[Pkg-samba-maint] Bug#445566: compiler bug pin-down

[Herbert Valerio Riedel]

hello!

I did some investigation, and the compiler bug seems to be related to
code inlining; I pinned it down to the following code:

/***********************************************************************/
typedef unsigned short u16;
typedef unsigned char u8;

static void
do_segfault(u8 in_buf[], const u8 out_buf[], const int len)
{
  int i;

  for (i = 0; i < len; i++) {
    //SSVAL(in_buf, 2*i, SVAL(out_buf,2*i)); // more or less just a byte-wise memcpy

    asm("nop\n");

    in_buf[2*i] = (   out_buf[2*i] | out_buf[(2*i)+1]<<8  ) & 0xFF; // in_buf[2*i] = out_buf[2*i];

    asm("nop\n");
	  
    in_buf[(2*i)+1] =  ( out_buf[2*i] | out_buf[(2*i)+1]<<8 ) >> 8; // in_buf[(2*i)+1] = out_buf[(2*i)+1];

    asm("nop\n");
  }
}

int main(int argc, char *argv[])
{
  u8 outbuf[32] = "buffer     ";
  u8 inbuf[32] = "\f";

  asm("nop\n");
  do_segfault(inbuf, outbuf, 12);
  asm("nop\n");

  return 0;
}
/***********************************************************************/

which when compiled with  4.2.3 20071123 (prerelease) (Debian 4.2.2-4),
causes a segfault when using -O2, but works when either removing the
'static' modifier and thus avoiding inlining of do_segfault, or by using
an optimization level which does avoid that... 

the generated assembler code is quite broken for the optimized case:

(I've only pasted and commented the relevant section containing the 5
nops)

// r4 points to outbuf (= source buffer)
// sp points to inbuf  (= target buffer)
#APP
        nop
        mov     r2, sp
        add     r1, sp, #56 // upper loop-bound; should have been #12
.L2:
#APP
        nop
        ldrb    r3, [r4, #0]
        strb    r3, [r2, #0]
#APP
        nop
        ldrb    r3, [r4, #1]
        strb    r3, [r2, #1]
#APP
        nop
        cmp     r4, r1 // upper loop-bound check
        add     r2, r2, #2 // increment target buffer (NB: source buffer is not incremented!)
        bne     .L2 // repeat loop if upper loop-bound not reached yet
#APP
        nop

cheers,
hvr