[Bug c/82187] New: missing optimization pointer to char*

[loader2x at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82187

            Bug ID: 82187
           Summary: missing optimization pointer to char*
           Product: gcc
           Version: 7.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: loader2x at gmail dot com
  Target Milestone: ---

I have this code: main.c:

int main(int c, char **v){
        char *s = v[0];
        while (*s++ != 0) {
                if ((*s == 'a') && (*s != 'b')) {
                        return 1;
                }
        }
        return 0;
}

which I compile generating the assembly code:

gcc -S -masm=intel -O3 main.c


main:
.LFB0:
        .cfi_startproc
        mov     rax, QWORD PTR [rsi]
        jmp     .L2
        .p2align 4,,10
        .p2align 3
.L4:
        cmp     BYTE PTR [rax], 97
        je      .L5
.L2:
        add     rax, 1
        cmp     BYTE PTR -1[rax], 0
        jne     .L4
        xor     eax, eax
        ret
.L5:
        mov     eax, 1
        ret


There are two accesses to the memory, it could be optimized to one access to
the memory.

When I set 'volatile char *s = v[0];', there are 3 accesses to the memory which
is good.