[Bug rtl-optimization/46598] [4.5/4.6 Regression] Miscompiles computed goto

[rguenth at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=46598

--- Comment #5 from Richard Guenther <rguenth at gcc dot gnu.org> 2010-11-23 
14:11:33 UTC ---
Even smaller testcase:

static int havege_bigarray [0x100000 + 16384];
static int andpt = 0;
static volatile int *havege_pwalk = 0;
static int PT = 0;
static int PT2 = 0;
void collect_ndrand(int xxx)
{
   static const int jumps[] = { &&loop40-&&loop40 };
   volatile int *const RESULT = havege_bigarray;
   volatile int *Pt0, *Pt1, *Pt2, *Pt3;
   int i, pt, havege_hardtick;
   i = 0;
   goto loop00;

loop40:
  pt = (PT >> 18) & 7;
  PT = PT & andpt;

  __asm__ __volatile__("rdtsc":"=A"(havege_hardtick));

  Pt0 = &havege_pwalk[PT];
  Pt1 = &havege_pwalk[PT2];
  Pt2 = &havege_pwalk[PT ^ 1];
  Pt3 = &havege_pwalk[PT2 ^ 4];

  RESULT[i++] ^= *Pt0;
  RESULT[i++] ^= *Pt1;
  RESULT[i++] ^= *Pt2;
  RESULT[i++] ^= *Pt3;

  PT = (((RESULT[(i - 8) ^ pt] ^ havege_pwalk[PT ^ pt ^ 7])) &
        (0xffffffef)) ^ ((PT2 ^ 0x10) & 0x10);
loop00:
   if (i < 0x100000) goto *(&&loop40+jumps[xxx]);
}

which shows, hah:

collect_ndrand:
.LFB0:
        .cfi_startproc
        movl    PT(%rip), %edx
        movl    $havege_bigarray, %eax
...

#APP
# 19 "pr46598_0.i" 1
        rdtsc
# 0 "" 2
#NO_APP
        movl    (%rax), %esi


Same thing after regalloc:

(insn 23 22 26 3 pr46598_0.i:19 (parallel [
            (set (reg:SI 1 dx)
                (asm_operands/v:SI ("rdtsc") ("=A") 0 []
                     []
                     [] pr46598_0.i:35))
            (clobber (reg:QI 18 fpsr))
            (clobber (reg:QI 17 flags))
        ]) -1 (nil))

of course the output is now completely unused, still "=A" should have
clobbered both ax and dx.