Re: Pathalogical divides

[Jeffrey A Law <law at cygnus dot com>]

  In message <10009211926.AA26210@vlsi1.ultra.nyu.edu>you write:
  > Consider the following program on x86:
  > 
  > int rem (int a, int b) { return a % b; }
  > 
  > int
  > main ()
  > {
  >   printf ("%d\n", rem (0x80000000, -1));;
  > }
  > 
  > When run, rather than producing zero, as expected, it gets a SIGFPE.
  > This is because the division of the largest negative integer by negative on
  > e
  > results in an overflow.
  > 
  > So the first question is whether this is valid C behavior.
Not valid C behavior.

  > Next, compile the above with -O3 on an x86 and notice that GCC gets a
  > SIGFPE when constant-folding.
  > 
  > Finally, consider:
  > 
  > int
  > foo (int a, int b)
  > {
  >   return (a - ((a == 0x80000000 && b == -1) ? 0 : a % b)) / b;
  > }
  > 
  > This program when passed "normal" arguments does not get an overflow.
  > But GCC pulls the conditional out of the subtraction and division and
  > causes the compiler to run into the SIGFPE above.
  > 
  > I think the compiler crash needs to be fixed.  We can do it either by
  > protecting the integer part of simplify_binary_operation against SIGFPE
  > just like the FP or explicitly testing for this case just like we
  > check for divide by zero.
We need to fix the compiler crash.
jeff