal` returns the address for a
different label, `&&test0`.

The code example shown makes use of an inlined nested function to hide the goto
from gcc. However, the error occurs before the nested function is called. Note
the printf %p printf statements. 

With small variations of this code, gcc will return the correct address for
`&&nominal`.  If gcc sees an explicit goto to the label for
`&&nominal`, then the printf %p will give the correct value.

See attached for .c .i .s gcc version and sys info.


/* broken.c ----------------------------------------------------------------

gcc -std=gnu2x -Wall -O0 -ggdb  -o broken broken.c 

*/

#include <stdlib.h>
#include <stdio.h>
#include <stdint.h>
#include <stdbool.h>

typedef void **CV·Ptr; 
CV·Ptr target_pt;
uint i = 0;

int main(){

  goto test0;

  inline void do_jmp(CV·Ptr target_pt){
    goto *target_pt;
  }

  test0:;

  i++; // gets optimized away
  printf("%x\n", i);
  goto report;

  nominal:
  i++;
  goto tests_finished;

  report:;
  target_pt = &&nominal;
  printf("test0: %p\n", &&test0);
  // this will be identical to &&test0 and we haven't even called do_jmp:
  printf("nominal: %p\n", target_pt); 

  if( i == 2 ){
    printf("foo!\n");
    goto tests_finished;
  }
  do_jmp(target_pt);

  tests_finished:;
}