[Bug c/96468] New: Warn when an empty while loop could have been a do-while

["josephcsible at gmail dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96468

            Bug ID: 96468
           Summary: Warn when an empty while loop could have been a
                    do-while
           Product: gcc
           Version: 10.2.0
            Status: UNCONFIRMED
          Keywords: diagnostic
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: josephcsible at gmail dot com
  Target Milestone: ---

Consider this C code:

typedef int sig_atomic_t;
volatile sig_atomic_t signaled;
_Bool getX(int *);
void processX(int);
void f(void)
{
    {
        int x;
        if(getX(&x))
            processX(x);
    }
    while(!signaled);
    /* do some other stuff */
}

There's two possibilities for what the author meant for it to do:
1. Do getX (and maybe processX) once (in a block just to minimize the scope of
x), then busy-wait until signaled becomes true, and then do some other stuff.
This is what it actually does.
2. Keep doing getX (and maybe processX) in a loop until signaled becomes true,
and then do some other stuff. This isn't what it actually does, because the
author forgot the "do" keyword.

We currently emit no warnings for this code, even when compiled with "-Wall
-Wextra". I propose that when we see "while(condition);", we warn that a "do"
may be missing, and if it's not, that you should use "while(condition){}"
instead (except in cases where it's impossible to just be missing the "do"
keyword, like if the "while" is at the beginning of a block).