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Created: Friday, April 25 2008 09:21.42 CDT

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Integer overflow

Author: EliCZ

# Views: 10532

How to write integer(32) division in i386 assembly language using idiv instruction so that it doesn't raise an exception?

mov   eax, Dividend
mov   ecx, Divisor
idiv  ecx

It can throw #DE, add Divisor == 0 check:

mov   eax, Dividend
mov   ecx, Divisor
jecxz DontIdiv
idiv  ecx

It can still throw #DE, edx must be initialized for (i)div.
AMD64 manual : "To avoid overflow problems, precede this instruction (idiv) with a CBW, CWD, CDQ, or CQO instruction to sign-extend the dividend."

mov   eax, Dividend
mov   ecx, Divisor
jecxz DontIdiv
cdq
idiv  ecx

It looks fixed (C compilers think so), can it throw #DE now?

Yes, it can - I didn't avoid overflow problems.
There is still one combination that will raise #DE:
Dividend = INT_MIN and Divisor = -1.

mov   eax, Dividend
mov   ecx, Divisor
jecxz DontIdiv
cmp   eax, INT_MIN
sete  dl
cmp   ecx, -1
sete  dh
test  dl, dh
jne   DontIdiv
cdq
idiv  ecx

Other integer arithmetic instructions set OF only and the result for 0-INT_MIN is INT_MIN.

How do C compilers (CPUs, OSes) handle the integer overflow?

neg32.c:
int neg32(int x) {
  return(-x);
}

neg64.c:
long long neg64(long long x) {
  return(-x);
}

idiv32.c:
int idiv32(int x, int y) {
  return(x/y);
}

idiv64.c:
#define __USE_ISOC99
#include <limits.h>
#ifndef LLONG_MIN
  #ifdef LONG_LONG_MIN
    #define LLONG_MIN LONG_LONG_MIN
  #else
    #define LLONG_MIN -9223372036854775808LL
  #endif
#endif
#include <stdio.h>

long long idiv64(long long x, long long y) {
  return(x/y);
}

int main(void) {
  long long x = LLONG_MIN;
  long long y = -1;
  long long r = 1234;
  r = idiv64(x, y);
  return(printf("%lli\n", r));
}

Sometimes a (CRT) function is used instead of division performing instruction,
especially for integer64 division in 32bit builds.
(int)-INT_MIN == INT_MIN always.

architecture/os/compilers/builds neg32 | neg64 | idiv32 | idiv64
i386/winnt/msc|dmc/32 INT_MIN | LLONG_MIN | crash | LLONG_MIN
i386/winnt/wc/32  INT_MIN | LLONG_MIN | print exception info and exit | LLONG_MIN
i386/linux/gcc/32 INT_MIN | LLONG_MIN | "Floating point exception" | LLONG_MIN
amd64/winnt/msc/64 INT_MIN | LLONG_MIN | crash | crash
amd64/linux/gcc/64 INT_MIN | LLONG_MIN | "Floating point exception" | "Floating point exception"
ia64/winnt/msc/32 INT_MIN | LLONG_MIN | long pause and exit | LLONG_MIN
ia64/winnt/msc/64 INT_MIN | LLONG_MIN | INT_MIN | LLONG_MIN
power/aix/gcc|xlc/32 INT_MIN | LLONG_MIN | 0 | LLONG_MIN
power/aix/gcc|xlc/64 INT_MIN | LLONG_MIN | 0 | 0
sparc/solaris/gcc|suncc/32|64 INT_MIN | LLONG_MIN | INT_MIN | LLONG_MIN

See you at caro workshop.

Blog Comments

GynvaelColdwind	Posted: Friday, April 25 2008 11:46.03 CDT
Woah... INT_MIN / -1 ... I've never thought of that! Good stuff mate!

nezumi	Posted: Monday, May 5 2008 05:31.56 CDT
clever post! I just would like to add: `-INT_MIN/-1; (INT_MAX + 1)/-1;` causes an exception as well. (yeah, I know that -INT_MIN/-1 and (INT_MAX + 1)/-1 it's the same, however they look different at source level before pre-processing)

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