[Bug target/59744] New: miscompilation of unsigned comparison on aarch64

[michael.hudson at linaro dot org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=59744

            Bug ID: 59744
           Summary: miscompilation of unsigned comparison on aarch64
           Product: gcc
           Version: 4.9.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: michael.hudson at linaro dot org

Hi,

This slightly strangely written program (it's distilled down from
frame_offset_overflow in the gcc source itself) should print "bigger" if the
first argument is bigger than 10 (or negative, but let's ignore that please):

#include <stdlib.h>
#include <stdio.h>

int a[2] = { 10, 20 };

int
is_bigger (long offset, int index)
{
  unsigned long size = -offset;

  if (size > a[index])
    {
      printf("bigger\n");
      return 1;
    }

  return 0;
}


int
main (int argc, char** argv)
{
  long v;
  v = atol(argv[1]);
  is_bigger(-v, 0);
  return 0;
}

When compiled at -O1 or above (and with inlining disabled at -O2 and above),
though, it bungles the 0 case:

(t-doko)mwhudson@arm64:~$ gcc-4.9 -O3 test.c -o test -fno-inline -Wall
(t-doko)mwhudson@arm64:~$ ./test 1
(t-doko)mwhudson@arm64:~$ ./test 11
bigger
(t-doko)mwhudson@arm64:~$ ./test 0 
bigger
(t-doko)mwhudson@arm64:~$ gcc-4.9 -O0 test.c -o test -Wall
(t-doko)mwhudson@arm64:~$ ./test 1
(t-doko)mwhudson@arm64:~$ ./test 11
bigger
(t-doko)mwhudson@arm64:~$ ./test 0
(t-doko)mwhudson@arm64:~$ 

What's going on?  Here's the disassembly of is_bigger (at O3):

0000000000400608 <is_bigger>:
  400608:       b0000082        adrp    x2, 411000 <_GLOBAL_OFFSET_TABLE_+0x28>
  40060c:       91010042        add     x2, x2, #0x40
  400610:       a9bf7bfd        stp     x29, x30, [sp,#-16]!
  400614:       52800003        mov     w3, #0x0                        // #0
  400618:       910003fd        mov     x29, sp
  40061c:       b8a1d841        ldrsw   x1, [x2,w1,sxtw #2]
  400620:       ab00003f        cmn     x1, x0
  400624:       540000a2        b.cs    400638 <is_bigger+0x30>
  400628:       90000000        adrp    x0, 400000 <_init-0x3f8>
  40062c:       911b6000        add     x0, x0, #0x6d8
  400630:       97ffff90        bl      400470 <puts@plt>
  400634:       52800023        mov     w3, #0x1                        // #1
  400638:       2a0303e0        mov     w0, w3
  40063c:       a8c17bfd        ldp     x29, x30, [sp],#16
  400640:       d65f03c0        ret

Basically it seems that the condition "-offset > val" is being compiled as "val
+ offset does not overflow", which is not valid for offset == 0.