[Bug c/80341] New: gcc miscompiles division of signed char

[vsevolod.livinskij at frtk dot ru]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=80341

            Bug ID: 80341
           Summary: gcc miscompiles division of signed char
           Product: gcc
           Version: 7.0.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: vsevolod.livinskij at frtk dot ru
  Target Milestone: ---

Created attachment 41140
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=41140&action=edit
Reproducer.

GCC generates incorrect code. Correct result is 90.

Reproducer:
>$ cat foo.c
extern const signed char var_25;
extern signed char var_465;

void foo() { 
    var_465 = (unsigned short) var_25 / -55; 
} 
>$ cat main.c
#include <stdio.h>

extern void foo ();

const signed char var_25 = -84;
signed char var_465 = 0;

int main () {
    foo ();
    printf("%d\n", (int) var_465);
    return 0;
}

Run:
>$ gcc main.c foo.c ; ./a.out 
1

GCC version:
Rev. 246681
>$ gcc -v
Using built-in specs.
COLLECT_GCC=gcc
COLLECT_LTO_WRAPPER=/home/vsevolod/workspace/gcc-dev/bin-trunk/libexec/gcc/x86_64-pc-linux-gnu/7.0.1/lto-wrapper
Target: x86_64-pc-linux-gnu
Configured with: /home/vsevolod/workspace/gcc-dev/trunk/configure
--prefix=/home/vsevolod/workspace/gcc-dev/bin-trunk --disable-bootstrap
Thread model: posix
gcc version 7.0.1 20170404 (experimental) (GCC)