[PATCH] Check if Signed Addition Overflows

[Marshall Clow]

On May 15, 2014, at 9:42 AM, suyog sarda <sardask01 at gmail.com> wrote:

> Hi,
> 
> Attaching patch for checking if Signed Addition overflows.
> This patch implements two things :
> 
> 1. If we know one number is positive and another is negative, we return true as signed addition of two opposite signed numbers will never overflow.
> 
> 2. Implemented TODO : If one of the operands only has one non-zero bit, and if the other operand has a known-zero bit in a more significant place than it (not including the sign bit) the ripple may go up to and fill the zero, but won't change the sign.
> e.x -  (x & ~4) + 1
> We make sure that we are ignoring 0 at MSB.

I suspect that you are thinking too hard about this.
a + b will overflow (both positive) if
	MAX_VALUE - a < b 

where MAX_VALUE is the maximum possible positive value for that type.

Both negative can be handled similarly.

Bonus is that this doesn’t require the assumption of twos-complement representation.

[ If course, if we don’t have MAX_VALUE for the type, or the compiler can’t do math on that type, then my suggestion won’t work. ]

— Marshall



> 3 Test case included :
> 
> 1. To check if 0 in one number is at more significant place than 1 in second number. We emit 'add nsw' as we are sure there will be no overflow as the ripple will die at that zero.
> 2. If 0 is at less significant place than one, then emit normal 'add' as we are not sure if the ripple will die before reaching Sign bit.
> 3. IF the two numbers are of opposite sign, we emit 'add nsw' as we are sure that it will never overflow.
> 
> Please help in reviewing the patch.
> 
> -- 
> With regards,
> Suyog Sarda
> <AddOverFlow.patch>_______________________________________________
> llvm-commits mailing list
> llvm-commits at cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/llvm-commits