[Bug c/92395] New: m68k-linux-gnu-gcc generates wrong code when the -mshort option is used

[i...@abp-labs.com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=92395

            Bug ID: 92395
           Summary: m68k-linux-gnu-gcc generates wrong code when the
                    -mshort option is used
           Product: gcc
           Version: 7.4.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: i...@abp-labs.com
  Target Milestone: ---

Created attachment 47189
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=47189&action=edit
C source file

The option -mshort makes GCC m68k to use int size = 16-bit instead of 32-bit.

But the pointers are also affected by this option and the asm code generated is
buggy. Compiling the following test case gives:

file a.c

void f(char *begin, char *end)
{
    do
    {
        *end-- = 0;
    }
    while (end > begin);
}


m68k-linux-gnu-gcc -S -O2 -fomit-frame-pointer a.c -o -

        .file   "a.c"
        .text
        .align  2
        .globl  f
        .type   f, @function
f:
        move.l 4(%sp),%d0
        move.l 8(%sp),%a0
.L2:
        clr.b (%a0)
        subq.l #1,%a0
        cmp.l %d0,%a0
        jhi .L2
        rts
        .size   f, .-f
        .ident  "GCC: (Ubuntu 7.4.0-1ubuntu1~18.04.1) 7.4.0"
        .section        .note.GNU-stack,"",@progbits

m68k-linux-gnu-gcc -mshort -S -O2 -fomit-frame-pointer a.c -o -

        .file   "a.c"
        .text
        .align  2
        .globl  f
        .type   f, @function
f:
        move.l 4(%sp),%d1
        move.l 8(%sp),%a0
.L2:
        clr.b (%a0)
        move.l %a0,%d0
        subq.l #1,%d0
        add.l #65535,%a0 // Wrong code!
        cmp.l %d1,%d0
        jhi .L2
        rts
        .size   f, .-f
        .ident  "GCC: (Ubuntu 7.4.0-1ubuntu1~18.04.1) 7.4.0"
        .section        .note.GNU-stack,"",@progbits

Another example with itoa function attached:

m68k-linux-gnu-gcc -c itoa.c -Os -m68000 -Wall -mshort -fdata-sections
-ffunction-sections -fomit-frame-pointer -fno-builtin -fno-ident -S -o -

        .file   "itoa.c"
        .text
        .section        .text.itoa,"ax",@progbits
        .align  2
        .globl  itoa
        .type   itoa, @function
itoa:
        movem.l #14384,-(%sp)
        move.w 24(%sp),%d0
        move.l 26(%sp),%a0
        move.w 30(%sp),%d1
        move.w %d1,%d2
        subq.w #2,%d2
        cmp.w #14,%d2
        jhi .L9
        cmp.w #10,%d1
        jne .L10
        move.w %d0,%d3
        jge .L4
        neg.w %d3
.L4:
        move.w %d0,%d2
        move.w %d3,%d0
.L3:
        move.l %a0,%a2
        moveq #0,%d3                 // moveq #-1,%d3 -> 32-bit OK
        not.w %d3                    // remove this instruction
        lea digits.1019,%a3
.L5:
        ext.l %d0
        divs.w %d1,%d0
        move.l %d0,%d4
        swap %d4
        lea (1,%a2),%a1
        move.b (%a3,%d4.w),(%a1,%d3.l)
        tst.w %d0
        jne .L11
        tst.w %d2
        jge .L6
        lea (2,%a2),%a1
        move.b #45,1(%a2)
.L6:
        lea (-1,%a1),%a2
        move.l %a0,%a3
        moveq #0,%d0                 // moveq #-1,%d0 -> 32-bit OK
        not.w %d0                    // remove this instruction
.L7:
        cmp.l %a2,%a3
        jcs .L8
.L2:
        clr.b (%a1)
        move.l %a0,%d0
        movem.l (%sp)+,#3100
        rts
.L10:
        clr.w %d2
        jra .L3
.L11:
        move.l %a1,%a2
        jra .L5
.L8:
        move.b (%a2),%d1
        move.b (%a3)+,(%a2)
        move.b %d1,(%a3,%d0.l)
        add.l #65535,%a2            // add.l #-1,%a2 -> 32-bit OK
        jra .L7
.L9:
        move.l %a0,%a1
        jra .L2
        .size   itoa, .-itoa
        .section        .rodata.digits.1019,"a",@progbits
        .type   digits.1019, @object
        .size   digits.1019, 17
digits.1019:
        .string "0123456789abcdef"
        .section        .note.GNU-stack,"",@progbits