[LLVMdev] APInt::sdivrem error?

[Preston Briggs]

OK, the code for sdivrem in APInt.h is wrong.
Here's what's written:

 static void sdivrem(const APInt &LHS, const APInt &RHS,
                     APInt &Quotient, APInt &Remainder) {
   if (LHS.isNegative()) {
     if (RHS.isNegative())
       APInt::udivrem(-LHS, -RHS, Quotient, Remainder);
     else
       APInt::udivrem(-LHS, RHS, Quotient, Remainder);
     Quotient = -Quotient;
     Remainder = -Remainder;
   } else if (RHS.isNegative()) {
     APInt::udivrem(LHS, -RHS, Quotient, Remainder);
     Quotient = -Quotient;
   } else {
     APInt::udivrem(LHS, RHS, Quotient, Remainder);
   }
 }


Here's what it should be:

 static void sdivrem(const APInt &LHS, const APInt &RHS,
                     APInt &Quotient, APInt &Remainder) {
   if (LHS.isNegative()) {
     if (RHS.isNegative())
       APInt::udivrem(-LHS, -RHS, Quotient, Remainder);
     else *{
**        APInt::udivrem(-LHS, RHS, Quotient, Remainder);
       Quotient = -Quotient;
**       Remainder = -Remainder;
     }*
   } else if (RHS.isNegative()) {
     APInt::udivrem(LHS, -RHS, Quotient, Remainder);
     Quotient = -Quotient;
   } else {
     APInt::udivrem(LHS, RHS, Quotient, Remainder);
   }
 }


What should I do to submit the correction?

Thanks,
Preston

On Sun, May 20, 2012 at 6:21 PM, Preston Briggs <preston.briggs at gmail.com>
wrote:
> I wrote the following bit of code
>
> static APInt FloorOfQuotient(APInt a, APInt b) {
>  unsigned bits = a.getBitWidth();
>  APInt q(bits, 1), r(bits, 1);
>  APInt::sdivrem(a, b, q, r);
>  errs() << "sdivrem(" << a << ", " << b << ") = (" << q << ", " << r <<
> ")\n";
>  if (r == 0)
>    return q;
>  else {
>    if ((a.sgt(0) && b.sgt(0)) ||
>        (a.slt(0) && b.slt(0)))
>      return q;
>    else
>      return q - 1;
>  }
> }
>
>
> but sometimes see surprising results.  Here are examples:
>
> sdivrem(9, -2) = (-4, 1)
> sdivrem(9, -3) = (-3, 0)
> sdivrem(10, -3) = (-3, 1)
> sdivrem(38, 2) = (19, 0)
> sdivrem(29, 1) = (29, 0)
> sdivrem(-8, -1) = (-8, 0)
> sdivrem(-9, -1) = (-9, 0)
>
>
> What's up with the last two?
>
> Am I doing something wrong (again)?
>
> Thanks,
> Preston
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20120521/d1a37577/attachment.html>