[Bug rtl-optimization/110791] New: [12/13/14 Regression] arm: Wrong code with -Os -march=armv8.1-m.main

["acoplan at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110791

            Bug ID: 110791
           Summary: [12/13/14 Regression] arm: Wrong code with -Os
                    -march=armv8.1-m.main
           Product: gcc
           Version: 13.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: rtl-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: acoplan at gcc dot gnu.org
  Target Milestone: ---

The following testcase is miscompiled since at least GCC 12 with -Os
-march=armv8.1-m.main:

void __attribute__ ((noinline))
foo (char *path, int *result)
{
  char *p = path;
  while (p >= path && *p != '/')
    p--;
  while (p > path && p[-1] == '/')
    p--;

  if (p < path)
    *result = 1;
}

int main(void)
{
  char path[4] = "usr";
  int x = 0;
  foo (path + 2, &x);
  if (!x)
    __builtin_abort ();
}

Below is the assembly we currently generate together with comments showing how
this goes wrong at runtime:

foo:
        mov     r3, r0         @ r3 <- (p = path)
        ldrb    r2, [r3], #-1  @ r2 <- *p; p--;
        cmp     r2, #47
        it      eq
        moveq   r3, r0         @ if (r2 == 47)  p <- path
        subs    r2, r3, r0
        cmp     r0, r3
        add     r2, r2, #1
        bhi     .L9            @ if (path > p) goto .L9 [taken]
        adds    r0, r0, #1
        bne     .L6
.L9:
        movs    r2, #1
.L6:
        subs    r2, r2, #1     @ r2 <- 0 [fall through from above]
        bne     .L3            @ [not taken, r2 was #1]
        bcc     .L4            @ [not taken]
        bx      lr             @ [return without setting *result = 1]
.L3:
        ldrb    r0, [r3, #-1]!
        cmp     r0, #47
        beq     .L6
        bx      lr
.L4:
        movs    r3, #1
        str     r3, [r1]
        bx      lr

At -O2 the code is both correct and much better quality:

foo:
        ldrb    r3, [r0]        @ zero_extendqisi2
        cmp     r3, #47
        itt     ne
        movne   r3, #1
        strne   r3, [r1]
        bx      lr