[Bug c/70477] New: -Wtautological-compare too aggressive?

[ktkachov at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70477

            Bug ID: 70477
           Summary: -Wtautological-compare too aggressive?
           Product: gcc
           Version: 6.0
            Status: UNCONFIRMED
          Keywords: diagnostic
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: ktkachov at gcc dot gnu.org
  Target Milestone: ---

I'm trying to perform an aarch64-none-linux-gnu bootstrap with -save-temps in
the BOOT_CFLAGS but I'm getting a -Wtautological-compare failure when compiling
c-family/c-pragma.c:

gcc/c-family/c-pragma.c:415:64: warning: self-comparison always evaluates to
false [-Wtautological-compare]
   if (BYTES_BIG_ENDIAN != WORDS_BIG_ENDIAN)
       ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~                    ^ 

For aarch64 these are defined as:
#define BYTES_BIG_ENDIAN (TARGET_BIG_END != 0)
#define WORDS_BIG_ENDIAN (BYTES_BIG_ENDIAN)

I think this can be reproduced also as:

int var;

#define A var
#define B var

int foo (int a, int b)
{
  if (A == B)
    return a;
  else
    return b;
}

gcc -Wtautological-compare -c warn.c
warn.c: In function 'foo':
warn.c:8:9: warning: self-comparison always evaluates to true
[-Wtautological-compare]
   if (A == B)
         ^~

Should we be warning on these things if they come from a macro expansion?