[LLVMdev] Add/sub with carry; widening multiply

[Domagoj Babic]

Jonathan,

On Nov 21, 2007 11:31 AM, Jonathan Brandmeyer <jbrandmeyer at earthlink.net> wrote:
> I've been playing around with llvm lately and I was wondering something about the bitcode instructions for basic arithmetic.  Is there any plan to provide instructions that perform widening multiply, or add with carry?  It might be written as:
>
> mulw i32 %lhs %rhs -> i64 ; widening multiply
> addw i32 %lhs %rhs -> i33 ; widening add
> addc i32 %lhs, i32 %rhs, i1 %c -> i33 ; add with carry

You would need 2 mulw instructions: signed and unsigned. Those
two functions would be useful.

addw can be easily split into several instructions that compute
the result and carry/overflow separately. Than you can add that
carry using plain add. For details, see:
Henry S. Warren: Hacker's Delight, A. Wesley 2002.

Cheers,

-- 
        Domagoj Babic

        http://www.domagoj.info/
        http://www.calysto.org/