Re: Help with integer division overflow issue

[Izumi Tsutsui <tsutsui%ceres.dti.ne.jp@localhost>]

(re-send in proper encoding)

thorpej@ wrote:

> 
> > # cc -o test-ccr test-ccr.c
> > # ./test-ccr
> > val = -2147483648 (0x80000000)
> > ccr = 0x80000008
> > # 
> 
> Wait, WAAAT?
> 
> So, not only is the V flag not set, but N is set (yah, I guess the result is negative, but it should have overflowed!) and some random upper bit in the "int ccr;" variable is set???
> 
> Help me to understand!

@moveccr said V was cleared by movel following divsl and
the upper bit is a garbage of the previous result in %d0:

    2698:       42ae fff8       clrl %fp@(-8)
    269c:       202e fffc       movel %fp@(-4),%d0
    26a0:       4c79 0800 0000  divsll 4a50 <divisor>,%d0,%d0
    26a6:       4a50 
    26a8:       2d40 fffc       movel %d0,%fp@(-4)
                                ^^^^^^^^^^^^^^^^^^
    26ac:       42c0            movew %ccr,%d0
                                ^^^^^^^^^^^^^^
    26ae:       2d40 fff8       movel %d0,%fp@(-8)
    26b2:       2f2e fffc       movel %fp@(-4),%sp@-
    26b6:       2f2e fffc       movel %fp@(-4),%sp@-
    26ba:       4879 0000 27bc  pea 27bc <_fini+0x1c>
    26c0:       4eb9 0000 24cc  jsr 24cc <printf@plt>


With the following asm, N and V seem set:

---
__asm __volatile(" \
                  movel %0,%%d0 ;\
                  divsl divisor,%%d0 ;\
                  movew %%ccr,%%d1 ; \
                  movel %%d0,%0 ; \
                  movew %%d1,%1"
                  : "=r" (val) , "=r" (ccr)
                  : "0" (val)
                  : "d0", "d1");
---

    2698:       42ae fff8       clrl %fp@(-8)
    269c:       202e fffc       movel %fp@(-4),%d0
    26a0:       2040            moveal %d0,%a0
    26a2:       2008            movel %a0,%d0
    26a4:       4c79 0800 0000  divsll 4a58 <divisor>,%d0,%d0
    26aa:       4a58 
    26ac:       42c1            movew %ccr,%d1
    26ae:       2040            moveal %d0,%a0
    26b0:       3241            moveaw %d1,%a1
    26b2:       2d48 fffc       movel %a0,%fp@(-4)
    26b6:       2d49 fff8       movel %a1,%fp@(-8)
    26ba:       2f2e fffc       movel %fp@(-4),%sp@-
    26be:       2f2e fffc       movel %fp@(-4),%sp@-
    26c2:       4879 0000 27c4  pea 27c4 <_fini+0x1c>
    26c8:       4eb9 0000 24cc  jsr 24cc <printf@plt>

---
milan-% ./test-ccr
val = -2147483648 (0x80000000)
ccr = 0x0a
milan-% 
---

---
Izumi Tsutsui