[Lightning] `modi' broken on PowerPC

[Ludovic Courtès]

Hi,

The `modi' instruction is apparently broken on PowerPC, as shown by the
test case attached below:

  $ ./lightning-modi 
  mod_eight (20420) = 20412 (vs. 4)
  lightning-modi: lightning-modi.c:42: main: Assertion `mod_eight (r) == (r % 
8)' failed.
  Aborted
  $ ./lightning-modi 
  mod_eight (216096) = 216088 (vs. 0)
  lightning-modi: lightning-modi.c:42: main: Assertion `mod_eight (r) == (r % 
8)' failed.
  Aborted

PowerPC apparently doesn't have any integer-modulo instruction (?!), so
Lightning generates relatively complex code:

  (gdb) disassemble result result+100
  Dump of assembler code from 0x10011518 to 0x1001157c:
  0x10011518 <buffer.2607+0>:     mflr    r0
  0x1001151c <buffer.2607+4>:     stwu    r1,-96(r1)
  0x10011520 <buffer.2607+8>:     stmw    r23,60(r1)
  0x10011524 <buffer.2607+12>:    stw     r0,100(r1)
  0x10011528 <buffer.2607+16>:    mr      r23,r3
  0x1001152c <buffer.2607+20>:    mr      r10,r23
  0x10011530 <buffer.2607+24>:    li      r24,8
  0x10011534 <buffer.2607+28>:    mtlr    r31
  0x10011538 <buffer.2607+32>:    li      r24,24
  0x1001153c <buffer.2607+36>:    divw    r31,r10,r24
  0x10011540 <buffer.2607+40>:    mullw   r31,r31,r24
  0x10011544 <buffer.2607+44>:    subf    r10,r24,r10
  0x10011548 <buffer.2607+48>:    mflr    r31
  0x1001154c <buffer.2607+52>:    mr      r3,r11
  0x10011550 <buffer.2607+56>:    lwz     r0,100(r1)
  0x10011554 <buffer.2607+60>:    mtlr    r0
  0x10011558 <buffer.2607+64>:    lmw     r23,60(r1)
  0x1001155c <buffer.2607+68>:    addi    r1,r1,96
  0x10011560 <buffer.2607+72>:    blr

I am not very familiar with PPC assembly so I did not try to investigate
it any further so far.  I'd be glad to help as much as I can though.

Thanks,
Ludovic.


#include <lightning.h>
#include <sys/time.h>
#include <time.h>
#include <stdio.h>
#include <assert.h>

typedef int (* mod_t) (int);

mod_t
generate_modi (int operand)
{
  static char buffer[1024];
  mod_t result;
  int arg;

  result = (mod_t)(jit_set_ip (buffer).iptr);
  jit_leaf (1);
  arg = jit_arg_i ();
  jit_getarg_i (JIT_R1, arg);

  jit_modi_i (JIT_R2, JIT_R1, operand);
  jit_movr_i (JIT_RET, JIT_R2);

  jit_ret ();
  jit_flush_code (buffer, jit_get_ip ().ptr);

  return result;
}

int
main (int argc, char *argv[])
{
  int r;
  struct timeval now;
  mod_t mod_eight = generate_modi (8);

  gettimeofday (&now, NULL);
  srandom (now.tv_usec + now.tv_sec);

  r = (random () % 231233) + 8;
  printf ("mod_eight (%i) = %i (vs. %i)\n",
          r, mod_eight (r), (r % 8));
  assert (mod_eight (r) == (r % 8));

  return 0;
}