[Bug c/94012] New: Difference in gcc warning generation with c file and preprocessed file

[vvijayan at mathworks dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94012

            Bug ID: 94012
           Summary: Difference in gcc warning generation with c file and
                    preprocessed file
           Product: gcc
           Version: 9.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: vvijayan at mathworks dot com
  Target Milestone: ---

Hi,

I am observing a bit strange warning  behaviour in gcc while compiling
preprocessed file.

Warning thrown is "Wint-in-bool-context"

Small testcase to reproduce the issue in given below

$cat test.c

#define my_strcmp(s1, s2) ((s1) == (s2) ? 0 : -1)

int next(const char* s1, const char* s2, int flag) {

    if(!my_strcmp(s1, s2))
       flag = 1;

    return flag;
}

When compiled with gcc-8.3.0 (built from sources) observed following behaviour.

Compiling c file with -Wall - No warnings are thrown
====================================================
$gcc -c test.c -Wall
...

Compiling with -save-temps flag
===============================
$gcc -c test.c -Wall -save-temps
test.c: In function ‘next’:
test.c:6:27: warning: ?: using integer constants in boolean context
[-Wint-in-bool-context]
    6 |     if(!my_strcmp(s1, s2))
      |         ~~~~~~~~~~~~~~~~~~^    

...

Try to compile the preprocessed file - Same warning is thrown
====================================
$gcc -c test.ii -Wall
test.cpp: In function ‘int next(const char*, const char*, int)’:
test.cpp:6:23: warning: ?: using integer constants in boolean context
[-Wint-in-bool-context]
    6 |     if(!my_strcmp(s1, s2))
      |         ~~~~~~~~~~~~~~^~~~     


gcc -v
Using built-in specs.
COLLECT_GCC=../gcc-8.3.0/bin/gcc
COLLECT_LTO_WRAPPER=../8.3.0/lto-wrapper
Target: x86_64-pc-linux-gnu
Configured with: ../gcc-8.3/configure --with-gmp=../gcc-8.3/gmp-4.3
--with-mpfr../gcc-8.3/mpfr --with-mpc=../gcc-8.3/mpc
--enable-languages=c,c++,fortran  --enable-shared --enable-linker-build-id
--enable-plugin --enable-checking=release --enable-multiarch --enable-gold
--enable-ld=default --enable-libstdcxx-time=no --prefix=/opt/gcc-8.3.0
--with-pkgversion='GCC 8.3.0-gold' --with-tune=generic --with-system-zlib
--enable-multilib --with-multilib-list=m32,m64 --with-arch-directory=amd64
--with-arch-32=i586 --with-abi=m64
Thread model: posix
gcc version 8.3.0 (GCC 8.3.0-gold)

The issue exists in case of gcc-9.2.0 also.
$gcc -v
Using built-in specs.
COLLECT_GCC=gcc
COLLECT_LTO_WRAPPER=../gcc-9.2.0/libexec/gcc/x86_64-pc-linux-gnu/9.2.0/lto-wrapper
Target: x86_64-pc-linux-gnu
Configured with: ../gcc-9.2.0/configure --with-gmp=../gcc-9.2.0/gmp-4.3
--with-mpfr=../gcc-9.2.0/mpfr --with-mpc=../gcc-9.2.0/mpc
--enable-languages=c,c++,fortran --enable-shared --enable-linker-build-id
--enable-plugin --enable-checking=release --enable-multiarch --enable-gold
--enable-ld=default --enable-libstdcxx-time=no --prefix=/opt/gcc-9.2.0
--with-pkgversion='GCC 9.2.0-gold' --with-tune=generic --with-system-zlib
--enable-multilib --with-multilib-list=m32,m64 --with-arch-directory=amd64
--with-arch-32=i586 --with-abi=m64
Thread model: posix
gcc version 9.2.0 


Encountered this issue in a distcc enabled build where the files are
prepossessed before they sent to remote machine.

Regards,
Vinitha Vijayan